DESIGN OF STRUTTING SUPPORT SYSTEM
TR 26: 2010 Tec h n i c al Ref Ref er en c e for Deep Ex c avatio vati o n
Publis ubl ishe hed d by Spring Singa ing apore por e
2
Partia rt iall Factor ct ors s for fo r Soil oi l Loa Lo ad The design values of the geotechnical parameters X d should be derived using Xd = Xk / m Xk is the moderately conservative estimate of the soil parameter and is the reduction factor for the parameter. So i l p ar am et er Angle of shear resistance a Effec Effectiv tive e cohesi cohesion on Undr Undrai aine ned d shea shearr stre streng ngth th Unconf Unconfine ined d streng strength th Weigh eightt dens densit ity y
Sy m b o l
’ c’ cu qu
m
GEO/STR Limit states Design Approach 1 (DA1): Combination 1: A1 + M1 + R1 Combination 2: A2 + M2 + R1
A
M
R
4
GEO/STR Limit states
5
• The term “moderately conservative” is taken to mean the “cautious estimate” of the value relevant to the occurrence of the limit state as specified in CIRIA C580. It is also equivalent to “representative value” as specified in BS 8002 and to the “characteristic value” as in EC7.
surcharge
Earth Pressure
pore water pressure Effective active pressure Effective passive pressure
Hinge Method
Analysis of strut forces
Additional Loads for the Design of Strutting System In addition to the excavation load, the following loads must also be considered •
A minimum surcharge of 10 kPa should be considered. Where there is vehicular traffic, a design surcharge load of 20 kPa should be used. Higher surcharge load (> 20 kPa) may be required if heavy construction equipments are employed.
•
Change of strut force due to temperature difference of ± 10 C should be considered.
•
Change of strut force due to the installation and removal of struts at any level.
•
Change of strut force induced by wall rotation and relative displacements between the supported ends, if any.
• Accidental impact load of 50 kN to be applied normal to the strut at any point in any direction, unless otherwise demonstrated by risk assessment. •
Axial force on the waler due to the inclined struts (in plan).
•
Accidental removal or failure of o ne strut/anchor or its connections.
Partial Factors for Loads Design li mit states
ULS
SelfWeight (Dead Load)
***Strut Force from Soil Analysis
*Imposed Load (Construction 1kN/m)
1.35
Design Strut Force
Leading live load = Accompany Temperature = 1.5x0.6 1.5
(
0=1.0,
1=0,
2=0.2)
Accompany live load = 1.5x0.7 ALS 50kN Point Load in y or z direction
1.0
ALS One Strut Failure (OSF)**
1.0
*Temperature Load
Accident Impact ( 0=0.6, 1=0.5, 2=0) Force (50kN)
Leading Temperature = 1.5
Characteristic Leading Live load Strut Force = 0.7* Accompany live load = 0.6
Accompany Temperature = 0 Leading Temperature = 0.5
Characteristic Leading Live load Strut Force = 0.7* Accompany live load = 0.6
Accompany Temperature = 0 Leading Temperature = 0.5
ULS: Table A1.2(B), EN1990-2002, pg 53 ALS: Table A1.3, EN1990-2002, pg 54
0
1.0
0
* values obtained Table A1.1, EN1990 and National Annex **requires soil-structure analysis for accuracy. Values not given under application for buildings. ***Soil-structure interaction analysis to satisfy both DA1-1 and DA12 for GEO/STR limit state
x
strut y
Examples on Load Combinations Case No.
