SIMPLE WAY OF CONSTRUCTING ROAD BRIDGES
P. SURESH. DEPUTY EXECUTIVE ENGINEER (R&B) SUB DIVISION, UDAYAGIRI. CELL NO: 9440818349
IMPORTANT NOTE
Respected Engineers, As the Booklet was prepared 10 years ago, some grades of concrete shown for Structures are now redundant and are to be modified as given below. The minimum grade for plain cement concrete : M15 The minimum grade for Reinforced cement concrete : M20 PLEASE CONSIDER MY REQUEST AND WISHNG YOU ALL THE BEST P. SURESH. DEPUTY EXECUTIVE ENGINEER (R&B) SUB DIVISION, UDAYAGIRI. CELL NO: 9440818349
Fundamentals & Classification A bridge may be defined as a “structure constructed providing passage over an obstacle without closing the way beneath”. It may be constructed, a) Across a stream or channel. b) To cross a railway line or traffic junction. c) Crossing of streams like aqueduct or super passage. In (R&B) Department we generally construct bridges either across a stream or to cross a railway line or traffic junction. The construction of aqueducts or super passages is related to Irrigation Department. First let us deal with construction of Bridge across a stream. The road bridges are generally constructed as per IRC codes and mostly we are adopting R.C.C bridges, pre-stressed concrete bridges, where as steel structures are generally adopted by Railways. Recently even in Railways, the bridges are constructed with pre-stressed and precast members with concrete. Before we go to the details of bridge, it is essential to know the definitions as per IRC 5-1998 and constructions of pipe culvers and slab culverts as per IRC code and definitions and terminology.
CULVERTS Culvert is a cross drainage structure having a total length of 6.0 m or less between the inner faces of dirt walls (backing walls). The culverts are constructed for minor discharges. It may be a Pipe culvert (or) Slab culvert. PIPE CULVERT: The pipe culverts are constructed for nominal flows like field channels, minor channels where the discharges are low. In this case R.C.C pipes are provided to allow free flow of water. They may be either NP2 class or NP3 class. In (R&B) Department we are adopting mainly NP3 pipes. NP4 pipes are being recently adopted in National Highways. N.P. means non pressure pipes and the No.1, 2, 3 etc. refers to the thickness of pipe. In roads we use only NP2 pipes in rural roads, where the shell thickness of pipe is 50 mm and NP3 pipes are used for major roads and State Highways. The shell thickness of NP3 pipes are 75 mm up to 750 mm dia and 100 mm for diameters more than 750 mm. The selection of diameter of a pipe is based on the level of road and discharges required. Further, the pipes may be placed over a C.C bed of (1:5:10) and a minimum cushion of 600 mm below road curst is required above the pipes, as per the IRC guide lines. The pipe culvert may be with one row or multiple rows. In case of pipe culvert with more than one row, the clear gap between the two pipes should not be less than ½ outer dia of pipe, subject to a minimum of 450 mm, to allow free flow of water. A typical section of pipe culvert is given below. The length of pipes (barrel) generally provided is 7.50 m / 10.00 m for rural roads and 12.50 m for major roads measured outer to outer of body walls. The pipe culverts can not function properly to drain off rainwater, as the vents get choked with jungle or other debris. They are normally provided with body walls on U/S & D/S in R.R masonry (or) in V.C.C (1:3:6).
7.50M / 10M / 12.50M 0.45
0.45 RCL of Road
0.30
C.C (1:5:10) Sand Filling
Internal Dia
Jeddy Stone
150mm C.C (1:5:10)
Sill Level
0.30
1.20M
0.15
H
Cushion < 600 mm below Road Crust
FIG No. 1 - Cross Section of Pipe Culvert The width of a body wall at sill level is generally fixed adopting the thumb rule (0.4 H+0.30) where H is the level difference between sill level and R.C.L. The top width is fixed as 0.45 m and the bottom width is calculated with the above two values by linear interpolation. The depth of foundation generally adopted 1.20 m below sill level and sill level is fixed at 0.15 m below existing bed level. Suitable slope of 1 in 1000 is adopted from U/S to D/S in pipes. The length of body wall is calculated as (2H + Outer to outer of pipes + 2H). The pipe may be either encased with cement concrete (1:3:6) (or) filled with gravel, as the case may be. C.C bed (1:5:10) of 150 mm thick has to be provided under pipes. In case of skew pipe culverts the length of barrel required will be more and has to be adjusted to full length of pipes (i.e. 2.5 m each pipe). In some cases, the pipe culverts are constructed from end to end of toe road embankment with small body wall at toe. The drawings for these culverts are readily available in IRC SP.13: Guide lines for the design of small culverts (single row and two rows).
SLAB CULVERTS: Slab culverts are generally proposed, where the discharge is considerable and when difference between sill level and RCL is too high or too low to accommodate the pipes. The function of slab culverts in discharging rainwater is good, compared to pipe culverts. That’s why slab culverts are preferred up to a linear water way of less than 6.00 m. A typical sketch showing the components of a slab culvert are given below.
3.50
Parapet Wall Deck Slab
Approach Slab
Backing Wall
Bed Block
H
Clear Span Abutment RCC (1:3:6)
Raft Foundation RCC (1:2:4)
500
Sand Filling
500
1.5
Sill
FIG No. 2 - Cross Section of Slab Culvert The sections of abutments, wing walls, details of Deck slab are given in IRC Sp.13 for different values of ‘H’ and span of culvert. When the Deck slab is supported on more than two supports the intermediate supports are called piers and sections for piers are also readily available. While constructing slab culverts the following points are to be kept in mind. 1) The sections given in IRC SP13 are based on S.B.C of 16.5 t/m2 and for other values of S.B.C. the sections are to be modified suitably, as per actual S.B.C. of soil at foundation level. 2) In case of B.C soils, the S.B.C. varies from 7.50 t/m2 to 10.0 t/m2, where individual footings under each abutment is not possible, a raft foundation in V.C.C (1:2:4) 500 mm thick may be provided.
3) The wings provided may be either box wings or fly wings. In case of box wings there should not be any connection between Abutment & Wings right from foundation level to deck slab. The reason is that the abutment is subjected to both vehicle loads and earth pressure where as the wing wall is subjected to earth pressure only. Thus there will be a difference in settlement and hence these two components are separated by placing a mastic pad right from foundation level. 4) Weep holes to drain off seepage water from earth behind abutment/ wing are to be provided at 1.0 m c/c staggered from the sill level + 0.20 m. Often many engineers feel that weep holes are to be provided above M.F.L only which is incorrect. 5) The slab culverts may be provided with approach slab in which case reinforced bed block and backing walls are to be provided. 6) Wearing coat of 100 mm at center and 50 mm at the end of carriageway may be adopted. 7) The width of slab culvert may be kept equal to carriageway width to 7.50 m and 12.5 m for major roads from outer to outer of parapets. 8) When approach slab is provided, the filling under approach slab may be done with stone dust only in layers of 225 mm thick, tamped layer by layer, instead of gravel to avoid subsequent settlement. If the filling is not done properly approach slab settles very soon after the traffic is allowed. 9) A craft paper / tar paper has to be placed over bed block as a bearing before laying deck slab concrete, to allow free movement of deck slab due to temperature variations. 10) The length of approach slab may be provided as 3.50 m from the back face of backing wall.
11) The length of box wing / fly wing will be provided depending on the difference in sill level and R.C.L which will be explained in detail in bridges, subsequently A drawing showing the reinforcement details of Deck slab for spans from 1 m to 6 m are given at the end, as per IRC SP13 – for ready reference. In the drawing the reinforcement was shown for mild steel. Similarly the sections for abutment & wing walls are also enclosed at the end for ready reference. The foundations of these sections are to be increased, when the bearing capacity of foundation soil is less than 16.50 t/m2. The concrete for abutment & wing wall shall be in V.C.C (1:3:6) and for Superstructure is either VRCC (1:2:4) or M20. However, as per IS 456-2000 the minimum grade of concrete for the structure nearer to coast (i.e. within 25 Kms from coast) is M25. This may be adopted for the culverts in coastal area. A mastic pad of 20 mm thick may be placed between slab & backing wall to act as expansion joint. The revised reinforcement details adopting tor steel is being enclosed at the end for deck slabs of culvert as available in the pocket book for Highway Engineers. (2002) for reference, as revised SP13 is not available.
FIELD DATA FOR BRIDGES Before we go for construction of bridges, it is essential to know the field details for design of bridge. The following details are required for design of a bridge. Maximum Flood Level: It is the level of the highest flood ever recorded (or) the calculated level for design discharge. The M.F.L. of drains, canals etc. are maintained by Irrigation Department and the values of M.F.L. and discharge can directly be obtained from Irrigation Department. In case such particulars are not available, the M.F.L. has to be obtained by local enquiry or flood marks over the nearby buildings (or) bridges. After knowing the M.F.L. we have to calculate the maximum discharge. Maximum Discharge: discharge.
There are two methods for arriving at the maximum
1) Area–Velocity method (given in Appendix – 2 of pocket book for bridge engineers). In this method the cross section of stream at site of crossing, 150 m upstream side and 150 m down stream side are taken and plotted. From the cross sections, the area of flow is calculated. For calculating the velocity Manning’s formula is used. 2/3
V = 1/n R S
½
Where n = the rugositiy coefficient R=
Hydraulic mean depth = A/P
P=
Welted perimeter
S=
Bed slope i.e. difference in level between the section on U/S & D/S V = Velocity in m/sec By adopting the formula Q = A x V, the flood discharge can be calculated.
2) By Catchment area Method: In this method the maximum discharge is calculated based on catchment area maps of Geological survey of India by adopting the Ryve’s formula. Q
=
CM2/3 (For Andhra Pradesh & Tamilnadu)
Where C
=
6.80 for areas within 25 Kms of coast
=
8.50 for areas between 25 and 150 Kms of coast.
=
10.0 for limited area near hills.
M
=
Catchment area in Sq.Km.
Q
=
Discharge in cum/sec.
The higher value of discharge obtained by the two methods is generally considered as maximum flood discharge. The design discharge arrived shall be increased for the design of foundations and protective work by 30% for small catchments up to 500 Sq. Kms and 25 to 20% for medium catchments of 500 to 5000 Sq.Km. LINEAR WATERWAY: For sections with defined banks the linear waterway is preferably kept equal to width of section at maximum flood level. For streams with alluvial beds (likely to be scoured during heavy floods easily) and no defined banks the linear waterway can be assumed W = C Q where C is usually taken as 4.80 As for as possible, the linear waterway shall be fixed, so as not to increase velocity. The linear waterway thus obtained my be compared with the bridges already constructed in the nearby vicinity by other departments like Railways, Irrigation. If the linear waterway is reduced, it may result in increase of velocity and maximum scour.
SCOUR DEPTH: The mean scour depth at a particular cross section is calculated with the formula. dsm =
2
1.34 (Db) Ksf
Where dsm = Db
=
1/3
mean scour depth discharge in cumecs per metre width which is given by total design discharge divided by effective linear waterway. (duly deducting pier widths)
Ksf
=
Silt factor depends on bed material varies from 0.50 to 2.00
MAXIMUM SCOUR DEPTH: The maximum scour depth for design of foundations for piers and abutments shall be considered as follows. (i)
for piers
=
2.0 dsm.
(ii)
for abutments
=
1.27 dsm.
(iii)
for design of floor protective works = 1.27 dsm to 2.00 dsm depending on the type of crossing.
