STRUCTURAL DESIGN AND ANALYSIS OF A REINFORCED BOX CULVERT (3M X3M)
BY
ODUNOLA, ADEJOLA SAMSON (10BC000338)
BEING SUBMITTED AS A PRELIMINARY FINAL YEAR PROJECT TO CIVIL ENGINEERING DEPARTMENT COLLEGE OF SCIENCE AND ENGINEERING LANDMARK UNIVERSITY, OMU-ARAN
IN PARTIAL FULFILLMENT OF THE REQUIRMENT FOR THE AWARD OF BACHELOR OF ENGINEERING (B.ENG.) IN CIVIL ENGINEERIN OF LANDMARK UNIVERSITY OMU-ARAN KWARA STATE.
JANUARY, 2016
TABLE OF CONTENTS 1.0.0. INTRODUCTON………………………………………………………………………2 1.1.0. BACKGROUND STUDY……………………………………………………………...2 1.2.0. SCOPE OF STUDY…………………………………………………………….………5 1.3.0. AIM…………………………………………………………………………………..…5 1.4.0. OBJECTIVES…………………………………………………………………………..6 1.5.0. JUSTIFICATION…………………………………………………………………….…6
2.0.0. LITERETURE REVIEW……………………………………………………………….7 2.1.0. DEFINATION OF TERMS…………………………………………………………….7 2.2.0. CULVERT DESIGN ITEMS…………………………………………………………...9 2.2.1. ENINEERING ASPECT………………………………………………………….9 2.2.2. SITE CRITERIA………………………………………………………………….9 2.2.3. DESIGN LIMITATIONS……...….………………………………………………9 2.2.4. DESIGN OPTIONS…..………..…………....…………………………………….9 2.2.5. RELATED DESIGN…..……..………………………………………………….10 2.3.0. TYPES OF CULVERT……………………………………………………………..…10 2.3.1. CLASSIFICATION OF CULVERT BASED ON FUCTION…………………...10 2.3.2. CLASSIFICATION OF CULVERT BASED ON SHAPE…………………...…11 2.4.0. DIFFERENCE BETWEEN A CULVERT AND A BRIDGE…………………...……12 2.5.0. WHY CULVERTS?.......................................................................................................13 2.6.0. ENVIROMENTAL IMPACTS OF CULVERTS…………………………………….14 2.7.0. PERFORMANCE CURVE……………………………………………………………17 2.8.0. HYDRAULIC DESIGN CRITERIA FOR CULVERTS……………………………...17 2.8.1. VELOCITY LIMITATION…………………...………………………………...17 2.8.2. ALLOWABLE HEADWATER……………………...………………………….17 2.9.0. COEFFICIENT OF EARTH PRESSURE………………………………………….…18 2.10.0. EFFECTIVE WIDTH………………………………………………………………..19 2.11.0. INLET CONTROL…………………………………………………………………..20 2.12.0. OUTLET CONTROL………………………………………………………………..21
1|Page
2.13.0. BRAKING FORCE…………………………………………………………………..22 2.14.0. IMPACT OF LIVE LOAD…………………………………………………………..23 2.15.0. CLEANING AND MAINTAINANCE………………………………………………24 2.16.0. MULTI-CELL CULVERT…………………………………………………………..27 2.16.1. EFFECTIVE USES AND LIMITATIONS……………………………………27 2.16.2. MATERIAL SPECIFICATONS………………………………………………27 2.16.3. INSTALLATION GUIDELINES……………………………………………..27
3.0.0. METHODOLOGY…………………………………………………………………….29 3.1.0. LOAD CASES FOR DESIGN………………………………………………………...30 3.2.0. LOADING…………………………………………………………………………..…30 3.3.0. LOADING CALCULATIONS………………………………………………………..30 3.4.0. MOMENT CALCULATION………………………………………………………….31 3.5.0. BENDING MOMENT ANALYSIS AND DIAGRAM………………………………31 3.6.0. REINFORCEMENT AND DETAILING……………………………………………..31
2|Page
LIST OF FIGURES FIGURE 1………………………………………………....STREAM CROSSING CULVERT FIGURE 2……………………………………….……DIFFERENT SHAPES OF CULVERT FIGURE 3……….A BOTTOM LESS ARCH CULVERT THAT ALLOWS FOR FISH PASSAGE FIGURE 4…………….DIAGRAM SHOWING VARIOUS INLET CONTROL METHODS FIGURE 5……………DIAGRAM SHOWING VARIOUS OUTLET CONTROL METHOD
3|Page
1.0
INTRODUCTION
1.1
BACKGROUND STUDY:
Current in-stream design projects are moving away from the use of hard structures, such as gabions, bank revetment, and culverts, and are increasingly employing a more natural, biotechnical engineering approach. While stream restoration and bank stabilization efforts may be prohibitive in terms of cost in the short run, ample evidence suggests that “natural stream channel stability is achieved by allowing the river to develop a stable dimension, pattern, and profile such that, over time, channel features are maintained and the stream system neither aggrades nor degrades”(Rosgen 1996). Environmentally sensitive design guidelines have recently been developed by several agencies, combining modern hydraulic criteria and economical construction and maintenance costs, with consideration of natural stream channel integrity, flood prevention, and habitat issues. Culverts have the potential to destabilize streams if capacity and stream morphology are not considered jointly, resulting in increased sediment supply and erosion, flooding, habitat loss, and property damage. By artificially narrowing a channel, structures and hardscape methods often have the unintended consequences of creating erosional eddies up and downstream of structures, or creating a down-cutting response in order to make up for the lost cross-sectional area (California Regional Water Quality Control Board, 2003). However, design alternatives and construction guidelines exist that increase the effective transport of varying flow events through culverts and under bridges, for use in situations where creating or modifying instream structures is necessary. The Maryland State Highway Administration (SHA) has created new design procedures that limit the impacts of constructing culverts and bridges in streams (Kosicki, Davis 2000). These guidelines have shifted from traditionally focusing solely on the relationship between 4|Page