Design limit State
Load Combinations
Case 1a
ULS: DA1-1: A1+M1+R1
1.35DL+1.5LL+1.35SF+0.9 TL
Case 1b Case 2a
1.35DL+1.5LL+1.0SF+0.9 TL ULS: DA1-2: A2+M2+R1 ALS: Accidental Loading in horizontal direction
1.0DL+0.5LL+1.0F+1.0SF
Case 2b
ALS: Impact Load in vertical direction
1.0DL+0.5LL+1.0F+1.0SF
Case 3
ALS: One Strut Failure
1.0DL+0.5LL + 1.0SF
Where;
DL = Self weight of the member SF = Strut Force from soil analysis LL = Live load along the strut, 1.0kN/m TL = Temperature load (axial force due to change of temperature) IF = Impact force
Design Against One Structural Component Failure TR 26: 2010 The system should not collapse due to the possible failure of any one structural component. The wall and the supporting structural members including their connections shall be capable of re-distributing the load from the failed member. The remaining structural system shall continue to remain safe without causing any danger to surrounding adjacent structures and properties.
11
One Strut Failure • TR26:2010 clause 3.7.4 “design for deep excavation should accommodate possible failure of any individual strut, tie rod, ground anchor, structural member or connection at each stage of the construction works.” • lack of clear authoritative guidance on appropriate design standards Absence of an industry-wide approach • 3 possible approaches are generally used as follow:
Design for One Strut Failure Approach 1 – One strut failure
• Use waler to distribute the strut force to left and right struts • Waler section becomes very large (M = Wl2/10). • Plastic design may be used for waler beam (if the section is plastic) • Use splay beams to improve the overall strutting performance. 13
Approach 1 – One strut failure
TD
T
TD
S
F
S
BD
B
BD
Without Splay beam – waler moment increases significantly
L
2L
With Splay beam –waler length after OSF remains as L
L
L
L
Approach 2 – one level failure •
One entire level of struts is assumed to fail and be removed.
•
Loads from the failed struts are distributed to adjacent top and bottom struts by the wall.
•
Wall is designed to withstand one level strut failure.
•
Plane strain 2D FEM analysis is usually performed to assess vertical bending moment in the wall.
•
Generally conservative with increase in wall thickness & reinforcement
Approach 3 – 3D analysis – Remove one strut – Perform 3D structural/soil interaction analysis – Allow plastic hinge formation – The structural system and wall shall continue to remain safe and without causing any danger to surrounding adjacent structures and properties. – One strut failure scenario may not always govern the design 18
One Strut Failure – Approach 3 •
Loads from the failed struts are distributed to surrounding struts taking into account of the three dimensional effect
•
Need 3D FEM analysis to determine the load re-distribution
Approach 4 – Alternate Strut Failure. This is not proven and therefore not recommended • In 2D-Plaxis, carry out the strutted wall analysis with all the struts in place, but model one of the strut layers with half the original stiffness(EA) to simulate one strut failing, i.e. the strut layer is not made to vanish but is modelled with an equivalent stiffness of that particular layer of struts being reduced to 50% of original EA. • Obtain the waler uniformly distributed load from the resulted strut force and design the waler accordingly. This is similarly repeated for other layer of struts in different runs. • The above seems to be equivalent to an alternate strut failure instead of a full layer strut failure (where EA=0), and is less critical than the latter. But this works only if the waler is infinitely rigid. 20
Approach 4 • Approach 4 - Residual stiffness effect (insufficient scientific research to back up this approach) P P Stiffness = 0.5EA EA
0.5EA
Limitations of 2D Analysis •
Results of lateral wall displacements show 2D analyses are inaccurate and always more conservative than 3D analyses.
•
When L/H < 4.5 and L/B < 3.5, 2D analysis may over-predict the wall displacement leading to uneconomical design. This is because corner effect (soil arching) becomes significant
• Shorter length L larger effect of soil arching • Corner stiffening effect is non-existent in 2D cases where excavation length is infinitely long. B
H
L
E x am p l e : S t r u t & Wa l e r D e si g n ( OS F C a s e ) B. Waler Section With OSF Condition Waler Section Strut Capacity
Stron g Waler Appr oach
One Level Failure Appr oach
Without OSF Consideration
200
1x UC 305 x 305 x 198
1x UC 203 x 203 x 86.1
1x UC 203 x 203 x 60
500
1x UC 400 x 400 x415
1x UC 305 x 305 x 198
1x UC 305 x 305 x 137
1200
2x UC 356 x 406 x467
1x UC 305 x 305 x 312.5
1x UC 305 x 305 x 312.5
OSF case did not affect the strut section
One level failure approach is adopted for the waler design. If this approach is adopted at first level strut large displacement will occur on the wall, which may caused inefficient design for the wall. Therefore, only for the first level the strong waler approach is adopted and concrete cap beam may be used to redistribute the load due to OSF.