AFFLUX: Afflux is the height by which the maximum flood level of stream rises at any point due to construction of bridge structure generally it is taken as 0.15 m. It shall be calculated depending on the linear water way etc. VERTICAL CLEARANCE: The minimum vertical clearance above HFL is given in IRC codes. Discharge in cumecs
Minimum vertical clearance (mm)
a) Up to 0.30
150mm
b) Above 0.30 and up to 3.00
450mm
c) Above 3.00 and up to 30.00
600mm
d) Above 30.00 and 300
900mm
e) Above 300 and up to 3000
1200mm
f) Above 3000
1500mm
The minimum vertical clearance shall be measured from the lowest point of deck structure inclusive of main girder to (M.F.L + afflux). From the above parameters the R.C.L of a bridge is calculated, as detailed below. R.C.L of a Bridge = M.F.L + AFFLUX + Vertical Clearance + Total Depth of Superstructure + Wearing Coat. In structures provided with metallic bearings no part of the bearings shall be at a height of not less than 500 mm above the design highest flood level and afflux. It is suggested that the difference between top of deck and affluxed H.F.L shall not be less than 1.75 m to safeguard road crust in approach embankment against capillary action of water. FIXING OF FOUNDATION LEVEL: After knowing maximum scour depth and M.F.L the depth of foundations shall be fixed as detailed below. Maximum scoured depth for pier
= 2.0 dsm.
Add grip length
=
Foundation level = =
1/3 (Max. scour depth)
M.F.L – Maximum scour depth – Grip length. M.F.L – (2.0dsm) – 1/3 (2.0dsm).
Once after fixing the foundation level, depending on the depth of foundations, we shall adopt either shallow foundations (Raft, individual footings) (or) deep foundations (Well foundations, Pile foundation).
Shallow Foundations: Isolated open foundations (individual footings) are generally adopted when S.B.C. of soil is around 15.0 t/m2 (or) more at shallow depths (i.e. around 3 to 5m depth below existing bed level). In cases where the S.B.C. of soil is less (like Black cotton soils, marine clays) smaller spans are economical, and raft foundations (or) Box structure with floor protection (to avoid scours) and cutoff wall are adopted. Deep Foundations: are generally adopted where suitable foundation soil is available at a depth of 6.0m or more, with substantial depth of water (or) large scour depth, because of which open excavation is not possible. Economical Span: The span arrangement generally depends on S.B.C of foundation soil. The following type of structures are generally considered economical for a particular range of span arrangement. 1) R.C.C single or Multiple boxes (Box culverts)
=
1.50 m to 15 m
2) Simply supported R.C.C slabs
=
3.00 m to 10.00 m
3) Simply supported R.C.C T-beam
=
10.00 m to 24.00 m
4) Simply supported voided slabs
=
10.00 m to 15.00 m
5) Simply supported pre-stressed Cement concrete girder bridges.
=
25.00 m to 45.00 m
6) R.C.C box sections simply supported And balanced cantilever (box girders)
=
25.00 m to 50.00 m
7) Cable stayed bridges (suspension bridges)
100.00 m to 800.00 m
=
MINIMUM DEPTH OF FOUNDATION: The minimum depth of foundation generally adopted is 2.00m below existing bed level. MINIMUM DISTANCE BETWEEN ROAD BRIDGE & RAIL BRIDGE: The distance between rail and road bridge should not be less than 400 m in any case. The
width of carriageway shall not be less than 4.25m for single lane bridge and 7.5m for double lane bridge. In National Highways an overall carriageway width of 12.0m from outer to outer of kerb is being adopted. The minimum width of footpath shall be 1.50m.
PROTECTIVE WORKS: The protective works shall include Jeddy stone apron on U/S and D/S for raft foundations, river training work, approach road protection. Generally we come across with raft foundation at shallow depth with cut off wall up to maximum scour depth and Jeddy stone apron. The protective works shall be completed before the monsoon, so that the foundations do not get undermined, during floods in rainy seasons. BEARINGS: Bearings are the vital components of a bridge which while allowing of longitudinal and or transverse rotations or movement of the superstructure with respect to substructure thus relieving stresses due to expansion and contraction and effectively transfer loads and forces from superstructure to the substructure. The following are the generally adopted bearings. a) For solid Deck slab / voided Deck slab, the superstructure resting on unyielding supports, no bearings need be provided when the span length is less than 10 m. A kraft paper is provided between Deck slab and Bed block to allow free movement. The top of pier caps (Bed blocks), Abutment caps are rubbed smooth with carborandum stone. b) For T-beam bridge with span more than 10m and resting on unyielding supports, neoprene or elastomeric bearings are provided. c) For spans larger than 25 m roller and rocker bearings are considered.
For spans in gradient like R.O.Bs the bearings shall be placed horizontally with suitable bearing pedestals (so that the loads are transferred to a horizontal plane), but not in gradient. EXPANSION JOINTS: Expansion joints are provided at the end of deck and cater for movement of deck due to temperature, shrinkage, creep etc. a) When the span is less than 10 m incase of solid deck slabs, a mastic pad of 20 mm thick shall be placed and top sealed with bituminous joint filler to act as an expansion joint, which can cater for a horizontal movement up to 20 mm. b) When the span is more than 10 m and less than 24 m elastomeric slab seal expansion joint is being adopted now, which can cater to a maximum horizontal movement of 40 mm. this gives very smooth ride to vehicles without any jerk and noise. c) When the span is large i.e. more 25 m strip seal expansion joint (with elastomer) shall be provided which allows movement up to 70 mm. This type of joints are giving jerks to the vehicles while moving over joints. Corrosion Protection Measures: In coastal regions, distress has been observed in many bridges due to corrosion of reinforcement which can be prevented by adopting adequate cover to reinforcement (not less than 40mm) and proper compaction of concrete. The corrosion of reinforcing steel does not occur. When the concrete is totally dry or totally submerged. The corrosion can be prevented by using protective coatings with the help of fusion bonded epoxy coatings. The cost of this coating is around Rs.9000/- per M. Tonne. Over the rate adopted as per S.S.R, which may be included in the estimates itself and can be insisted during execution, as is being done in Railways.
Approach to Bridge: The approaches on either side of a straight bridge, shall have a minimum straight length of 15 m. However, in usual practice, 15 m straight and level portion is being provided on either side of bridge. The free board for the approaches to a high level bridge shall not be less than 1750 mm. It is important to know the difference between vertical clearance and free board. The term vertical clearance is referred to bridge i.e. the level difference between M.F.L and bottom of bridge superstructure, where as free board is referred to approaches of bridge the level difference between M.F.L and R.C.L of approach road. Gradient and Vertical Clearance in R.O.Bs & R.U.Bs: In case of road under bridges, a minimum vertical clearance of 5.50 m in urban areas and 5.0 m in rural areas shall be provided. For road over bridge, a minimum vertical clearance of 5.87 m for electric traction and 4.875 m for non electric traction. The gradients generally adopted is 1 in 50, wherever possible. In case of congestion due to Built up area etc., may be increased up to 1 in 40. No level portion need be provided, on either side of duct portion. The approach slab shall be laid in gradient only, as per approved L.S of road, but not level. Approach Slab: The purpose of laying a approach slab is to transfer the loads coming from approaches in a smooth way to bridge structure without any impact. The length of approach slab generally adopted is 3.50 m from the back of backing wall
and 7.50m width. A typical section showing the position of approach slab and reinforcement details are given below. Approach Slab 300
Width of Backing wall
100
3.50m
C.C (1:4:8)
WMM
GSB
Stone Dust Filling 300
12mm Ø @ 150mm c/c Both ways
12mm Ø @ 150mm c/c Both ways
FIG No. 3 - Details of Approach Slab The approach slab shall rest on backing wall, as shown in drawing. A lean concrete (1:4:8) 100 mm thick under approach slab shall be provided. It is important to note that the filling under approach slab is to be done with gravel, as per IRC 78. However when gravel is used for filling, the approach slab can not be laid immediately, without allowing for traffic, as there will be settlement in gravel. When adequate time is available for laying approach slab, it is advisable to lay the approach slab after one rainy season duly allowing the traffic. When it is not possible to lay the approach slab later, as in the case of of elastomeric slab seal expansion joint, it is generally adopted to fill the portion below approach slab with stone dust right from bed level. The stone dust shall be laid in thin layers of 150mm thoroughly watered and compacted with rammers layer by layer. If the filling is done with stone dust, the settlement under approach slab can be prevented without any trouble. This method proved to be effective. Hand Rails & Hand Posts: The purpose of providing hand rails is to safeguard the vehicles and passing public falling from top. In case of bridges in nonurban areas, Type-I railing with two rows of railing with a height of 770 mm is adopted. In case of bridges in urban areas Type-II railing with three rows of railing
with a height of 1050 mm is adopted. All the railings will be precasted and fixed in position, and laid concrete for hand posts. Both the railings and posts are supported with the kerb of 275 mm depth normally. Now a days in N.H. the system of providing hand rails and hand post is replaced by crash barrier type protection. Perforated kerbs are provided, when railing is provided for causeways, when the level difference between bed level and R.C.L of bridge is more to allow free flow of flood waters above the bridge deck. Wearing Coat: The wearing coat shall be provided over the superstructure with M30 concrete to protect the Superstructure. The thickness of wearing coat at centre is 100 mm and at ends is 50 mm. Reinforcement to cater tot the temperature effect shall be provided with 6 mm at 200 mm c/c both ways. However at joints location extra rods to a length of 500 mm on either side shall be provided at 200 mm c/c, in order to maintain 100 mm spacing at joints. This type of wearing coat is now dispensed and Bituminous wearing coat is being adopted, in main roads like National Highways and State Highways, explained subsequently in bridges.
COMPONENTS OF BRIDGES Introduction: The various components of a bridge structure is shown below.
Approach Slab PCC Bed 3.50M
Wearing Coat
RCL
Superstructure Elastomeric Bearing Bearing Pedestal
Backing Wall Filter Media Behind Abutment (600mm)
Weep Holes (100mm dia) Abutment
GL Well Cap Top Plugging Well Steining
Sand / Water Filling Bottom Plugging of Well
Bowl
Well curb Well Cutting Edge
FIG No. 4 - Sketch Showing Bridge Structure We can divide any bridge work in to the five components. (A) FOUNDATIONS. (B) SUB STRUCTURE. (C) SUPERSTRUCTURE. (D)APPROACHES. (E)PROTECTIVE WORKS. Let us discuss in brief the functions of each component. (A) FOUNDATIONS: It is a component of bridge, which transfer the loads coming from Substructure to the foundation soil. So the foundation is mainly based on the S.B.C. of foundation soil and the pressure on foundation soil shall be less than S.B.C. of soil. The foundations are of two types. (i)
Open foundation [Raft (or) individual footings].
(ii)
Well foundations (or) Pile foundations.
In case of open foundation the process is simple. First we excavate openly to the required foundation level (normally less than 5.0 m) and lay concrete for foundations, as per the design necessary. In case of well foundations, the following is the mode of transfer of load to the foundation soil. As seen in the drawing shown above the load from substructure is transferred to the well cap (i.e. a top member of foundation). From well cap the load is transferred to well steining. The load from well steining is transferred to the foundation soil through bottom plugging. A schematic representation of transfer of load incase of well foundations is given below. Load of vehicles Superstructure Substructure Well cap Well steining Bottom plugging Foundation soil (B) SUBSTRUCTURE: It may be defined as a structural member which transfers the load coming from superstructure to foundations. This can be divided into pier & abutment. It may be (i)
Wall type structure.