the stream and the highway structure for major flood events, to adopting a design process that maintains “the consistency of dimension, pattern, and profile of the stream with particular attention given to maintaining bankfull width and width/depth ratio”(Kosicki, Davis 2000). This agency has also experienced successful results in using additional floodplain culverts in order to “relieve the hydraulic load on the main channel culvert so as to limit downstream scour and erosion” (Kosicki, Davis 2000). By emphasizing stream geomorphology in their structural design process, the SHA predicts a reduction in future maintenance problems and flood hazards. However, the use of large bores or multiple culverts is not a solution in itself; without applying a geomorphic approach, oversized culverts or auxiliary cells can become sediment traps that clog one or more culvert barrels. If the stream passage is larger than the bankfull width, a stream will ultimately change to reestablish bankfull flow conditions (Kosicki 2003). An approach incorporating the Rosgen’s Stream Classification system (Rosgen, 1996), as well as conventional hydraulic design tools such as HY-8 and HECRAS, has facilitated the SHA’s permit approval process while creatively addressing common problems such as scour, degradation, head-cutting, and lateral movement (Kosicki 2003, Similarly, the Maryland Department of the Environment, Water Management Administration, has created a set of guidelines for the waterway construction process (Maryland’s Waterway Construction Guidelines, 2000). These efforts are also in response to a growing need for the stabilization, modification, or rehabilitation of streams due to the effects of urbanization or previous channel construction. Their design recommendations incorporate a consideration of the Rosgen Stream Classification, as well as an understanding of the root causes of the channel instability. In some situations, various culvert designs are suggested which can facilitate the flow of flood waters across a floodplain, and promote the conditions for improved fish passage. 5|Page
Maryland’s Department of Permitting Services, and Department of Public Works and Transportation in Montgomery County, MD, have also published a set of environmentally sensitive guidelines for the design of culverts. These collaborative guidelines were developed in an effort to overcome some of the severe problems that have been associated with past construction design practices, particularly: 1. Degradation of the stream habitat, aquatic life, and water quality. 2. Physical blockages of water flows and impediments to the passage of fish and other aquatic life. 3. Excess construction and maintenance costs, and the burden on taxpayers. 4. A need to streamline the requirements of the various regulatory agencies involved in the monitoring of stream crossings. After developing design goals that address the above problems, these agencies generated recommendations and considerations for various stream structure options which, regardless of structure type, will convey the flows generated by the 100-year storm event, maintain the channel’s existing water depth and velocity for the normal flow channel, and provide for the unobstructed flow of the bankfull storm event without increasing or decreasing velocity by more than 5% (Mongomery County, 1998). (Watershed sciences, 2007) When a roadway is constructed across a natural stream, a major problem is detected, during periods of high rainfall, the water level tends to rise and overflow the roadway causing a lot of damage to both the roadway and the natural life that habits the stream or water body. Now the practical solution to this problem is too provide a way for the water from the stream flowing under the road to pass safely under the road without spilling onto and flooding the road and also to cater for increases in the water level during periods of extended rainfall. This type of structure that is constructed so as to allow the passage of water under a roadway is called a 6|Page
culvert. The purpose of this project is to design and analyse a reinforced box culvert. A culvert is a covered channel of relatively short length designed to pass water through an embankment (e.g. highway, railroad, and dam). The design requires a hydrological study of the upstream catchment to estimate the maximum (design) discharge and the risks of exceptional (emergency) floods. The sizes of the culvert are based on hydraulic, structural and geotechnical considerations. Indeed, the culvert height and width affect the size and cost of the embankment. The culvert impact on the environment must also be taken into account, e.g. flooding of the upstream plain. The design process is a system approach. The system must be identified, as well as the design objectives and constraints. A detailed analysis of it must be conducted and questions should be asked at the end if the final design meets the objectives. The culvert design begins with the report from a survey and hydraulic design reports, this report is used in conjunction with existing roadway plan to then accurately specify the culvert length, design fill and other items relating to the completed culvert plan.