Accidental Load 50kN is applied in one direction at a time
Spacing of struts Side view
Vertical clear spacing of strut should be at least 3.5m (preferably 4m) because the height of an excavator is about 3.5m. Horizontal spacing of strut depends on the dimension of excavator (excavator plan dimension is about 3.5m x 4m).
>3.5m Wall
l >4m strut l
Waler Plan view
Strut Force P1 Strut force F
l h P2
strut l Wall
Side Elevation
F = 0.5(P1 + P2)h x l
Waler Plan View
Design Moment of Waler Beam •
Simply support beam M = wL2/8
•
Plastic design
M = wL2/16
(sagging)
Continuous beam M = wL2/10 (hogging) WL2/10` 0.08WL2 0.4WL L
WL2/10
0.025WL2 1.1WL
0.08WL2 0.4WL
1.1WL
L Continuous beam
L
Waler Beam Wall
Mx strut
Mx
Beam-column problem
N Ed
major axis buckling
N b , y , Rd
minor axis buckling
N Ed N b , z , Rd
k yy k zy
M y , Ed M b , Rd M y , Ed M b , Rd
k yz k zz
M z , Ed M z , Rd M z , Ed M z , Rd
1 1
Axial Force in Waler Beam • Diagonal struts will induce axial force in the waler. The waler needs to be designed for both axial compression and bending. • The axial force in the waler may be transmitted to the wall provided that 1) shear connectors are installed between the wall and the concrete backing and 2) shear connectors are provided between the waler and the concrete backing.
Shear Connectors for Diagonal Strut •
To transfer shear force from diagonal strut to wall
•
Axial force on waler will not accumulate
View B
Shear Studs •
Axial force to be transfer from waler to wall = F
•
Shear resistance of one stud = PRd CBP
No of studs required = F/ PRd
CBP CBP
Axial Force = F
Design resistances P Rd (kN) of shear connectors to BS EN 1994-1-1 Concrete strength (N/mm 2)
Stud di ameter and height (mm)
C20/25
C25/30
C30/37
C35/45
19 mm dia 100 mm
63
73
81
81
22 mm dia 100 mm
85
98
108
108
16 mm dia 75 mm
45
52
57
57
Design resistance = Characteristic resistance/1.25 For concrete grade greater than C35/45, failure of shear stud is governing
32
Axial Force on Waling Beam
Axial Force Transferred to Waler F
F
F
F/3
2F/3
2F/3
F/3
(F/3)cos si n
(F/3)/sin
(F/sin * cos
(F/sin * cos
Axial force distrib ution
(F/sin * cos
Diagonal Struts
F
Strut with Splays
Load Transfer in E-W Direction High Concentr ation Axi al Load at Waler
Earth Pressure Strut Force Waler Axial Force
35
Shear Stud at Diagonal Zone
36
Waler Details Stiffeners are required to ensure stability
Eccentric Loads • The design of struts, walers and strut/waler connections shall take into account of eccentricity in transfer of load from the waler to the strut. • For walers made from a single section UC or UB, the eccentricity shall be taken as 10% the depth of the strut, but not less than 30mm of the overall dimension of the strut in the vertical plane.
Design strut for load eccentricity M=Fe e = 10%d > 30mm
Single waler
e = 10%d > 30mm
Double waler
F1 D F2
39
Design Waler for load eccentricity M=Fe e = 10%d > 30mm
F Single Waler
Stiffeners may be needed to prevent side sway
M=Fe
F/2 +Fe/D = F1 F
D
F/2 - Fe/D = F2 Double Waler This is because rotation of wall (deflection of wall) is now limited to 0.05H% and hence one waler failure scenario is not possible.