(ii)
R.C.C. circular pier
(iii)
A set of piers connected by beam (pier cap) called “Trestle”
The wall type structure shall be in plain concrete, generally adopted for raft foundation (or) causeways with deep foundations. The other two types are provided in R.C.C. with adequate reinforcement as per design. The substructure includes pier cap, bed block and backing wall over Abutments. Generally these members are provided in R.C.C. It may be a simple wall type pier cap, Hammer Head pier cap. Pier cap in the form of a beam supported on 3 to 5 Nos. of R.C.C pipes. Further the substructure includes bearing pedestals which accommodate bearing, through which the load from superstructure is transferred to substructure. (C) SUPERSTRUCTURE: It is a component of bridge, which directly takes the load of vehicles allows movement of vehicles directly. This may be a (i) Solid Deck Slab (ii) T-Beam type superstructure (iii) Voided Deck slab (iv) Box girder. The first two are solid sections, where as the remaining are Hollow sections in R.C.C. (D) APPROACHES: It is general practice to provide a straight and level portion of 15m in approaches on either side of bridge structure. The soil used for formation generally should have dry density of 1.52 gms/c.c up to an embankment height of 3.0 m and dry density 1.60 gms/c.c for embankment height more than 3.0 m. The carriageway width of double lane approach is 7.0 m and formation width is 12.0 m. Extra width shall be provided inside portion of curve as per designs. The crust for approaches shall be designed, as per C.B.R. of sub grade soil. The C.B.R. of sub grade soil used for embankment formation shall not be less than 5%. Minimum Radius of Curve Maximum gradient permissible
= =
90 m 1 in 30
Necessary revetment shall be provided for embankments having a height of more than 1.50 m. It is very important to provide a fold in revetment to a width of 0.60 m in to the formation width on either side. Chutes in revetment is to be provided at 15 m intervals, to drain off the rainwater, without damaging the revetment. The toe wall shall have to be taken into the bed level to a depth of 0.60 m. Toe walls should
never be laid on filled soils (or) by refilling above the natural bed level. If necessary a C.C retaining wall shall be provided to avoid failure of revetment. (E) PROTECTIVE WORKS: Protective work include bed protection against scour in the form of Jeddy stone apron on U/S and D/S, River raining works, Quadrant Revetment around wing walls etc. Based on the above parameters, the designs, wing finalises the sections and communicate the drawings. The job of construction Engineer is to study the drawings thoroughly and to place the structure on ground in a correct way. The points discussed so far are to give a basic idea of various design parameters.
SUB SOIL INVESTIGATION BEFORE START OF WORK Before start of bridge work, confirmatory of bores shall be taken to ascertain the type of foundation soil and their properties. For Minor Bridges: at least one bore at each of the abutment location and one in bed of stream shall be taken. For Major Bridges: at least one bore shall be taken at each of the abutment and pier locations. The depth of sub soil investigation shall be based on foundation level and the bore holes shall extend up to at least 1 ½ times the width of foundation below the foundation level to ascertain the variation in soil properties if any. The diameter of well is considered for width in case of well foundation. The soil properties should be same up to 1.50 times the dia of well below the foundation level. If there is any week soil like marine clays are noticed, settlement analysis has to be done to arrive at the likely settlement after loading. Disturbed / undisturbed soil samples shall be taken at every 1.5 m intervals. In case where undisturbed soil sample can not be taken (like sands etc.) the standard penetration test (at field) has to be conducted and SPT values noted. For cohesive soils like clays, gravel etc., the lab values of C, Ø are obtained at each stage. The safe bearing capacity at foundation shall be calculated as per IS.6403-1981, code of practice for determination of bearing capacity of shallow/ foundations and compared with design pressure on foundation soil. The S.B.C. obtained from C, Ø values shall be more than the pressure on foundation soil. For cohesion less soils (where Ø=O) like sands, it is not possible to take the undisturbed soil sample. Hence standard penetration test shall be conducted at site and from SPT values the S.B.C. of soil shall be arrived and compared. In case of presence of any marine clays (or) inferior soils between layers of sand at a depth of less than 1.5 times the width of foundation below foundation level consolidation test shall have to be conducted and settlement analysis done. The
permissible value of settlement in clays is 76 mm. This case is generally encountered in structures near sea coast. A consolidated bore chart shall be prepared duly giving the various types of soils, at different levels. This will be useful during the well sinking operations, to know the problems and to take remedial measures for reducing excess tilts and shifts.
PERMANENT BENCH MARK Immediately after ground marking of Bridge it is essential to establish a permanent Bench mark constructed in concrete and connected to the G.T.S. bench mark nearby. It is highly essential and take all precautions, in carrying out bench mark to the bridge site. In a hurry, the B.S. & F.S. are erroneously noted, when the sun sets in evening due to in adequate lighting. A small error in reading metres of staff lead to total mistake and went unnoticed until the substructure is completed. It is highly essential to check the levels twice before fixing the value of B.M near bridge. The pedestal constructed for bench mark shall be away from the alignment of bridge work and approaches, so that it will remains until the bridge opened to traffic. In case of R.O.Bs and R.U.Bs the bridge portion across the railway track shall be constructed by Railway authorities and R&B department will form approaches on either side of bridge. Some times error takes place in the bench mark of Railways and (R&B) authorities due to some errors in instruments. Therefore, in order to over come this problem, a common bench mark shall be established accessible for both the departments and the value shall be arrived by joint inspection. It must be on record, to avoid future problems. This common bench mark shall be constructed in concrete and far away from the line of alignment, and must be safeguarded for years until the bridge is opened to traffic. This may be connected to other points over buildings also to use on alternative or to check the level from time to time. The value of bench mark may be painted neatly on the top of pedestal.
FOUNDATIONS (A)
OPEN FOUNDATIONS
Earthwork Excavation:
In case of open foundations the earthwork excavation
can be started immediately after the completion of marking and construction of pedestals. The depth of foundation is generally around 3 m to 5 m from the existing bed level. The excavation may be taken up in the form of steps as shown in the drawings up to foundation level.
1.0
Existing Bed Level
1.50
1.0
1.0
B
1.0 Foundation Level Pit for Dewatering
Pit for Dewatering
FIG No. 8 - Earth Work Excavation for Open Foundation This type of excavation is possible only in cohesive soils like B.C soils in dry condition gravel, other hard soils. In case of cohesion less soils like sand, this type of excavation can not be excavated. Hence shoring and shuttering is to be arranged to arrest falling of sand from top. If water table is present above foundation level, dewatering should be done to lay the concrete. This has to be done by digging pits at corners, beyond the foundation width as shown in the sketch and should be pumped out simultaneously to reduce the water table level below the foundation level. In case of sandy soils, the dewatering leads to sand blows, finally collapsing large heaps of sand. Hence the excavated width should be large enough to avoid filling of foundation due to sliding of sand. Immediately on reaching foundation level bed concrete shall be laid without any delay. The S.B.C. of soil at foundation level has to be ensured by taking undisturbed soil samples in case of cohesive soils. In case of cohesion less soils standard penetration test may be conducted at site, and from S.P.T.
values the bearing capacity of foundation soil can be calculated as per IS code. The S.B.C. of soil thus obtained should be more than the design pressure on foundation soils. The extra depth if any excavated below foundation level shall be filled with sand only but not with excavated earth. The pits excavated for dewatering shall also be filled with sand only after laying concrete. In order to avoid delay in obtaining laboratory C, Ø values, the samples may be collected at two or three locations by digging pits up to foundation level manually and sent to laboratory well in advance before start of work. This procedure avoid keeping excavation open for weeks, which may create problems due to unexpected rainfall, for want of laboratory C, Ø values. In case of open foundations in rocks the annular space around the footing shall be filled with cement concrete of M15 grade up to top of rock. For laying the foundation concrete, when it is not possible for dewatering due to heavy percolation of water, it shall be laid by tremie pipe method (or) skip boxes. No pumping of water shall be allowed from the time of placing of concrete up to 24 hours after placement as it may result in sucking of cement from concrete by pumps. Pumping during concreting leads to loss of cement and causes failures, if neglected. Hence pumping during concreting operations shall not be allowed under any circumstances. It is suggested that the last 300 mm of earthwork excavation shall be done just before laying of lean concrete below foundation.
Reinforcement of open foundations A typical sketch showing the reinforcement of R.C.C raft is shown below.
Distribution Reinforcement
Cover 50mm
Main Reinforcement
Length of Raft
Main Reinforcement
FIG No. 9 - Reinforcement Details of Raft
Reinforcement of skew R.C.C raft: In case of skew R.C.C rafts, the reinforcement in span direction i.e. main reinforcement should be kept perpendicular to the pier line and distribution reinforcement parallel to pier line. A typical sketch showing the reinforcement in R.C.C skew raft is given below. In case of skew R.C.C. rafts the reinforcement is kept erroneously parallel to the sides of a parallelogram formed which is incorrect. The construction joints after days work shall be kept at the location of pier only, not in the mid span when the raft is to be laid for more days. Laps to the reinforcement shall be provided as 56 times dia of rod. Necessary shear keys in the form of the steel from the bottom of raft to a length of 0.50 m above raft, at pier location shall be provided to have good connectivity with raft. In case of P.C.C. raft, the shear keys may be made in the form of holes to a depth of 0.30 m at pier locations. This can be achieved by keeping some precast blocks in fresh concrete and removing the same after one hour.
In case of R.C.C. circular piers, the main rods of piers must be brought from the bottom of raft with necessary development length. The position of pier reinforcement should be carefully marked with adequate cover. Otherwise the bars will be out of pier position or the cover to reinforcement would be less. The coarse aggregate for plain raft may be 40 mm graded metal and 20 mm graded aggregate for R.C.C. raft. The foundations for wings are generally kept at the same level of raft while laying the footings for wings, a mastic pad shall be kept between raft and footing of wing, as there will be differential settlement between raft and wings. It is to be clearly noted, that there should not be any connection between abutment and wing right from foundation level to R.C.L. to allow differentiate settlement if any due to loading.
Abutment Wing
Wing
Raft
Wing
Wing Position of Mastic Pad at Foundation Level
FIG No. 10 - Plan of Raft & Wing Foundation
WELL FOUNDATIONS As already shown in the drawing, the following are the main components of well foundations. (a) Well cutting edge:
It is the lowest portion of well foundation, which
induces maximum stress on foundation soil to permit the well to sink. Generally it contains two angles and one plate welded for well foundations in soils as shown below.
Well Curb
M.S Plate I S Angles
FIG No. 11 - Section of Well Cutting Edge in Soils
The weight of cutting edge shall not be less than 40 Kgs per metre length, as per standards. In case of wells likely to pass through hard strata like rocks a steel plate cutting edge with lining to the well curb shall be adopted, as shown below.
M.S Liner outside of Kerb
Well curb in concrete
M.S Liner inside of Curb
Steel Plate Sharpened M.S Liner
FIG No. 12 - Section of Well Cutting Edge in Rocks
The fabrication of cutting edges shall be carried out to the dimensions specified in the drawings. The steel sections should not be heated for bending to a circular shape. However ‘V’ cuts may be made in the horizontal portion throughout the length, to facilitate cold bending. After bending the ‘V’ cuts shall be filled by welding. The cutting edge shall be placed in true level position at exact location. Any slope of bed should be trimmed to level position. The cutting edge shall be kept at 300 mm above existing water level (if water is present). The steining rods shall be bonded to cutting edge, to prevent separation of cutting edge during sinking operations. For this purpose, the end of the steining road shall be threaded with a lathe machine. A hole is made in the cutting edge angle and a check nut would be welded to the angle at the bottom. The steining rods with threads should be tightly fixed to the nut and welding, also done to the cutting edge angle. Simple welding of steining rod with angle without above arrangement will not be sufficient and may get separated during sinking operation. (b) Well Curb:
It is the portion above the well cutting edge in R.C.C. The
slope is like a trapezium in cross section, and circular. This particular shape (like a truncated cone) is necessary to do bottom plugging and to stop further sinking of well beyond foundation level, due to subsequent live loads etc. The outer face of well curb is generally kept vertical. All concreting in the well curb must be completed in single operation irrespective of depth of curb. The inside faces and out side faces of well curb shall be protected with M.S. liners for the wells to pass through boulders hard rock etc. The sinking operation of well shall be started after a minimum 3 days of curing. The following formula would be useful for computing concrete quantity in well curb.