1.2
SCOPE OF STUDY:
This project is limited to the structural design and analysis of a reinforced box culvert, no attempt will be made to discuss the hydrological aspect of the design, but hydrological parameters will be discussed in the literature review section. 1.3
AIM:
The aim of this project is to analyse and design a reinforced box culvert according to AASHTO LRFD Bridge Design Specifications, and in accordance with the British Standard (B.S) code.
7|Page
1.4
1.5
OBJECTIVES
Determine the different components of a culvert.
Determine the total load acting on the various parts of the culvert
Analyse the culvert and come up with bending moment and shear force diagram.
Design reinforcement steel for the culvert.
JUSTIFICATION:
Failure of culverts occur for various reasons, this includes maintenance, environmental and installation related failures. But the major type of failure related to culverts are road collapses, if the failure is sudden and catastrophic it can lead to loss of life. The dominant reason for collapse of culverts is poor or inadequate design and analysis of the culvert. The purpose of this project cannot be over emphasised as accidents due to failure of culverts can be lead to loss of life and properties. In the hydrological analysis of culverts taking into account factors like head flow, discharge, etc. are highly important in the effectual design of a culvert as any error in the hydrological design can cause damage to the environment, Undersized culverts can cause problems for oceanic life and also affect the quality of water available in that area via erosion. Poorly designed culverts tend to become packed with sand and other unwanted rubble during periods of medium to high rainfall which can lead to flooding of the road way above the culvert. Therefore it is crucial for a culvert to be sufficiently designed both structurally and hydrologic ally according to standards to withstand any unexpected environmental trials
8|Page
2.0.0 LITERATURE REVIEW:
2.1.0 DEFINATION OF TERMS: CULVERT: According to Wikipedia a culvert is a structure that allows water to flow under a road, railroad, trail, or similar obstruction from one side to the other side. Typically embedded so as to be surrounded by soil, a culvert may be made from a pipe, reinforced concrete or other material. APRON SLAB: It is a smooth (generally concrete surface) that is placed between the culvert and the channel to improve efficiency and reduce erosion. FLAP GATE: It is a passive "trap door" device placed on culvert outlets to prevent inflow. The hinge can be on the top or side of the culvert. HEADWALL: It is a wall built at top and sides of a culvert end to secure adjacent soil. DEPTH OF COVER: This the depth of earth fill that is to be placed above a culvert. SLUICE GATE: a manually or automatically operated sliding or rotating panel to restrict flow into or out of a culvert. WING WALL: a flaring vertical wall on either side of a culvert. SURCHARGE: a condition in which the water elevation at the upstream end of a culvert exceeds the culvert obvert. OBVERT: It is interior top of a culvert, equal to the invert plus the culvert diameter. INVERT: This refers to the bottom of a culvert. ROUGHNESS: It is a way of quantifying the degree of drag on flowing water by a surface. Most commonly expressed as a dimensionless Manning’s number. 9|Page
INLET STRUCTURE: An arrangement of wing walls and apron that smoothens the hydraulic transition from open channel to culvert flow and increases maximum capacity. It may also be the mounting point for a trash rack. OUTLET STRUCTURE: An arrangement of apron, wing walls and sometimes energy absorption structure at the end of a culvert. (The pacific stream keeper’s federation, Al jonsson 2001) PIPING: This refers to water flowing along the outside of a culvert. This can lead to erosion and failure. (The pacific stream keeper’s federation, Al jonsson 2001) SLOPE: It is the measurement of the change in elevation with distance. (The pacific stream keeper’s federation, Al jonsson 2001) TRASH RACK: It is a metal grate placed at the upstream end of a culvert to prevent woody debris, rocks etc. from entering the culvert. . (The pacific stream keeper’s federation, Al jonsson 2001) BOX CULVERT: It is a culvert of rectangular cross section, commonly of precast concrete. (The pacific stream keeper’s federation, Al jonsson 2001) BEDDING: It refers to the fine gravel or crushed rock placed around culverts to evenly distribute load. (The pacific stream keeper’s federation, Al jonsson 2001) CRITICAL DEPTH: Critical depth can best be illustrated as the depth of water at the culvert outlet under outlet control at which water flows are not influenced by backwater forces. Critical depth is the depth at which specific energy of a given flow rate is at a minimum. For a given discharge and cross-section geometry, there is only one critical depth. (Iowa storm water management manual, 2009).