40
Minimum Load Eccentricity A minimum load eccentricity of 30mm may be used with the following conditions: • Use of proper installation method (use to temporary guide plate) to control and ensure the eccentricity is kept to minimum • Include eccentricity in the checklist. Eccentricity to be checked prior to approval.
Buckling length of strut
Buckling length of strut
Horizontal Bracing
UC152x152x37kg/m
45
46
Check Local Buckling of I-Section under pure compression
t D
d
T b
Co m p r es s i o n el em en t
Rat i o
L i m i t i n g v al u es Cl as s 1
Outst ut stand and element element of compression flange
b /T
Cl as s 2
Cl as s 3 d/t <
120 1+ 2.0 r
Internal element element of compression flange
b /T
2
Design of Lateral Restraints q d L N Ed 8
e0d
q L
; e0d = L/500
If q = L/2000 (assume nominal deflection of rigid support)
NP Ed
q d L 2% N Ed If q =0,
q d L 1.6%N Ed N Ed
F
F = 1.6 to 2.0%NEd
N Ed
Lateral Restraint to Strut to be designed for 2% NEd
Strut to Runner Beam Connection & Runner Beam Bracket •
•
Strut to runner beam –
Angle members on runner beam restraint strut from buckle vertically and horizontally
–
Strut is allow to move its longitudinal axis
Runner beam bracket –
To transfer load from runner beam to kingpost, including axial force from runner beam
–
Angle members provide vertical restraint to prevent uplift of the runner beam
Side View
Side View
Bracing system that provides restraint to more than one member shall be designed to resist the sum of the restraint forces from each member reduced by the factor m
1 m 0.51 m m =number of parallel members to be restrained.
Example: m = 4
m = (0.5 (1+1/4))0.5 = 0.79 F= 0.79 x 0.02P R 51 Liew
Assume q = L/2000 deflection of rigid support)
Design of braced frame to support horizontal struts Design br aced frame for the hori zontal loads F1 and F2: Equal to 0.5% (1.35 Dead Load + 1.5 Imposed Load) (This is equivalent hori zontal force account for frame imperfections) Plus Restrained fo rce = 2.0% (sum of strut force at that level) x m
PLAN BRACING LAYOUT
Can be replaced by vertical bracing
TEMPPERATURE EFFECTS ON STRUTS L=tL L = change in strut length
L = strut length t = change in temperature from the installation temperature
= thermal coefficient of expansion = 1.2 x10 -5 per steel.
= L/L = /E = P/EA N = EA L/L
Force induced:
N=
t EA
E = Young's modulus A = cross-sectional area of strut N= change of strut force due to thermal effect
C for
°
TEMPERATURE EFFECTS N=k
t EA
• Temperature load shall be added to the strut loads. • In Singapore, a change in temperature of 10oC is expected. A conservative assumption is to assumed both ends of the strut are fixed. • k = 1.0 is for a fully restrained strut where both ends are prevented to expand freely. If the degree of restraint of the strut allows some expansion, lesser strut load due to temperature effect will result. • In the absence of rigorous analysis, k = 0.6 is recommended for flexible sheet pile walls and k = 0.8 for stiff wall with stiff soil condition. Temperature effects are normally added to the predicted strut loads after the analysis is completed.
Diaphragm wall (stiff wall) Example: UC 305x305x158 S355, A= 201 cm2, Le = 10m, T = 10o C, L ey = 10m, Ncy =1780kN N= t EA= 494kN (28% of Pcy ) Actual value will be less due to movement of wall. For diaphragm wall (stiff wall) N=k t EA= 0.8x494kN = 395 kN
Measured strut loads and temperature change with time at site Strut temperature
Strut Force
Measured strut force/temperature versus time
Example Strut size used was H400x400x172 kg/m with sectional area of A= 219 cm². E, Young’s modulus of steel E is equal to 205x106 kN/m² and thermal coefficient of expansion for steel is equal to 12x10-6 per oC.