Π/4 (D2) – (Π/12)* (a2 + ab +b2) x H
Where ‘D’ is the outer dia of well, ‘a’ dia at top of cutting edge, ‘b’ inside dia well and ‘H’ is height of well curb. (c) Well Steining: It is the portion of well foundation, above the well curb and up to the bottom of well cap. This transfers the load coming from well cap to the foundation soil, through bottom plugging. The first lift of steining shall not be more than 2.0m. The first lift of steining shall be cast only after sinking the well curb, at least partially for stability. The other lifts may be 2 to 2.50 m. The steining concrete of well shall be executed in one straight line with the help of a straight edge. A scale should be marked right from the bottom of cutting edge in all four corners (2 Nos. in traffic direction and 2 Nos. in perpendicular direction) after laying the curb with a steel tape. The marking of scale should be done with a precision steel tape, by a responsible person. The zero of scale starts at the bottom of cutting edge. These scales will be useful in calculating the tilts and shifts of well, after sinking of each lift of steining. The steining rods should be positioned with uniform spacing, alround the well. It may be noted that the steel provided in well steining is only a nominal steel (0.12% of gross cross-sectional area), but not structural steel. Adequate care must be taken of maintain cover to the reinforcement both in inside and out side of well. The steining concrete shall be laid using 40mm metal only, even though some steel is provided and is called V.C.C only. The steel provided in well steining is only to prevent separation of different lifts of steining during sinking operation. The steining rods shall be taken up to the top of well cap, with adequate development length.
(d) Bottom Plugging:
After the well was taken to the required foundation
level, the portion between the two inner faces of well curb shall be filled with concrete, which would be like a truncated cone, as shown below.
Well steining
Well steining
0.6
b
Well Curb
Bottom Plugging a h< d/6 a
Well Curb Foundation Level Bowl portion of well
FIG No. 13 - Section of Bottom Plugging of Well
It performs two functions i)
It transfers the load from foundation to sub soil.
ii)
It stops the well at foundation level without permitting for further sinking due to further live loads etc. The bottom plugging shall be done in one continuous operation. When water
is present it is not advisable to resort to dewatering operation. In case of sandy soils as this may lead to sand blows and induces tilts and shifts in wells. In such a case bottom plugging shall be done using a tremie pipe. No dewatering shall be allowed during concreting and up to 24 hours after placing concrete in position. The cement content shall be increased by 10% for respective grade of concrete for under water concreting. Often, a small mistake is committed while preparing the estimate. The bowl portion below foundation level is also to be filled with bottom plugging concrete. This
bowl portion concrete quantity shall be added in estimate. The depth of sump below foundation level shall not be more than d/6 ini general. In sands it shall be around (d/12). The following formula would be useful in computing the bottom plugging concrete quantity. Qty. in truncated cone
=
II (a2 + ab +b2) x H 12
Quantity in bowl
=
II x h x ( a2 + h2) 8 6
(Segment of a sphere)
The height of bottom plugging shall be measured with soundings using a steel tape when water is present. (e) Sand filling / Water filling: After the well was taken to the required foundation level, bottom plugging would be done as explained above. The portion above bottom plugging shall be filled with sand. The sand filling shall be started after 3 days of laying of bottom plugging. This shall be carried out up to the bottom level of top plugging. Some times, when the foundation soils are too weak even after taking the well to the required foundation level, the sand filling shall be replaced by water filling to reduce the load and thus the pressure on foundation soil. (f) Top Plugging: It is a thin layer of plain concrete laid above sand filling and under well cap. The purpose of top plugging is to continue the sand filling and to act as base for well cap when water filling is done in place of sand filling, no top plugging shall be possible. (g) Well Cap: It is the top most portion of well foundation (which is a structural R.C.C member), which transfers the load coming from substructure to well steining and then to foundation soil, as explained above. It is generally in circular shape.
The reinforcement contains one mat at bottom and one mat at top with reinforcement in both directions. The steel of well steining shall be brought up to the top of well cap to have adequate bond. The reinforcement of well cap is shown below.
Two mats one at Bottom and one at Top
FIG No. 14 - Reinforcement of Well Cap (Circular)
All the wells are generally designed for a resultant tilt of 1 in 80 and resultant shift of 150mm. CALCULATION OF TILT& SHIFT OF A WELL The wells during sinking operation are likely to get some tilt and shift. Let us know what is a tilt and shift. Tilt may be defined as the inclination of the well to plump line. Shift may be defined as the movement of centre point of well horizontally. The calculation of tilt and shift of well is given below for reference.
A proforma for calculation of tilts and shifts of well is given in appendix 1200/II of “Specification for Road and Bridge works”. The same is enclosed for ready reference.
PROBLEMS IN SINKING OPERATION:
The sinking of a well is
done by removing the material from inside of the well and thus inducing maximum stress at cutting edge portion, thus allowing the well to go down. During this process the wells are likely to get some tilt and shift in both X – X direction and Y – Y direction. After sinking one lift of steining (generally 2.0m) the tilt of a particular well is calculated.
Y1
Y1
Ty
X1
X2
0
Ly
Y2 Y2 Y-Y Direction X1 Lx
Tx
X2
X-X Direction
FIG No. 15 - Sketch Showing Tilts & Shifts of Well
From the above figure, we can say that the well has tilted in both directions. The tilt along X – X direction (Tx) is given by [Lx / outer dia of well] and similarly tilt along Y – Y direction (Ty) is given by [Ly / outer dia of well]. After knowing these two values the resultant tilt is given by (Tx 2 + Ty 2). Now this tilt is to be rectified. To rectify the tilt, first of all we must know the plane where the resultant tilt is acting.
From the above figures we can say ‘X’ is higher in one direction and ‘1/2’ is higher in the other direction. That means the resultant tilt acts in (X1 O Y2) plane. Now we have to keep additional weight in the form of sand bags in the plane (X1 O Y2) during further sinking. Similarly in the opposite plane (Y1 O X2) we have to keep some sort of obstruction at the bottom of cutting edge under the well, so that this will not tilt further. And at the same time the earth under the cutting edge in the plane [X1 O Y2) is excavated more, so that the well comes down in this plane. This can be achieved with sinkers, who can go up to the cutting edge, with helmets if it is under water. This procedure is repeated in sinking of each lift of steining, so that the resultant tilt is minimized when the well reaches its foundation level. During this process of rectifying tilts the well is likely to get shift from the alignment. Rectification of tilt only can be done by remedial methods mentioned above. That means, during the process of rectification of tilts the shifts are also to be rectified. From this we can say without tilt, there would not be any shift. Therefore, there are not separate methods for rectification of shifts. Other methods of sinking like kinematic sinking and mild blasting under the cutting edge of well are also adopted for rectifying the tilts of well, by experience. In case of wells resting over rock blasting under cutting edge and chiseling of rock under cutting edge are adopted. The measurement for payment of well sinking shall be taken from the level at which cutting edge is placed actually to the foundation level. This shall be from bed level, when the bed is dry and 300m higher than the water level if water is present. In case when the bed is too high from average be level, at abutment location. It is economical to remove the earth and placing the cutting edge at average bed level, so that earthwork payment shall only be effected instead of well sinking from that higher level. It should be noted the earth shall be totally removed but not like a hole. The cutting edge should never be placed in a pre defined hole, in any case.
Even after adopting all the methods, if the well is having more tilt and shift, we have to go for revised design of well cap and the pressure on foundation soil on [X2 O Y2] plane will be more. So the pressure on foundation soil has to be checked. If the pressure on foundation soil is more than the S.B.C of the soil at foundation level, the well has to be taken down further, so that either the tilt can be reduced or till the required S.B.C of soil is reached. The shape of the well cap will be different from circular shape in this case, when the well exceeds the permissible limits of tilt and shift. All the design aspects would be examined by (D&P) wing and we are supposed to give the correct picture of tilt and shift of a particular well. While making payment for well sinking a recovery has to be effected from the bills of contractor, for excess tilts and shifts as detailed below. S.No.
Amount of Tilt and Shift
Percentage deduction on the Rate for sinking of whole well
1.
Tilt exceeding the specified permissible value (1 in 80) but equal to or within 1 in 60.
5%
2.
Tilt exceeding 1 in 60, but equal to or within 1 in 50.
10%
3.
Tilt exceeding 1 in 50
20%
4.
Shift exceeding the specified permissible value (150mm) but equal to or within 200mm.
2%
5.
Shift exceeding 200mm but equal to or within 300mm
5%
6.
Shift exceeding 300mm
10%
If the well exceeds the tilt of 1 in 50 and shift of 300mm it shall be considered as substandard work.
All the excess items due to the revised design of well cap shall be borne by the contractor in addition to the penalty mentioned above. The entire depth of sinking if any required, shall be borne by the contractor and no extra payment shall be made to the contractor for this. In case of skew bridges and curved bridges with wall type piers, the Y-Y axis is taken normal to the traffic direction and line of pier will be different from Y-Y axis adopted for calculation of tilts and shifts as detailed below.
Y
Pedestals for Piers
Pier Line
X X
Traffic Direction
Well Cap
Pedestals for Calculation of Tilt & Shift
Y
FIG No. 16 - Plan of Well for a Skew Bridge (or) Curved Bridge
That means for the purpose of calculating tilt and shift the axis perpendicular to the traffic direction is only considered irrespective of line of pier. It is very important to note that shear keys in the form of steel (either 20mm or 25mm spaced at 1.0m c/c) shall be provided in the pier location (in case of wall type P.C.C piers) right from bottom of well cap to a height of 0.50mabove well cap. In case of R.C.C piers, the main reinforcement of pier shall be kept from the bottom of well cap with adequate bond length. In case of a trestle (a set of two or more piers) the marking of pier should be done carefully and main reinforcement of pier shall be placed in the correct location with due care.
The top level of well cap shall be kept generally at a higher level than the bed level incase of canals, where water is continuously present (genrally above the water level), so that the concrete can be laid without any difficulty. Incase of dry beds, where flow is nominal, the bottom of well cap may be kept at the existing bed level. The well caps placed at higher level in dry beds gives ugly appearance and hence this should be avoided as far as possible. The typical shape of a well cap with tilt exceeding 1 in 80 is given below for information. X
Shifted Position of Well Original Centre of Well
No Change in Pier Position Shifted Centre of Well
Original Position of Well
X
FIG No. 17 - Well Cap Redesigned for Excess Tilt & Shift
MARKING OF SUBSTRUCTURE: As already discussed, the substructure may be (i)
Wall type pier.
(ii)
R.C.C circular pier with hammer head bed block.
(iii)
A set of two or more R.C.C piers connected by beam (trestle)
(iv)
V.C.C wall type abutment.
(v)
R.C.C spill through abutment.
(vi)
Wing walls either box type wings or fly wings.
The selection of a particulars type of pier is mainly based or design parameters. The abutments are generally in plain concrete using 40mm HBG metal. The box wings are preferred to fly wings, when the height of fill is less. Weep holes shall be provided at 1.0m c/c in both directions, staggered in successive rows as shown below from the level of O.F.L + 0.30m. The weep holes should have slope towards drain face, as shown in drawings subsequently. After laying well cap, the marking of pier should be done with the help of permanent pedestals in transverse direction. It is to be noted, that the position of pier remains unaltered even though the well has tilt and shift. Often it is of wrong opinion that the position of pier will be changed, when the well is having tilt any shift. Because of the tilt and shift, the centre of well will not coincide with the centre of pier position, as shown below, thus the loading will be eccentric.