10 | P a g e
2.2.0 CULVERT DESIGN ITEMS According to the Iowa storm water management manual, the following should be considered for all culverts where applicable 2.2.1. Engineering aspects: a. flood frequency b. velocity limitations c. buoyancy protection 2.2.2. Site criteria: a. length and slope b. debris and siltation control c. culvert barrel bends d. ice buildup 2.2.3. Design limitations: a. headwater limitations b. tailwater conditions c. storage – temporary or permanent
2.2.4. Design options a. culvert inlets b. inlets with headwalls c. wingwalls and aprons d. improved inlets e. material selection f. culvert skews g. culvert sizes and shapes h. twin pipe separations (vertical and horizontal) 11 | P a g e
i. culvert clearances
2.2.5. Related designs: a. weep holes b. outlet protection c. erosion and sediment control d. environmental considerations The designer must incorporate experience and judgment to determine which of the above items listed need to be evaluated and how to design the final culvert installation.
2.3.0 TYPES OF CULVERT Culverts can be classified based on a variety of criteria’s e.g. shape, function, etc. 2.3.1 CLASSIFICATION OF CULVERT BASED ON FUNCTION:
STREAM CROSSING CULVERT: A stream crossing culvert, as the name implies is a culvert that is provided when a roadway crosses a stream, it is built to allow water to pass to the downstream. For this type of culvert it is important to align the culvert with the natural river or stream, it is also necessary for the cross sectional area of the culvert to be the same size as the width of the stream and the centre of the stream to be aligned with the centre of the culvert so as to reduce interference with the natural marine life, stream crossing culverts are built so as to blend in with the existing stream or river.
12 | P a g e
Figure 1: Stream crossing culvert
RUNOFF MANAGEMENT CULVERT: This culverts are strategically placed to manage and route roadway runoff along, under, and away from the roadway. They are sometimes called cross drains.
2.3.2 CLASSIFICATION OF CULVERT BASED ON SHAPE:
BOX CULVERT/ RECTANGULAR CULVERT: This refers to a culvert in which the barrel is in the shape of a rectangle or a box, it is the most common type of culvert, it can be precast or cast-in-situ.
CIRCULAR CULVERT: This is a culvert that is in the shape of a circle, Circular culverts are mostly made of steel, and it is mostly used in swampy areas.
ARCH CULVERT: Just as the name implies, arch culverts are culverts with the barrel shape of an arch, they could be of two types (1) Full arch culverts; which have a bottom and hence when placed on a river, do not allow the passage of natural marine life (2) arch culverts without bottom, they only consist of the top arch and so they allow the flow of aquatic life through the culvert. 13 | P a g e
SPRUNG ARCH CULVERT: It is simply the combination of an arch culvert and a box culvert, they are rarely used.
Figure 2: Different shapes of culverts
NOTE: The shape of a culvert may differ from one place to another, as the culvert type and shape is based on a number of design factors e.g. road embankment height, requirements for hydraulic performance, environmental impact, limitation on upstream water surface elevation. (Wikipedia.com)
2.4.0 DIFFERENCE BETWEEN A CULVERT AND A BRIDGE A bridge and a culvert can be hard to differentiate from each other by just looking at it, but in terms of engineering there are clear cut differences between them. So in order to properly differentiate them we need to look at both a culvert and a bridge critically:
First of all, a bridge is a structure built across a physical obstruction like a river, mountain etc. usually for the transportation of humans and goods, while a culvert is simply a passage built to allow the flow of water through a barrier or obstruction.
A bridge basically uses a system of columns (piers) and beams to transfer load from the main deck of the bridge to the foundation and down to the earth while a culvert does not 14 | P a g e
make use of any beam whatsoever, it consists of a top slab, a bottom slab and side walls which can be designed as retaining walls.
If the size (height) of the structure in question is greater than 20ft (>20ft) the structure is a bridge but if less it can be classified as a culvert. (iamcivilengineer.com, 2015).
Most bridges do not have a floor i.e. they are not joined at the foot of the piers, while culverts have floors(bottom slab).(iamcivilengineer.com,2015)
2.5.0
WHY CULVERTS?
Harvesting or other agriculture based tasks can do a lot of damage to stream habitat and affect the water quality. Workers who need to move vehicles and equipment across streams must consider how they can do so and still protect the natural stream and aquatic life. For this reason a culvert is best suited to tackle the situation. Culverts as hydraulic structures have a number of advantages which are outlined below
Prevent Erosion
Prevent flooding
Allows water to flow unobstructed
Divert water for farming/engineering purposes.
Another major advantage/reason why a culvert should be used is the ease of construction and installation, culverts could either be cast-in-situ or precast, but for economic reasons, a precast culvert is advised. Culverts are also very portable and are usually readily available locally. Operators can install and remove them quickly.
15 | P a g e
2.6.0 ENVIROMENTAL IMPACTS OF CULVERTS Culverts whether stream crossing or runoff are generally designed to blend in with the natural slope and shape of the land, but when the site is not put into consideration in design, it can directly affect the marine/aquatic life in the river or stream. This chapter is dedicated to investigating how culverts can affect the environment. D. Mace Vaughn (2002) identified some environmental impacts of culverts as follows:
A culvert may break the continuity of water in a stream if its outflow is lifted above the water level downstream of the culvert.