Measured temperature t = 48.5 – 26 = 22.5 oC Measured change in force
N = 900 kN A = 219 cm2 N= k
t EA
k = 0.74 (which is between 0.6 and 0.8) Use k = 0.8 is conservative.
King Post & Decking Kingposts and decking structures should be designed with the appropriate loads and load factors in accordance with the relevant codes of practice to achieve robustness and adequate factor of safety such that no disproportionate catastrophic collapse would occur. Where appropriate, anticipated retaining wall movements under the most onerous conditions should be considered in the design and detailing of kingposts and decking structures. TR26: Technical reference for deep excavation, 2010
Kingpost Design
Purpose 1. Kingpost helps to provide support to struts and runner beams. 2. It reduces the effective length of the struts about the x-x axis.
Design Considerations 1. ensure joints are capable of transmitting forces 2. ease of installation and handling. 3. ensure economic design. 4. effective length of an embedded king post should be determined from analysis to derive the position of fixity below the ground. The analyses should also include construction stages when the temporary support members or struts are removed.
60
EXAMPLE ON DESIGN OF KINGPOST
Maximum unbraced length of King Post = 5m
Effective Length = 5m + 2m (below formation level)
5000m - Maximum unbraced length after excavation
7m
5m
Design of King Post • Vertical load increases progressively as struts are installed level by level • Need to design for moment due to eccentricity of load or horizontal load • King post provides vertical restraint to struts, hence add 1.0% strut force acting vertically • Depth of embedment of king post = shaft friction + end bearing
VERTICAL LOAD RESISTANCE OF KING POST Pdesign < R Resistance of a steel pile R = Rs/( s) + Rb/ ( b) • Rs is the ultimate shaft friction resistance • Rb is the ultimate base resistance •
Material factors for driven pile = 1.5, s= b=1.3
VERTICAL LOAD RESISTANCE OF KING POST
R = Rs+ Rb = qs As + qb Ab qs = unit shaft friction. For more than one soil type, the average value of q s over the length of the pile is taken As = surface area of the pile in contact with the soil qb = End bearing resistance Ab = steel cross-section area of the base of the pile or plug cross-sectional area
• Cohesive Soil qs =Cu
0.3 < Cu < 0.6 (Undrained shear strength)
qb =9Cu
Cu = 20kN/m2 for soft clay to 400 kN/m2 for hard clays
• Cohesiveness Soil (e.g. Sand) qs =2Nb
Nb = average standard penetration (STP) value of each soil layer
qb =400Nb Nb = STP value at the pile tip
eccentricity If designed to provide lateral restraint to horizontal strut, add 1.0% strut force to king post.
Avoid transferring of horizontal load to king post
Bracket Design • Detail of Bracket on Kingpost
Bracket Design • Detail of Bracket on Kingpost with ECC
King post provides restraints to struts
P1
1%P1
P2
1%P2
strut
Add 1% strut force horzontally and vertically strut
P3
1%P3
strut wall
King post
Stability of Node Point
0.002P
0.002P
0.001P
Web bearing and buckling check for each layer of strut to waler connection
DESIGN OF CONNECTIONS Strut-Waler Connection
15mm Thk Stiffner Plate With 12mm Fillet Weld all round
Waler
Strut
Waler 20mm Thk End Plate 80x80x8mm Angle Bracket
20mm Thk End Plate
Waler
Strut
18mm Thk Flange Plate
4M 20 Grade 8.8 Bolt & Nut
10M 20 Grade 8.8 Bolt & Nut 10mm Thk Web Plate
Typical Splice Detail
Note: All steel plates and sections shall be S355 unless otherwise stated.