Y
Pier Position fixed irrespective of centre of well
Centre of Shifted Well
Original Position of Well
X
X
Shifted Position of Well Centre of Pier and Centre of original well
Y
FIG No. 18 - Sketch Showing Position of Shifted Well
The design of well shall be checked for this eccentric loading.
In case of open foundation, the marking of pier shall be done, with the help of pedestals in both directions. Incase of skew bridges the length of pier shall be increased to accommodate semicircular cut waters as given below. Incase of P.C.C piers / abutments, shear keys in the form of holes, shall be provided in each horizontal layers, to keep the substructure monolithic. This may be staggered. The pier cap / abutment cap will be in R.C.C adopting 20mm HBG graded chips. It may be a) Bed block over wall type pier. b) Hammer head bed block for single R.C.C pier. c) A beam connecting set of piers (Trestle) d) Bed block & backing wall over abutment.
Backing Wall
Bed block over Abutment
> 150mm height
1.0
Bearing Pedastal Hammer head Pier cap
1.0
Main reinforcement of pier Pier
weep holes
Well cap
Well cap
Le ng th
Single pier with Hammer Ex
tra
Head Bed Block
Stirrups of Backing wall
Ex
tr a
Le ng th
Stirrups of Bed block
Reinforcement details of Bed block & Backing Wall
Cut waters for Pier in Skew bridge
FIG No. 19:- Substructure Drawings
The main reinforcement of pier shall extend into the pier cap up to the top with adequate bond. The reinforcement details and position of bearing pedestal is shown in the sketch given below. Laps in main reinforcement of pier shall be staggered and not more than 1/3 of total bars should have lap at one point. The lap length shall not be less than 56d. The minimum clear distance between each lap shall be not less than 3m. The each lift of concrete for pier shall not be more than 2.50m except in case of shutter vibrators are used. Incase of skew slab bridges, the bearings shall be kept normal to the girder positions and the width of pier cap shall be provided extra, as shown below.
X
0
Normal Bearing Pedastal Pier cap
<1 50 mm
Bearing Pedastal
X
FIG No. 20 - Plan of Pier Cap in Skew
If Ø is the skew angle, the width required in normal will be multiplied by Sec Ø to arrive at the width of pier. Suitable clearance of not less than 150mm beyond the edge of bearings shall be provided. In case of gradient slabs like R.O.Bs it is very important to note that the pier cap and bearing pedestal shall be provided horizontally level but not in gradient, as shown below.
irder
G Main r Girde Main
Pier Cap Pier
FIG No. 21 - Pier Cap in Gradient Thus the shutter at the raising end shall be raised according to gradient and the quantity of concrete is slightly increased. If B is the width of bearing, and gradient is 1 in ‘S’ the raise at outer end will be B/S. Thus the shutters at the lower end will be joined to bearing and raised by (B/S) at higher end with reference to lower end. Incase of curved bridges without bearings (spans less than 10m) super elevation required will be provided in pier cap only, by raising the outer edge of pier cap. In this case also, the top surface of pier will be in level portion and super elevation in outer direction as detailed below.
ex
Horizontal Plane
FIG No. 22 - Side View of Pier Cap
The bearings are fixed to the bearing pedestal by using epoxy compound, so that the bearing will not get displaced during concreting of superstructure. A false flooring will be made all-round the bearings incase of horizontal bottom of voided deck slab and box girders so that all the required reinforcement will be placed with cover block and concreting to superstructure shall be laid. After curing of superstructure is completed form work will be removed and at the same time the false flooring shall also be removed, so that the superstructure rests on bearings.
Super Structure
Bearing
Top of Pier cap
False Flooring
Bearing
Bearing
FIG No. 23 - Sketch Showing Bearing Pedestals
The reinforcement of superstructure shall be placed as usual with adequate number of cover block and concrete shall be laid. After removal of centering for superstructure, the false flooring adjoining the bearings shall also be removed, thus the superstructure finally rests on bearings. Similar method shall be adopted for T-beam superstructure in which case the false flooring will be restricted to beam width only. The concrete for bed block and backing wall over abutment shall be laid in single operation duly keeping the stirrups as described above to be monolithic in action. The backing wall (Dirt wall) shall be laid up to a level of 0.25m below R.C.L
to accommodate approach slab over it. That means the approach slab shall be laid on the top of backing wall.
150 100
100
Expansion Jiont Wearing Coat
3.50M
Super Structure Bearing Bearing Pedestal
FIG No. 24 - Sketch of Bed Block & Backing Wall over Abutment Wings behind abutment may be either box wings (or) fly wings. In case of box wings separate footing shall be laid for foundations of box wings. There should not be any connection between abutment and wing wall right from foundation level, as there will be differential settlement. In case of fly wings reinforcement shall be provided from backing wall and laid simultaneously along with abutment and backing wall. The length of wing wall shall be calculated as detailed below. Wing walls shall be taken to a level of 100mm above the top of slope of embankment to prevent any soil from being blown or washed away by rain. The length of fly wings shall not be more than 4.0m. The clearance between bottom of superstructure and top of pier cap shall be not less than 150mm. The purpose of providing bearing pedestals is to keep the bearings in exact position and also to provide adequate working space, to lift the superstructure with the help of jacks for replacement of damaged bearings in future if necessary. The superstructure will be lifted with the help of jacks and bearings shall be replaced after few years. In such a case, all the bearings in that line shall be replaced (generally 3 Nos under main girders) but not only one bearing even though only one bearing is
damaged. Bearing mesh shall be provided both at bearing position and jack position. The jack position shall be marked with paint over pier cap. The reinforcement details of fly wing is given below for information.
FIG No. 28 - Bearing Mesh (6mm) Weep holes shall be provided in wing walls using 100mm dia A.C pipes at 1.0m intervals in both directions staggered in row. The volume of weep holes need not be
1.0 0.5 1.0
1.0
0.15
1.0
1.0
deducted in concrete.
1.0 1.0
1.0
Low Water Level
FIG No. 29 - Weep Holes Position
Filter media to a width of 0.60m using 50% 150mm HBG stone and 50% of 40mm HBG metal shall be provided both behind abutment, wings and retaining walls. Incase if the embankment formation is proposed with sand, gravel backing to a width of 0.60m shall be provided adjoining filter media to avoid scooping out of sand along with seepage water. The provision of gravel layer prevents the sand displacement, along with seepage water. For retaining walls mastic pad shall be
provided at 30m intervals (or) when the section changes depending on height. It is usual practice to provide different sections for different height and the section is generally adopted for 25m length. Instead of changing the sections abruptly at one point, it is convenient to vary the section above sill level gradually form one end of block to the other end of block.
Filter Media (600mm)
Sand Formation
Grav el Ba ckin g
Gravel Backing
Weep holes @ 1.0m c/c
Slope (1:20)
SILL
RETAINING WALL
FIG No. 30 - Sketch Showing Filter Media
This facilitates providing filter media in one line, and proves to be economical duly satisfying the design requirement.
Incase of curved bridge with bearing pedestals, superstructure shall be provided by varying the levels of bearing pedestals from inner and to outer and, as shown keeping the pier cap in level position. Level Footpath
Level Footpath
Super Ele
vation
Super Structure
Bearing
Bearing Pedestal
Level Pier Cap
FIG No. 31 - Bearing Pedestals In Curve Incase of skew bridges, the wing walls are to be provided parallel to traffic axis, the length of wings at the obtuse angled corners will be have to be more than the other wing wall to ensure that the quadrant revetment does not protrude into the waterway. If O is the skew angel and Lx is the length of wing wall in normal calculated as shown above the length of wing wall in skew on L/S in figure is given by (Lx Cos O) and on right side (Lx SecO)
Short wing Wall (Lx Cos 0)
Longer wing Wall (Lx Sec 0)
FIG No. 32 - Sketch Showing Wings in Skew Bridge
It is to be noted that bearings of different sizes must not placed next to each other to support a span. They shall be in a single line of support and identical dimensions. The earth face of abutment and wing wall shall be kept rough and smooth finished on drain face. The top surface of the pier cap may be finished smooth by rubbing with Carborundam stone. Incase of singular circular pier with hammer head bed block, for a skew bridge, the pier cap must be oriented in skew direction only, with extra length to give a normal carriageway width of 7.50m. DIFFERENTIAL BED BLOCKS (OR) PIER CAPS: This is also another type of pier cap with one half being at one level and other half laid at higher level as detailed below:
Expansion Joint RCL
RCL
Large Span
Small Span
Bottom of Superstructure Bottom of Superstructure
Differential Pier Cap Pier
FIG No. 33 - Cross Section of Differential Bed Block
This type of pier cap will be necessary in the following cases 1) When the span is different on either side of pier, the depth of superstructure will be different. Hence in order to accommodate the superstructure of different spans a differential bed block shall be provided. 2) In case of R.O.B also, in order to maintain vertical clearance and to avoid interruption to rail traffic the railway authorities adopt unsupported span in the double track, which involves advanced method of construction like, prestressed cement concrete and precast members. The adjoining span of (R&B) different will be less compared to railway span and naturally the depth of superstructure will be different. In this case also a differential pier cap shall be adopted. This pier cap shall be constructed by railway authorities and we must lay over superstructure over this. The levels at pier cap top shall be verified by us before concreting is done. Erros may take place in this case. 3) In case of curved bridges in order to provide super elevation at top of pier cap, supporting at T-beam type superstructure, the pier cap, some frames shall be provided in stepped manner with different levels.
4) In case of bridges in gradient in order to meet the gradient requirement and to transfer the loads horizontally, a differential pier cap shall be provided, to meet the gradient requirement in width of pier cap from one end to other end.
SUPERSTRUCTURE Superstructure is a component of bridge that directly supports loads of traffic and facilitates smooth uninterrupted passage. The superstructure may be solid deck slab, T-beam superstructure, voided deck slab (or) Box girder, depending on the span. Generally in our departments, we are adopting simply supported superstructure. For superstructure, standard drawings are readily available for solid slabs upto 10m and 10m to 24m for T-beam structure. These standard drawings are redily available for normal crossings and for skew crossing of different angles. The maximum skew angle allowable is 450. For other type of superstructures separate designs are to be prepared. First let us discuss with solid slabs up to 10m span. The carriageway width normally adopted in state roads is 7.5m between kerbs and 12.0m from outer to outer of bridge railing for National Highway. As the solid slabs are simply supported main reinforcement shall be provided at bottom only and nominal steel shall be provided at top. The following important points are to be kept in mind, before executing superstructure. 1) Cover bocks of 50mm x 50mm shall be kept ready well in advance, properly cured by the time the steel is placed. The thickness may be not less than 40mm in coastal areas and 25mm in other areas. 2) While placing bottom shutters, they must be plain. The joints of steel shutter may be closed with adhesive tape of 2” width, before placing the steel. 3) Lubricants either Diesel / Other oils shall be invariably applied to the top of shutters. Otherwise, ugly finishing will be noticed at the bottom of slab after removal of shutters. 4) Reinforcement schedule should be studied thoroughly and in case if the length of main reinforcement available is not of required length laps may be provided as 56d.
5) The laps shall not be done at middle third of span (maximum bending moment zone) under any circumstances. All laps should be staggered. 6) The main reinforcement is kept at the bottom and distribution reinforcement over it. 7) The top mat is only a nominal steel and it is difficult to keep it at top, while laying concrete. Chairs of suitable height shall be placed at smaller spacing, so that the top reinforcement does not bend down. 8) The position of drainage spouts should be clearly marked at exact locations and kept properly tied to the reinforcement, before laying concrete. The distance between drainage spouts should not be more than 10m and staggered. The top of drainage spout shall be higher by 50mm to be in flush with wearing coat subsequently. 9) In case if foot paths are provided, the inner kerb steel shall be kept at exact location with cover. The distance from outer end of outer kerb to inside face of inner kerb is 1.775m. So, the inner kerb steel may be kept at 1.725m, thus leaving a cover of 50mm. 10) Aggregate used must be graded aggregate of 20mm. The 20mm aggregate available in market is observed to be over size. It must be added with 10mm aggregate to give a dense concrete. 11) Craft paper may be placed in two layers, at supports to prevent undue thrust on the substructure during the expansion of slab. 12) Mastic pads of 20mm shall be placed to full height, before start of concrete. The main problem during concreting of superstructure is that the aggregate falls down into the expansion gap, thus making the joint wider. This gives a zigzag expansion gap if unnoticed. This can be prevented by taking care, while concreting at joint locations.