The water velocity in a culvert may be higher than in the natural stream because the culvert is straight and constricts the stream into a narrower channel. Also, if the culvert contains little or no substrate (e.g. gravel, rocks, or cobbles), then the smoother bottom and sides will offer less resistance to the flowing water.
A culvert may break the continuity of the stream’s substrate. It may have less, if any, substrate along its stream bottom and, presumably, the ground underneath the culvert would be compacted as a result of construction.
Culverts channelize the stream and do not allow it to migrate laterally across its floodplain. This channelization may cause increased erosion and sedimentation.
Culverts serve as an entry point of pollutants (e.g., salt, silt, or soot) that accumulate from water that runs off of roads into roadside ditches.
Culverts may change the temperature of the stream water. If the area around the culvert and road receives more energy from the sun because the tree canopy was removed, water temperatures may be elevated. However, if the stream is slow relative to the length of the culvert (i.e., if the stream in the culvert is very shallow, slow-moving, and has to travel
16 | P a g e
Over a long distance), then the water may be cooled.
Figure 3: A bottomless arch culvert that allows for fish passage
According to the centre for environmental excellence by AASHTO the following methods may be applied to limit the negative environmental effects of culverts:
Culvert size: Culvert size may be increased to decrease water velocity.
Culvert shape: A different culvert shape (e.g., ellipse, culvert arch, or box culvert) may be chosen to achieve fish passage requirements.
Invert level: The invert level at an inlet or outlet is very important for managing flow effects at contractions (inlets), expansions (outlets), and flow regime in a culvert barrel. Invert levels affect habitat upstream and downstream of culverts. Lowering the invert may be necessary to allow the placement of natural substrate on the culvert bottom. Care should be taken to ensure a stable channel upstream and downstream of the culvert
17 | P a g e
because erosion due to increased flow velocities can progress in both directions and create barriers to fish passage.
Roughness: Changes in culvert roughness may effectively decrease water velocities to acceptable levels. For example, corrugated circular culverts can be chosen with large, helical corrugations to provide greater overall roughness and provide for a larger low flow water depth suitable for fish. Concrete box culverts can be modified by using oversized aggregate or grouted riprap. The addition of energy dissipaters can control the hydraulic regime and thereby reduce velocities.
Grade Control: Artificial resting areas upstream or downstream of a culvert can mitigate many adverse conditions in the culvert barrel and at the inlet or outlet. Weirs or sills downstream of a culvert can be used to maintain adequate water depth and prevent scouring of a plunge pool. An upstream resting pool can trap sediment while allowing recuperation time for 710 migrants. Combined with proper in stream cover, culverts may provide migrants some protection against predators. Classification
Stream Characteristics
Class 1 - Major fish habitat
Large named permanently flowing stream. Aquatic vegetation present. Known fish habitat. Smaller named permanently or intermittent flowing stream. Aquatic vegetation present. Known fish habitat. Named or unnamed watercourse with intermittent flow. Named or unnamed stream with flow during rain events only.
Class 2 – Moderate fish habitat
Class 3 – Minimal fish habitat Class 4 – Unlikely fish habitat
Minimum Preferred Structure Bridge
Large box culvert or bridge
Box / pipe culverts
Ford or culverts
Table 1: Minimum preferred structure for fish passage (Goulburn broken catchment management authority)
18 | P a g e
2.7.0 PERFORMANCE CURVE A performance curve is used to determine the effects of a high flow rate on the culvert at the site and any other problems that may arise. A performance curve should be drawn for a culvert to access various head waters and the hydraulic capacity of the culvert under this head waters, an engineer cannot access the effect of any slight increase in the head water using only the design peak flow. 2.8.0 HYDRAULIC DESIGN CRITERIA FOR CULVERTS This project is limited to the structural design of culverts alone, but below is a brief explanation on various hydraulic conditions’ to be put in cognisance when carrying out the hydraulic design of culverts, they include:
2.8.1 VELOCITY LIMITATION Velocity limitations include the maximum and the minimum velocities that should be considered when designing a culvert, the outlet velocity affects the stability of the culvert, the greater the outlet velocity the greater the need for stability, there is no specified maximum velocity for reinforced concrete box culverts, but there should be provision of outer protection when the velocity is an erosion risk. 2.8.2 ALLOWABLE HEADWATER Headwater is the height of water above the invert of a culvert at the entrance and exit of a culvert. The allowable headwater can be gotten from the evaluation of the use of the land upstream of the culvert, the slope of the area where the culvert is to be placed.