13) The concreting of superstructure deck slab shall be completed in one continuous operation as far as possible. In case, due to unprecedented rain or other reasons beyond our control if the concrete is to be stopped for more than 24 hours. Epoxy compounds may be used for bonding old concrete with fresh concrete. 14) Pin vibrators of larger diameters are used, for easy and effective vibration. It is dangerous to keep these higher diameter vibrator needles for more time because more scum will come to the top (forms a weak layer) leaving aggregates at bottom. As such, vibration should be stopped once the aggregates are just disappearing from top. Each layer of concrete shall not be more than 300mm. 15) Sand may be filled in drainage spout openings at top and a gunny bag placed to prevent falling of concrete into the spout to avoid closure due to falling of concrete. 16) The top grill can be lifted, with the help of crow bars if it is bend down, due to movement of men and machine while laying top layer of concrete. 17) Water used shall be having a PH of more than 6 and minimum water only shall be used to give good vibration and dense concrete. Usage of excess water is preferred by masons for working convenience, and it must be controlled to avoid segregation of concrete. In order to have good vibration and good finishing minimum water shall only be allowed. 18) In case of large spans of 10m, the depth of slab would be 740mm and concrete quantity is too high even for two mixers. If it is preferred to lay the concrete in two days, total depth of concrete to half width shall be laid and other half in the next day. Some people lay half thickness to full width one day and other top half on the next day, which is incorrect.
19) In case of skew bridges, the distribution reinforcement shall be kept parallel to the pier and main reinforcement perpendicular to it as shown below.
X
Pier
Distribution Reinforcement Parallel to Pier
Main Reinforcement Perpendicular to Pier
Pier
Normal Width 7.50M
X
FIG No. 34 - Reinforcement of Skew Slab
The normal width perpendicular to the direction of traffic shall be 7.50m between kerbs. Some times it is done unknowingly the skew width kept as 7.50m, which reduces the normal carriageway width considerably depending on skew angle. 20) In case of curved bridges, while super elevation is required, the required super elevation shall be provided in pier cap itself. The levels must be carefully marked on shutters before concreting. The cross section is given below for information. It is to be noted that no super elevation is to be provided in foot path portion. Some times, in approved drawings, these lines will be drawn showing super elevation erroneously.
Level Foot path Level Foot path
Uniform Wearing Coat
Superstructure (Uniform Depth) Super elevated Pier Cap Pier Cap Without Pedestals Level Pier
FIG No. 35 - Pier Cap in Curved Bridge Further in calculations the R.C.L of Deck slab wearing coat of 100mm thick shall be deducted for level slabs. But in this case where super elevation is provided there will not be any camber in wearing coat and hence 75mm thick uniform wearing coat shall only be deducted for arriving at the level of deck slab. 21) The top surface may be slightly rough to receive wearing coat subsequently. 22) Curing shall be started immediately after 12 hours. Often curing is being delayed, as cross bunds with lean cement mortar are laid in the next day and curing can not be started, until these bunds are set sufficiently. This can be avoided if the bunds are laid in the evening itself, so that curing can be started next day morning. It is very important to note that more water is required in the first 24 hours after laying concrete. 23) Curing compounds are sometimes sprayed over the concrete surface which forms a thin film and prevent loss of water from concrete. This procedure is adopted in case of pre-stressed and precast members curing. Compounds are not permitted when subsequent concrete is to be laid. 24) Centering for deck slabs can be removed after 14 days and curing must be continued even after removal of form work up to 28 days invariably.
25) Reinforcement for hand posts shall be kept at exact location not exceeding 2.00m, before start of concrete for deck slab. The calculations of spacing is detailed below. For solid slabs, the bearing width shall be 760mm and for 10m span c/c distance will be 10.76m. The end posts will be 300 x 200 in cross section. The intermediate post will be 180 x 200 in cross section. The length after deducting expansion gap = 10.74m (20mm for solid slabs) Deduct end posts ½ width on either side = 10.74 – 0.15 – 0.15 = 10.44 No. of intermediate post assuming 2m spacing = 10.44 = 5.22 say 6 Nos. 2.0 Spacing from centre to centre of intermediate post = 10.44/6 = 1.74m c/c 26) The height of Hand posts in non urban areas adopted is 770mm above kerb with two rows of hand railing (Type-I). In case of urban areas with foot paths the height of hand posts adopted is 1050mm above kerb with 3 rows of hand rails. 27) The clear width of foot path generally adopted is 1.50m. It may be either solid foot path (or) Hollow with precast slabs. In case of solid foot paths, PVC pipes of 150mm dia in three rows are provided to run the service lines, openings are provided at 15m intervals, in the form of precast slab shall be provided. The top of foot path shall be made skid resistant by simply tapping wire mesh over the concrete when it is green. T-BEAM SUPERSTRUCTURE When the span of bridge is more than 10m T-beam superstructure is provided. The T-beam superstructure consists of three main girders, two end girders at supports and one centre girder supporting a deck slab. The width of main girders is 300mm width of cross girders is 250mm. In this case the width of support is kept as 1.0m or more depending on span, where as 0.76m provided in case of solid slabs. The following points are to be kept in mind before concreting for a T-beam superstructure.
1) The main reinforcement at bottom of main girder will be in 4 layers (4 tier) of 28mm dia. These layers must be separated by spacer rods kept horizontally, so that the concrete surrounds all layers of steel. 2) Because of large spans, the length of steel rods will not be sufficient to place them as a single rod from support to support. Hence, lapping of rod is essential. Because of small width of main beam large depth of beam and multi layered steel, the lapping of steel creates very little space available between rods to do concreting. This is the main problem in executing T-beam type superstructure. To over come this congestion of steel problem the following methods are adopted to avoid honey-combing of concrete and concrete not surrounding the reinforcement in practice. a) Lower size of aggregate of 10mm may be used in place of 20mm aggregate with excess cement content for concreting in web portion up to top layer of steel. b) Plasticizers may be added to increase workability without increasing water content. These palsticisers gives more workability i.e. free flow of concrete into the web portion, without reducing the strength of concrete. c) Staggering the laps at different locations not falling in the middle third of span. d) Even though it is not appropriate the lapping rods may kept over the rod, so that the horizontal clearance for flow of concrete is not reduces. This method helps in avoiding honeycombs. e) Smaller quantities shall be laid and small needle vibration of 20mm shall be used to vibrate concrete. Leaving a cover of not less than 40mm adjoining vertical shutter, may be helpful if the concrete using 10mm aggregate is laid in that gap, and needle vibrator of 20mm size may be taken up to the bottom of beam in this gap to have good compaction and to avoid honey combing.
f) A person with hammer may be deployed under the beam, while concreting beams. This man strikes the vertical shutter of beam with hammer frequently. Any loose sound during hammer stroke indicates hollow portion in beam and can be rectified by re-vibrating with due care until the sound is satisfactory. g) Laying of concrete in web portion should be done with close supervision with spare vibrator needles in day time only. The top rods may slightly be separated with the help of a crow bar to insert the needle during laying concrete in the bottom. These top rods may be tied again in correct position once the concrete up to top layer of bottom steel is completed. h) Concreting up to 100mm below the web portion of all beams shall be done in one day. The balance concrete of beams and deck slab shall be executed in the next day. i) The reinforcement in cantilever portion shall be kept at top only duly placing cover blacks / chairs at end so that they will not be displaced, due to the movement of men while concreting. j) The main beams of deck shall be given upward camber while keeping shuttering to account for dead load deflection as given in standard drawing.
Varies from 9mm to 23mm depending on span
Main Beam Centering
FIG No. 36 - Sketch of Centering in Span Direction
This upward camber is given to allow for deflection due to dead load, so that the beam will be horizontal after the shuttering is removed. k) Bearing meshes in two layers one at 20mm and the other at 10mm from the bottom concrete of main girder at bearing position shall be provided similar meshes in two layers shall also be provided in bearing pedestals. If there is no bearing pedestal the bearing mesh shall be provided in pier cap / bed block of abutment. l) The thickness of slab in cantilever portion from 1.0m from end cross girder on either side of main girder shall be varied from 275mm to 350mm to meet the shear requirement as shown below.
130mm
350mm
130mm
275mm
at 1.0m from end
at the face of cross girder
FIG No. 37 - Sketch Showing Varying Slab Thickness
That means the thickness along span direction shall be varied gradually from 275mm to 350mm by lowering the centering in the end 1.0m from cross girder. A typical section showing the reinforcement details of main girders and slab is given below for information. m) The failure of concrete incase of T-beam bridges mainly due to a) Concrete not coming down to the bottom of beam due to congestion of reinforcement leaving large gaps in the bottom portion. b) Inadequate cover to the reinforcement in vertical face of beam which leads to corrosion of steel and spalling of concrete due to expansion of corroded seal. c) Minor cracks in concrete at bottom due to tension. This is being unnoticed for many years and finally exposing the steel corroded. d) Use of salt water during mixing or curing due to non availability of suitable water near sea coast and in dry areas during construction. e) Improper gradation of aggregates, which leaves voids in concrete resulting in interior quality of concrete with inadequate density.
f) Presence of salt in sand or the sand is too fine in coastal areas of Andhra Pradesh. g) Non application of corrosion protective epoxy coatings to the steel in bridges located nearer to sea coast. h) Marine atmosphere containing acidic properties and wetting of concrete due to back water of sea. The above problems are naturally creating a sort of panic nature in the construction engineers as T-beam bridges are giving troubles in little span of 10 to 15 years in spite of experienced engineers executed the work. Even though there are some rehabilitation methods are available like pressure grouting for filling voids and epoxy compounds for application of fresh concrete after removal of all loose concrete, the construction of T-beam bridges must be executed with extreme care to avoid subsequent problems. VOIDED DECK SLAB As detailed above, when the span is more than 10m the construction of solid slab superstructure is uneconomical and we have to go for T-beam type superstructure. Because of the practical problems discussed above, incase of T-beam bridges, the other alternative is voided deck slab. Voided Deck slab is a hollow section in reinforced cement concrete. In this case, the concrete is laid with direct vision, like solid slab, eliminating the problems of concreting in beam portions. A typical section generally adopted for voided Deck slab is given below.
0.50
0.50
200 800 200
Top Slab 700
700
700
1200
7.50
Bottom Slab 5.0M
FIG No. 38 - Section of Voided Deek Slab The section contains a box of 5.0m and two cantilevers of 1.75m on either side. In the box portions R.C.C pipes in 3 rows (700mm) are placed in span direction, to reduce the concrete quantity. In contains (i)
Soffit Slab of 200mm thick.
(ii)
Middle portion containing pipes 700mm inside dia.
(iii)
Top slab with cantilevers 200mm thick.