19 | P a g e
2.9.0 COEFFICIENT OF EARTH PRESSURE The earth can exert pressure, minimum as active and maximum as passive, or in between called pressure at rest. It depends on the condition obtained at site (Terzaghi4 and Gulati5). For example in case of a retaining wall where the wall is free to yield and can move away from the earth fill the pressure exerted by the earth shall tend to reach active state and thus be minimum. As to reach active state only a small movement is required which can normally be achieved in case of a retaining wall, also before failure of the wall by tilting, the back fill is bound to reach active state. The wall thus can safely be designed for active pressure of earth, with co-efficient applicable for active pressure. In case of an anchored bulk head, the earth pressure on the anchor plate will tend to achieve passive state because the anchor plate is dragged against earth and large displacement can be allowed, one can consider passive coefficient for the design of anchor, of course, some factor of safety need be taken as required displacement to achieve passive state before the bulk head gives way may not be practical. In cases where the structure is constructed before back fill earth is placed in position and the situation is such that structure is not in a position to yield on either side, the earth pressure shall reach a state at rest. In such situation the co-efficient of earth pressure shall be more than the active condition. In case of box since it is confined with earth from both sides the state of earth shall be at rest and a co-efficient more than the active pressure is normally adopted in the design. The earth is filled after construction of the box further the box is not in a position to move/yield therefore the pressure shall be at rest. The value is designer’s choice. The co-efficient of earth pressure in case of box is taken to be 0.333 for a soil having ф = 30º equivalent to active condition by many authors in their books of design. Some authors take this value = 0.5 for normal soil having ф = 30º. (B.N.Sinha & R.P. Sharma October – December 2009)
20 | P a g e
2.10.0 EFFECTIVE WIDTH Effective width in the run of culvert (length across span) is expected to be affected by a moving live load. This width plays a significant role as far as consideration of live load in the design of culvert. Where however, there is large cushion the live load gets dispersed on a very large area through the fill and the load per unit area becomes less and does not remain significant for the design of box, particularly in comparison to the dead load due to such large cushion. In case of dead load or uniform surcharge load the effective width has no role to play and such loads are to be taken over the entire area for the design. Effective width plays an important role for box without cushion as the live load becomes the main load on the top slab and to evaluate its effects per unit run for design as a rigid frame, this load is required to be divided by the effective width. As such evaluating effective width correctly is of importance. The relevant IRC Codes, other Codes, books, theory/concepts are at variance as far as effective width is concerned and requires discussions at some length. It is required to understand the concept behind effective width. Basically, it is the width of slab perpendicular to the span which is affected by the load placed on the top of slab. It shall be related to the area of slab expected to deform under load. It can be well imagined that this area of slab which may get affected will depend on how the slab is supported whether in one direction or both directions and secondly on the condition of support that is whether free or continuous or partially or fully fixed. It can also be imagined that the width shall be larger if the slab is allowed to slide over support under the load as in case of freely supported, and the same will reduce if the slab is restrained from sliding and more the restraint the less shall be the width. In this view the effective width shall be least for fully fixed and gradually increase for partially fixed, increase further for continuous slab and shall reach maximum for slabs freely supported at ends. Where support on one side is different than on the other side the effective width should be obtained taking this fact in consideration. The distance of the load from the
21 | P a g e
near support affects effective width, more the distance larger will be the effective width and will reach highest when the load is at centre. The ratio of breadth (unsupported edges) and the span also affects effective width. All factors mentioned above need to be taken into account while obtaining the effective width. The IRC: 21-20006 Clause 305.16 gives an equation for obtaining effective width for simply supported and continuous slab for different ratio of overall width verses span for these two kinds of supports. The Code does not provide if one of the support is continuous while other is simply supported. The Code is silent for other types of supports such as fixed or partially fixed. Some designers use this formula and factors for continuous slab is taken valid for partially restrained support in a situation like box culvert. This does not appear to be in order. The reasons for this can be better realized by the explanations given in sub para 3 above. Nevertheless, effective width need to be obtained in box type structure also to evaluate affected area by moving load for considering these in the design. The AASHTO9 for Standard Specifications for Highway Bridges 17th Edition 2002, provides at para 16.6.4.3 under RCC Box that “The width of top slab strip used for distribution of concentrated wheel loads may be increased by twice the box height and used for the distribution of loads to the bottom slab”. This confirms what is mentioned in sub para 5 and is alright. However, any such dispersal for bottom slab different than top slab shall not be practical when braking force effect is to be taken, which shall have to be for the same run of the box structure as a whole. (B.N.Sinha & R.P. Sharma October – December 2009). 2.11. 0
INLET CONTROL
If the culvert is operating on a steep slope it is likely that the entrance geometry will control the headwater and the culvert will be on inlet control. Inlet control for culverts may occur as unsubmerged or submerged. For the unsubmerged condition, the culvert invert slope is supercritical and the culvert acts like a weir. For the submerged condition, the culvert doesn’t flow full and acts like an orifice. (Robert Duane Nickols) 22 | P a g e
2.12.0 OUTLET CONTROL If the culvert is operating on a mild slope, the outlet characteristics will probably control the flow and the culvert will be on outlet control. There are three types of outlet control conditions.
The headwater is submerged and the outlet is submerged with the culvert flowing full.
The headwater is submerged and the outlet is unsubmerged. The headwater is unsubmerged and the outlet is unsubmerged.
The culvert slope is sub-critical and the tail water depth is below the pipe critical depth.(Robert Duane Nickols)
Figure 4: Diagram showing various inlet control methods
23 | P a g e
Figure 5: Diagram showing various outlet control method
2.13.0 BRAKING FORCE This is another area where opinion of the designers vary in two ways firstly, whether braking force caused by moving loads shall deform the box structure and should therefore be considered in the design of box. Secondly, if it is to be considered what effective width should be taken to obtain force and moment per unit run of box. Of course the braking force will affect the global stability and change the base pressure to some extent. The IRC Code is
24 | P a g e
silent as far as box is concerned. It will be in order to neglect effect of braking force on box having large cushion. In such situation the braking effect will be absorbed by the cushion itself and no force will be transmitted to the box beneath. Question will, however, arise up to what cushion height no braking force need be taken. This height generally is taken to be 3 m. Thus no braking force for cushion height of 3 m and more and full braking force for no cushion, for intermediate heights of cushion the braking force can be interpolated. Braking force by the moving loads on top slab of box having no cushion shall act on the box structure and shall deform the box. The question is what length of box can be considered to share this braking force. In another words what effective width of box shall be taken to obtain braking force per unit run of box. One way is to take the effective width of box same as considered for vertical effect of moving loads. (B.N.Sinha & R.P. Sharma October – December 2009).
2.14.0 IMPACT OF LIVE LOAD
Moving loads create impact when these move over the deck slab (top slab). The impact depends on the class and type of load. The IRC:6-2000 Code gives formula to obtain impact factor for different kind of loads by which the live load is to be increased to account for impact. The box without cushion where the top slab will be subjected to impact is required to be designed for live loads including such impact loads. Any such impact is not supposed to act on box with cushion. Hence no such impact factor shall be considered for box with cushion. The impact by its very nature is not supposed to act at lower depth and no impact is considered for the bottom slab of the box. It does not affect the vertical walls of the box and not considered in the design. The IRC:6-200010, Code Clause 211.7 specifies that for calculating pressure on the bearings and on the top surface of the bed blocks, full value of appropriate impact percentage be allowed. But for design of pier, abutment below the level of bed block, the appropriate impact percentage shall be multiplied by the factor given therein. 25 | P a g e
Accordingly, the impact is to be reduced to 50% below bed block and zero at 3 m below, proportionately reducing between this heights. Although these provisions are for bridges but can be applied in case of box structure in absence of any specific provision in the Code for box in this regard. The AASHTO9 at para 3.8.1.2 specifies that impact shall not be included for culverts having 1m or more cover. This, however, will be on lower side compared to considering zero impact for a cover (cushion) of 3 m. It is, therefore, suggested that considering full impact on top slab without cushion and zero impact for 3m cushion and interpolating impact load for intermediate height of cushion is on conservative side and can be safely adopted. (B.N.Sinha & R.P. Sharma October – December 2009). 2.15.0 CLEANING AND MAINTENANCE One method to account for all culverts is to maintain an inventory of culverts and under-drains and use a checklist from this inventory to account for culverts during inspections. Inspect culverts often, especially in the spring and autumn, and after storm events, checking them for signs of corrosion, joint separation, bottom sag, pipe blockage, piping, fill settling, cavitation of fill (sinkhole), sediment buildup within the culvert, effectiveness of the present inlet/outlet inverts, etc. Check inlet and outlet channels for signs of scour, degradation, agradation, debris, channel blockage, diversion of flow, bank and other erosion, flooding, etc. Practice preventive maintenance to avoid clogging of pipes and other situations which may damage the culvert or diminish its design function. If a culvert is plugged with sediment, flush it from the outlet end with a high pressure water hose. Take measures to reduce downstream sedimentation and clean debris and sediment from the outlet ditch afterwards. When replacing damaged culverts which handle the flow adequately, use the same size, shape, and type of pipe. Changing any of these criteria may adversely affect the established stability of the ditch, stream, and/or roadway.