In this case the soffit slab is first laid duly keeping the reinforcement as per design. After laying the soffit slab pipes are kept on a set of seating chairs at 0.50m interval. The second stage of concreting is laid after keeping the pipes in position, up to a 100mm less than the top of pipes. In this case, the gaps between pipes 2 No. and at ends 2 No. are provided with stirrups and top & bottom reinforcement, which acts like a beam. After completing the second stage concreting the reinforcement for top slab is kept in position as per design. The third stage concreting is done up to 200mm above the pipes. The total depth of superstructure is less i.e. 1.20m compared to 1.875m for a T-beam bridge of span of 18.75m effective. From the above, we can say that the voided deck slab is nothing but two solid slabs one soffit slab and one top slab connected by 4 beams. The execution of voided deck slab is very simple, as all concrete laid in 3 stages can be seen and effective compaction can be achieved. Thus the problems of holes in concrete of T- beam girder bridges are eliminated. The reinforcement provided is not having any
congestion. Subsequent problems of corrosion of steel and spalling of concrete is eliminated. The Box type structure gives good appearance of superstructure. It gives more vertical clearance due to reduction in depth of superstructure. Due to the above advantages the voided deck slab structure is convenient for executing safe bridge structure. Let us forget about the economics of this type of structure, compared to the T-beam bridges in the interest of our jobs. These are suitable for spans ranging from 10 to 25m. This type of structure can be used for causeways also without any anchoring arrangement of superstructure considering the effect of Buoyancy also. RCC BOX GIRDERS When the span of Bridge structure exceeds 25m, the T-beam bridges are not suitable due to large depth of girders. The other alternative is R.C.C box girder. It is also another type of hollow R.C.C section. The construction of this type of superstructure is easy and the structure looks good. A typical section of R.C.C box girder is given below for information.
0.50
0.50
<1.50M
7.50
FIG No. 39 - Section of Box Girder In this case also, the soffit slab (bottom) concreting is laid first and then two sloped faces concreting. And finally the top reinforcement kept in position with two cantilevers on either side, concreting shall be done. The difference between voided
deck slab box girder will be only in the middle portion i.e. placing of R.C.C pipes. The remaining process is same for both type of superstructure. The depth inside of the box shall be not less then 1.50m to allow for inspection of superstructure periodically. So far, for bridges with 7.50m carriageway the standard drawings shows level slab at top irrespective of type of superstructure. The required camber to drain off rain water is being given in wearing coat only 100mm at centre and 50mm at end abutting kerb. The revised drawings recently issued by M.O.R.T & H, the camber is being provided in the deck of superstructure itself at 2.5% and uniform wearing coat is provided (75mm). This is the new procedure being adopted recently. While fixing R.C.L we must take into consideration the extra thickness of superstructure at centre and the superstructure shall be laid with camber. While laying superstructure it is very important to leave recess at the joint location for a width of 600mm on either side to a depth of 75mm so that the sinusoidal rod and tie rods of expansion joint are welded to main reinforcement, subsequently. The expansion joint steel section may be welded in correct position, just before laying the wearing coat and the levels of expansion joint angle arrangement are kept at the level of wearing coat. The volume of reinforcement steel provided in reinforced concrete members need not be deleted from the concrete quantity (as some persons questioned me about this deduction). The reinforcement of hand posts shall be kept along with the superstructure steel, duly calculating the spacing in the procedure already discussed. In order to maintain the clear gap of expansion point, Thermocoal sheet may be provided which can be removed subsequently. The widening of expansion gap due to falling of stones and distortion of Thermocoal sheet should be avoided to maintain a straight line. The
recess for expansion joint may be kept for the carriageway of 7.50m only. But a clear gap shall be maintained at kerb locations. While laying kerb for a retaining wall of lengths exceeding 25m, a clear gap shall be provided with mastic pad to allow for expansion right from foundation level and the kerb over it should be given a gap for expansion. At this position reinforcement for end posts is to be provided. Often mistakes are committed, making the hand railing continuous without any gap for the entire length which may lead to cracks due to expansion in summer. In case of causeways superstructure a perforated kerb with hand railing shall be provided instead of continuous kerb when the level difference between bed level and RCL is high. The reinforcement for these perforated kerbs may be kept duly marking it at correct location. When perforated kerbs are provided, there is no need to provide drainage spouts. In this case as water drains out without interruption. The perforated kerbs provide support to railing and allows surplus water to drain off during heavy floods. Anchoring arrangement in the form of 32mm rods may be kep in pier cap and also kept in the superstructure to prevent uplifting of slabs during heavy floods. The sloped portion of slab of ends (in cross section) shall be made semicircular incase of causeways with deck slabs. For causeways, when the difference between bed level and R.C.L is low stream lined R.C.C guide, posts shall be provided at 3.0m c/c. These guide posts are semicircular in one half and the other half with parabolic surface to provide free flow of water during high floods. A sketch is given below for information. Generally in causeways the vented portion is provided level with R.C.C slab and gradient on either side not steeper than 1 in 30 are provided with body walls. The length of body wall at higher end is fixed as (M.F.L + 0.15m) and the body wall length is calculated accordingly.
Guide Posts 1 in 30
H
0.15
MFL 1 in 30
OFL
F.L
F.L Anchoring Arrangements Raft F.L
FIG No. 41 - L.S of Vented Causeway
If ‘H’ is the level difference between OFL and M.F.L the length of sloped portion will be (H + 0.15) x 30, on either side of level portion. When bearings are provided, no anchoring arrangements shall be made between pier cap and superstructure. But the buoyancy effect must be checked in such a case. (This is generally checked by D&P wing before finalization of designs). Bed protection in the form of Jeddy stone apron shall be provided for the total length of causeway including sloped portion to avoid scouring action from down stream side to upstream side which is called undermining. Expansion Joints: As already discussed a recess or gap is given while laying concrete of deck to a width of 600mm and depth of 75mm duly exposing the main reinforcement. The purpose of providing expansion joint is to allow free movement of superstructure due to high temperature in summer and to relieve the stresses induced due to this expansion. It also protects the edges of superstructure due to movement of vehicular traffic. The most commonly adopted expansion joint now a days is slab seal elastomeric expansion joints which serves longer periods and allows smooth flow of traffic for spans between 10m to 20m to cater to a movement upto 40mm. firstly,
the angle attachment of expansion joint with stiffener plates, tie rods, sinusoidal rods are welded to the main reinforcement of superstructure with required level. They are readily available in two bits of 3.75m each to meet the carriageway requirement of 7.50m. It contains two angles as shown below and tie rods.
Elastomer Fixed with ISA
nuts & Poly Sulphide Wearing Coat
75
75
Wearing Coat
600 Super Structure
600
Reinforcement of Bridge
Super Structure Clear expansion gap (40mm)
FIG No. 42 - Enlarged Details of Expansion Joint
In the gap left for fixing the joint the concrete shall be laid upto bottom of wearing coat first. Then the wearing coat shall be laid up to the level of angle attachment. The elastomers shall be available in the lengths of 1.0m and 0.75m, 0.50m as required. Note that the elastomer shall be fixed to angle attachment in dry condition. Hence the elastomers are fixed after curing of wearing coat is completed. The elastomer shall be fixed with epoxy to the angle attachment. Also lock nuts fixed, not to allow dislocation due to traffic. As this joint provides elastomer at top, this gives smooth riding and noise free expansion joints. These elastomers if spoiled due to traffic they can be removed and new elastomer can be placed if required (generally after 20 to 25 years). The process of fixing joints shall be demonstrated by company technician for one joint and the remaining joints can be fixed easily.
It is being given to provide joints in the kerb location also on front face. It is of my personal opinion that expansion joint (with elastomer) need not be placed in front face of kerb as no edge protection is necessary at this location and a clear gap of 40mm left will meet the requirement.
WEARING COAT The wearing coat is an important item which is directly observed by the road users and must be laid with due care. The wearing coat is generally of two types. (i) Bituminous Wearing Coat: It consists one layer of mastic asphalt 6mm thick / 12mm thick after applying prime coat over the deck, followed by 50mm thick (in 2 layers) of ashpaltic concrete. For high traffic intensity the wearing course comprises of 40mm bituminous concrete followed by 25mm thick bitumen mastic layer. (ii) Cement Concrete Wearing Coat: This is the wearing coat that is being adopted for state roads. This shall not be laid monolithic with deck concrete. The thickness of wearing coat shall be 75mm in VRCC M.30 and water cement ratio of 0.40. As per the latest specification the cross camber to drain off the rain water shall be given in deck itself at 2.5% and uniform wearing coat shall be provided in the two methods specified above. It is clearly specified that for providing corss camber no variation in thickness of wearing coat shall be permitted. Let us discuss about the VRCC wearing coat 100mm thick at center and 75mm at ends abutting kerb in level slabs longitudinally. In case of curved bridges super elevation shall be provided in deck itself and uniform wearing coat of 75mm shall be provided. It contains reinforcement of 6mm at 200mm c/c in both directions. Extra rods of 500mm length on either side shall be provided at 100mm c/c as shown below.
0.475
0.475 7.50 Expansion Joint
Extra rods at Jionts 200mm c/c
KERB
Mastic Pad at Kerb
KERB
Mastic Pad at Kerb
Expansion Joint 6mm @ 200mm c/c Bothways
FIG No. 43 - Reinforcement Details of Wearing Coat
The reinforcement shall be kept at centre of wearing coat. The concrete shall be in VRCC M.30 using 25mm, 12mm, 6mm chips. Over sized aggregate in 25mm may be not allowed as the thickness is less. The best method to lay wearing coat is to lay small blocks of concrete, as per levels at 1.0m intervals and keeping the reinforcement at centre. Over these small blocks of green concrete when we adopt wearing coat with different thickness the process of laying wearing coat shall be taken up for half of carriageway in alternate panels. At centre no mastic pad need be placed. However the two faces of panels shall be painted with bitumen to avoid friction. A mastic pad of 20mm thick shall be placed adjoining kerbs to allow for expansion movement. At joint locations, the expansion joint angle attachment shall be welded to the main reinforcement of deck, to the required camber, one day in advance of laying wearing coat concrete. The concrete upto deck slab level shall be laid at joint location after welding and wearing coat shall be laid as usual. Minimum water shall be used in concrete. The finishing of wearing coat shall be considered, as the testimony of Engineerin-Charge. After vibrating concrete (in this case external vibrator is preferred) a thin layer of mortar in the form of scum appears at the top. After the concrete is just set this thin layer of mortar shall be removed by brushing with coir brushes until the
stones in concrete are just touched. This job require good experience if this job is done either earlier or late gives very ugly appearance of wearing coat. In the absence of skilled labour (or) when the brushing is not desired by higher authorities the best way is to finish the surface with small flat trowel, when the concrete is just getting hard. This method even though gives very smooth surface, being appreciated by some engineers is easy and safe to the field engineers. A slight slope shall be given at end of carriageway towards the drainage spout to avoid stagnation of water and free flow of rain water. The top of drainage spout shall be flush with wearing coat. The working labour should not be allowed to walk over fresh concrete to avoid unwanted foot impressions. As rich concrete is placed for wearing coat, cracks are likely to appear over the surface in short time. Hence slight sprinkling of water may be done after 6 hours, depending on the site condition. The weavy surfacing of concrete must be made truly level with the help of twine from expansion joint to expansion joint. Other wise a bump will be observed by the travelers during passage of vehicles, causing inconvenience. Minor depressions may be filled with concrete / mortar within 24 hours, before curing. As this is the surface directly observed curing shall be done continuously for 21 days to have good results. In case of perforated kerbs mastic pad shall be placed on all the three corners of kerb
to
allow
for
any
expansion
transversely.