26 | P a g e
2.16.0 MULTI-CELL CULVERT Multi-cell culverts provide a method of permitting bank full and lower flow to be conveyed through a single culvert and storm flow to be conveyed across the floodplain without constriction. 2.16.1 EFFECTIVE USES & LIMITATIONS Multi-cell culverts permit flood waters to flow essentially unimpeded across a floodplain. Multi-cell culverts should not be used in Rosgen Type a streams due to steep slopes, in excess of 3%. They should also not be used in Type D streams due to high bed loads. Placement of culverts in Types A or D streams would likely obstruct fish passage. Single-cell culverts should be used rather than multi-cell culverts in incised (Types F or G) channels since these channel types do not have a well-developed floodplain. If these channels are actively incising, the channels must be stabilized prior to culvert construction; a culvert placed in an actively incising channel will likely result in a perched culvert. Multi-cell culverts are most effective in Types C and E channels since these channels tend to have a well-developed floodplain. Floodplain cells are highly susceptible to debris accumulation; therefore, in stream corridors with a significant debris jam potential, a moderate to heavy accumulation of various size debris, present multi-cell systems may not be appropriate. 2.16.2 MATERIAL SPECIFICATIONS Most culverts are constructed from either corrugated metal pipe (CMP) or concrete. CMP is the preferred material to maintain slower velocities for fish passage but may have a shorter design life than concrete. 2.16.3 INSTALLATION GUIDELINES Construction of multi-celled or single barrel culverts should proceed the same as for standard culverts as detailed in MGWC 4.3: Culvert Installation. The following are general guidelines for design and installation of single or multi-cell culverts: 1. Assess the Rosgen stream type and the channel stability prior to designing the culvert system. Alternatives to culverts should be considered for Types A and D channels. For all remaining channel 27 | P a g e
types, assess the channel stability to determine whether or not the channel is degrading or widening. If the channels are unstable, widening, or degrading, a culvert system should not be used unless the channel can first be stabilized. 2. For incised stream types F or G which have been stabilized, a single cell culvert which can convey the design storm flow can be designed and constructed. 3. For stable stream types C or E in which debris jam potential is not significant, a multi-cell culvert system should be constructed where practical. One cell is placed within the bank full channel which is designed to carry the bank full flow. The invert of this barrel should be depressed according to MGWC 4.5: Depressed Culverts. One to three cells are placed on either side of the floodplain to convey the design storm flow with minimum constriction of the flow. All erosion and sediment control devices, including dewatering basins, should be implemented as the first order of business according to a plan approved by the WMA or local authority. (See the 1994 Maryland Standards and Specifications for Soil Erosion and Sediment Control.)(Watershed Sciences, 2007)
28 | P a g e
3.0.0
METHODOLOGY
According to (oyenuga. O. victor, 2001), a box culvert should be analysed as a rigid structure with moments occurring at the corners. The Hardy Cross method of moment distribution is best suited for the culvert analysis or the Kani’s method of moment distribution. 3.1.0
LOAD CASES FOR DESIGN
Culvert empty: Full load on top of the slab, surcharge load and superimposed surcharge load on earth fill.
Culvert full: Live load surcharge on top slab and no superimposed surcharge on earth fill.
Culvert full: Live load surcharge on top slab and superimposed surcharge load on earth fill.
3.2.0 LOADING
Top Slab: The load include, slab own weight, imposed load ad weight of earth fill. In cases where the depth of the earth fill is greater than three times the width of the culvert, the earth load can be assumed to be equal to earth loads of height three times the culvert. When a point load such as wheel loads incident on a culvert without earth fill, the dispersal should be based on tyre width. For a wheel load on a fill of height,h, the load should be should be spread over an area of 4h 2 , that is 2h, by 2h. When h equals or slightly(B.S. 5400 Part 2: 1978)
Walls: Loads on walls include own weight, effect of active pressure, effect of any surcharge any pore water pressure. When the culvert is full, there will be water pressure on the inside wall and wall should be designed to resist this pressure and assuming no back fill. The walls need not be designed as tank walls. That is, no need to check for
29 | P a g e
stresses in the steel as well as checking for crack widths, the walls should simply be designed for flexural(bending, shear and axial pull).
Bottom slab: The top slab and all its imposed load, the walls and pressures on them produce an upward pressure (reaction) from the ground and causes moment. The weight of water in the culvert and weight of the bottom slab should be considered when determining the maximum pressure on the ground but since they are borne by the ground, directly, they do not generate moment.
3.3.0 LOAD CALCULATION TOP SLAB: The top of the culvert is designed as a slab, first of all, the total load acting on the slab is determined, taking into account the live load and the dead loads. This is done with the formula F 1.4 Gk 1.6 Q
k
Dead load: Self-weight: thickness of slab cover x unit weight of concrete (KN/m 2 ) Earth load: height of road fill x unit weight of earth (KN/m 2 )
Live load : Wheel load: wheel load x 2 (see B.S 5400 part 2:1978) (KN/m 2 )
The total load acted is then gotten from the addition of the live and dead load F= (DL + LL) (KN/m 2 )
BOTTOM SLAB:
Load acting on bottom slab include load transferred from the top slab and the upward pressure (reaction) from the walls which causes moment.
WALLS: The force exerted by the pressure of the earth is determined.
30 | P a g e
3.4.0 MOMENT CALCULATION Top slab: The fixed end moment due to the total load is determined using the formula
M
wl 2 12
Wall: The moment in the wall is determined using appropriate formula Bottom slab: The moment in the slab is determined using appropriate formula.
3.5.0 BENDING MOMENT ANALYSIS AND DIAGRAM The entire culvert is then analysed as a rigid body using Hardy cross’s moment distribution method, the bending moment and shear force in all the members are determined and the bending moment and shear force diagram is drawn. 3.6.0 REINFORCEMENT AND DETAILING The arrangement of reinforcement steel and the area of steel to be used is determined using the appropriate formula and the bar bending schedule is also provided.
31 | P a g e
32 | P a g e