APPROACH SLAB: As per the revised specifications of MORTH the minimum length of approach slab shall be 3.50m and minimum thickness shall be 300mm. This may be adopted for new bridges. Often many of the approach slabs laid, are settling because of gravel filling behind abutments. It is convenient to adopt stone dust filling duly watered thoroughly to take up the approach slab immediately. When this is not accepted it is safe to lay the approach slab after allowing the traffic for one rainy season. In this case, as traffic can not be stopped for 3 weeks the approach slab shall be laid in two halves. But the reinforcement of approach slab shall not be cut to half width. The reinforcement may be bent vertically to allow the traffic till the approach slab is laid and curing completed. This will be straightened and the concreting for other half of approach slab is laid. This is because not to allow differential settlement in two helves of approach slab (if the reinforcement is cut at centre this is not possible). The wearing coat shall also be laid in two halves and reinforcement for wearing coat can be cut at centre. The wearing coat shall not be laid monolithic with approach slab concrete. One day gap may be given between concreting of the two items. The approach slab shall be laid in level for level slabs and in gradient for R.O.Bs etc. kerb shall be laid as usual. Before laying kerb it is necessary to keep reinforcement of pedestals to be constructed for inauguration. The construction of pedestals over approach slab kerb is convenient and gives good appearance. FORMATION OF APPROACHES The formation of approaches to a bridge forms a part of the bridge work. The following points may be kept in mind before staring approaches. 1) The earth used for sub-grade shall have a minimum C.B.R of 5, preferably 7 and the dry density of not less than 1.75gm/cc. The earth used for formation of embankment shall have a maximum dry density of not less than 1.520 gm/cc. The
toe point shall be marked first and each layer of not exceeding 225mm thick loose shall only be laid. 2) The crust in approaches shall be designed based on C.B.R value and traffic intensity as per IRC 37. In general the following layers are adopted.
a) Granular sub base = 150mm (or) 225mm compacted. b) WBM Gr.II (2 layers) = 150mm compacted. c) WBM Gr.III (1 layer) = 75mm compacted. d) B.T. Single Coat Surface Dressing. e) B.M / D.B.M = 50mm f) SDBC = 25mm g) Sand Gravel shoulders = 1.0m wide on either side (or) to the full formation width. First let us start from construction of embankment. The lab value of maximum dry density and O.M.C shall be obtained for the soil proposed for embankment. Mark the alignment as per drawing and the toe points, as per the cross sections may be peg marked. To obtain the required compaction, watering shall be done in the evening at 2% more than the O.M.C value. The next day the compaction of earth may be done. Once after the layer is compacted the field density of earth after compaction shall be tested for maximum dry density by sand replacement method. The dry density obtained shall not be less than 98% of lab value. If it is satisfied the second layer shall be started. The width of embankment shall be reduced layer by layer depending on slope. The slope generally adopted is 2:1 (Horizontal, Vertical) for cohesive clayey soils and (1½:1) for gravelly soils. When the slope is protected with revetment, the slope may be modified to (1 ½:1) in all cases. In the same process the sub grade is laid upto level of bottom of granular sub base. The width at this level can be worked out as detailed below. For a double lane road, the formation width adopted is 12.0m, let us say that the total crust is 525mm and embankment slope is (2:1).
The width required at top of sub grade Or bottom of granular sub base
=
12.0 + 2 x 0.52 = 13.05m.
A slight extra width shall be provided (say 14.0m) to achieve compaction to the required width at ends also. Required camber of 1 in 33 (3%) shall be given from the first layer of earthwork itself. After the construction of embankment is completed, the granular sub base shall be laid. It is very important to note than no gravel is being allowed either in base or for blindage as all the gravel available are found to be unsuitable. The required value of liquid limit for gravel shall be less than 25% and plasticity index shall be less than 6. The liquid limits observed for natural gravel are around and plasticity index around 13% not compatible to the standards. Thus granular sub base shall be designed to have a C.B.R of not less than 30 is obtained from lab with the following materials. 40mm HBG metal =
35%
20mm HBG metal =
25%
stone dust
40%
=
The loose quantity required is 1.28 M3 for one cubic meter finished item of granular base. The field C.B.R test shall be conducted after compaction practically this mix is giving a C.B.R of around of 39 even after 3 days soaking. At present, as MORT & H datas are being adopted all the payments including granular sub base, WBM etc., are being effected by level payment by taking levels at 10m intervals. Hence suitable bench marks may be fixed at 200m intervals with a precision instrument. It is usual practice to keep a straight and level portion of 15.0m on either side of bridge as shown below.
Level 15M
Level 15M Bridge
Gradient not Steeper than 1 in 40
Gradient not Steeper than 1 in 40
FIG No. 44 - L.S of Bridge Approaches
The minimum radius of horizontal curve for bridge approaches shall be kept as 150m and 90m in exceptional cases due to limited land in built up area etc. Necessary transition curves may be provided between straight portion and circular curve, duly raising the outer edge. The maximum super elevation permitted is 7%. The minimum width of carriageway in approaches shall be kept 7.0 m without kerbs. If kerbs are provided in urban areas the carriageway width shall be kept as 7.50 m. Suitable vertical curve may be provided to join the approaches with bridge portion. It is very important to join a new bridge approach constructed to the old road, without
End of Bridge WBM Crust 225mm Granular sub bare 225mm
B.T Crust 75mm
any jerk as detailed below.
Gradient is 1 in 90 (assumed)
P
FIG No. 45 - Sketch Showing Joining New Approach To Existing Road
If ‘P’ is the joining point of new approach to the old road, assuming the crust of 525mm and gradient of 90. The granular sub base shall be laid up to
The first layer shall be laid up to
=
P – 90 x 0.30 (Crust above gravel Base) P – 27m.
= =
P – 90 x 0.225 (Crust above) P – 20.25m.
The second layer shall be laid up to
=
=
P – 90 x 0.15 = P – 13.50m.
The third layer shall be laid up to
=
P – 90 x 0.075 = P – 6.75m.
The B.M. layer shall be laid up to
=
P – 90 x 0.025 = P – 2.25m.
Finally the SDBC layer will be joined smooth, to the existing road, thus giving no jerk. Often mistake is committed such that all the layers are laid up to point ‘P’ and then joined with B.T layer only. Which gives impact and inconvenience to the fast moving vehicles. So the above procedure may be adopted to join the old road. The slopes of embankments are generally provided in 2:1 (Horizontally : Vertical). When the height of embankment is more than 1.50m, Revetment shall be provided to protect the slope of embankment with gravel backing duly adopting (1 ½:1) slope. The gravel backing provided shall be 150mm and thickness of revetment shall be provided not less than 225mm. A fold may be given at the top end of road to a width of 0.60m to avoid scouring of revetment and a toe wall shall be provided at toe (0.60m x 0.60m) to avoid sliding due to scouring at toe.
RCL
Fold 0.60
(1½ : 1) 225mm Revetment 150mm Gravel backing G.L Toewall (0.60x0.60)
FIG No. 46 - Sketch Showing Revetment & Toe Wall
It is to be noted that the toe wall shall be kept outside the line of revetment and also below existing ground level as shown above. Drainage chutes shall be provided in revetment at 15.0m intervals without fail to allow for drainage of rainwater. When cohesion less soils like sand are to be used for formation of embankment (where C = O), the slopes shall be protected with casing to avoid scouring in sandy formation due to rains either with cohesive soils or grave to a thickness of not less than 0.60m. However, the compaction can not be done with roller, as roller can not be moved over sandy soils. This gravel casing may be compacted in thin layer of 150mm manually or by other means of compaction. When cohesive soils like clay are used for casing the rolling width of 1.80m shall be filled with cohesive soil only as shown below.
1.0
7.0M
1.0
Sand
SDBC BM WBM WBM GSB
Sand
Farmation Width
1.80
Ea
(2 : 1)
rth sin Ca
1.80
1.80
Ea
rth
Ca
sin
g
g
Sand Filling
Earth
Sand
Earth
FIG No. 47 - Sketch Showing Casing For Sand Formation
This method works out to be cheaper and effective, when the lead for cohesive soil is less compared to gravel and good compaction can be achieved in slopes due to roller compaction and no scouring of slope can take place. Necessary camber may be given in sand formation itself. When the road formation is on a bund of drain or canal, it is not even possible to provide a slope of (1 ½:1) as the drain width will be reduced. In such a case a concrete toe wall may be provided up to a height of 1.0m above bed level and then rough stone revetment with C.C grouting can be adopted with a slope of (1:1). Never keep the toe wall on a made up soil or by filling with soil up to a level higher than the existing bed level, which simply get washed off during floods causing failure of revetment. Even if the embankment is not properly compacted, the sliding of revetment takes place due to formation of slip circle in expansive soils (B.C. soils, marine clays etc.). The interstices of revetment stone shall be packed with smaller size of stones with hammer so that the revetment acts as a monolithic unit.
After completing the formation of embankment granular sub base having a C.B.R of not less than 30% shall be laid. This can be achieved if grave is not allowed in sub base. A mix of 40mm, 20mm and stone dust is giving good results. Movement of traffic over this shall be permitted by keeping the crust continuously wet. Other wise it will be disturbed. After completing granular sub base WBM Grade-II may be laid. Here also for blindage screenings of 12mm shall be used. It is better to delete the addition of natural gravel in any layer. Two layers of Grade-II shall be laid generally 75mm compacted each layer. For grade-III crushed metal only shall be allowed. Don’t get carried by the notion that crushed metal gives less thickness and do not have any interlocking. It gives dense packing. Here also stone dust shall be used for blindage. After completion of Grade-III layer either prime coat or single coat surface dressing shall be laid and traffic may be allowed for few weeks, so that any loose pockets in embankment may get settlement. It is general that the approaches of bridge settles due to their high embankment. All loose pockets shall be cut with vertical faces and filled with 40mm metal and stone dust. Over this one layer of D.B.M 50mm may be laid and SDBC 25mm shall also be laid. The gravel shoulders may be laid for a width of minimum 1.0m on either side, with sand gravel mix. This is the only item where grave is being allowed in total road items. Drainage of rainwater, both in carriageway and in shoulders is very important, which is the main reason for failure of approach road. Even if some excess moisture is present in gravel shoulders, may cause settlement at ends and raising berms. This depression it left un-required, is enough to spoil the road. So in case of new approach roads the drainage is to be verified in rainy season. When the embankments are high it is usual practice to fix guide posts. This may be in R.C.C. While fixing guide posts, they may be kept at 0.30m inside of
formation of end. The top level shall be fixed with instrument precisely, such that all the guide posts take the gradient of approach road. They may be kept at 3.0m c/c they may be painted black & white for distinct vision. Further the joining of approach to bridge shall be done as detailed below. The slope for embankment i.e. (2:1) (or) (1 ½:1) where as the slope of quadrant is (1:1). So, the width of formation may be gradually reduced from 30m to join the bridge. The
pe Slo ing ary
gS lop e ryin
V
Va
1 in 20 (<90M)
Grouted Quadrant Revetment (1 : 1)
7.50m
Flaring not more than
Bridge
Kerb
Kerb
slopes also are gradually modified from (2:1) slope to (1:1) slope in 30m length.
(2 : 1)
Approach Road
(2 : 1)
12.00m
Guide Posts
FIG No. 48 - Sketch Showing Joining of Approach Road
Kerbs shall be painted yellow and black 0.20 m wide. Painting to railing is discretionary. Often we provide Ivory for hand rails and Golden brown for Hand posts. Never go for too dark colour for posts, as it gives bad appearance. You can choose colour of your own choice. In some bridge in urban areas (R.O.Bs etc.) lighting arrangement shall be made by our department where as the electricity charges and other maintenance shall be borne by the local bodies. In such a case arrangement for fixing iron electrical poles may be made before casting hand posts. The spacing shall be preferably 10m. The cost of this arrangement would be Rs.10.00 lakh/Km. This may be included in estimate. Raised pavement markers / cat eyes may be provided at centre and at kerbs of bridge
to guide the traffic. In approaches it shall be provided at 15m intervals. They would be fixed with adhesive and gives beautiful appearance during nights. The cost of each one is around Rs.350/-. With this I conclude my presentation. This may be up dated by any one with their knowledge so that it will be use full to all the department engineers.