TYPICAL DETAILS OF WALER (2)
Typical shear studs, dia. 19mm (300mm embedment depth) at 200mm spacing Waler Bracket
CBP wall
Design of Temporary Platform
Temporary Platform
20 kN/m2
Typical Mobile Crane (25 Tonne DEMAG Model)
CONTRUCTION DECK DESIGN
MACHINERY SPECIFICATION MODEL: SCX- 800
WEIGHT: 80 TONNE SIZE= 6.06m x 4.83m TRACK SIZE: 2NOS. X(5.15m x 0.8m) TRACK PRESSURE: 90 kPa PROPOSED WORKING RADIUS: 15m LIFTING CAPACITY JIB LENGTH= 7 TONNE CLAMSHELL= 6.5 TONNE
CONTRUCTION DECK DESIGN ENLARGED CONSTRUCTION DECK PLAN METRO DECK= 200mm THK LTA STANDARD DETAIL OF GUARD RAIL AT EDGE MAIN BEAM= 610 X 324 X 195 kg/m UB SECONDARY BEAM= 610 X 229 X 140 kg/m UB
CONTRUCTION DECK DESIGN
SECONDARY BEAM (SB) LOADING CONSIDERATION
CASE 1: DESIGN FOR 20kPa LIVE LOAD
CASE 3: TRACK PRESSURE PARLLEL TO SB (MID SPAN)
CASE 2: TRACK PRESSURE PERPENDICULAR TO SB
CASE 4: TRACK PRESSURE PARLLEL TO SB (END SPAN)
CONTRUCTION DECK DESIGN
MAIN BEAM (MB) LOADING CONSIDERATION
CASE 1: DESIGN FOR 20kPa REACTION FORCE FROM SB
CASE 2: TRACK PRESSURE PARALLEL TO MB
CASE 3: TRACK PRESSURE PERPANDICULARTO MB (MID SPAN)
Loading Imposed Load x 1.5 • Wheel load x 1.3 (for Impact) • Construction load = 20kN/m2 for constructional activity; stacking of materials, equipment and plant.
Dead load x 1.35 • Weight of Platform, any temporary works connected to the platform structure.
Dynamic Loading due to Lifting load moving vertically • The static loading of the moving item should be increased by 25% when 1. 2.
using mechanically operated lifting gear . Otherwise use 10% for manually operated lifting gear
Decking Loading Cases Consideration 80ton crawler with 20ton Lifting •
Case 3: Front Lifting – Boom is perpendicular to crane shoe
•
Shoe A and B carry equal loading. 900
50% of crane load
A
4500
900
B
6300
50% of crane load
83
Decking Loading Cases Consideration •
Case 2: Side Lifting – Boom is perpendicular to crane shoe
•
Shoe A carries 75% of the load while Shoe B carries 25%.
6300
75% of crane load
A
900
4500
25% of crane load
B
900
Secondary Beam
6300 mm
Lsb = 9.5m
• •
Secondary beam is designed for 9.5m span. Metro deck provide lateral restraint for Secondary Beam Ly = 2m
85
Main Beam P1
P2
4500
4500
Lmb
a) Crane shoe at center
P2
P1
Lmb
b) Crane shoe at end
• Main beam is designed for 11.5m span length. • The loading from secondary beam is acting as a point load to the main beam. • Main beam is laterally restrained by secondary beam resting on top. • The lateral buckling length of the main beam is 2m. 86
Vehicle Load on Platform 20m3 concrete truck
For crane moving in a direction parallel to the beam
For crane moving in a direction perpendicular to the beam
CONTRUCTION DECK DESIGN
MAIN BEAM (MB) LOADING CONSIDERATION
MAIN BEAM SUPPORTED BY DECK POST AND TOP FLANGE IS RESTRAINED BY SB
CONTRUCTION DECK DESIGN
TYPICAL SECONDARY BEAM TO MAIN BEAM DETAIL
Connection Details - metro deck
92
Typical Connection of MB to Deck Post/King Post
93
CONTRUCTION DECK DESIGN
TYPICAL SECONDARY BEAM TO MAIN BEAM DETAIL
CONTRUCTION DECK DESIGN
SECONDARY BEAM TO CAPPING BEAM DETAIL
CONTRUCTION DECK DESIGN
SECONDARY BEAM TO CAPPING BEAM DETAIL
Slotted holes to facilitate movement
Homework 4 Q2 Propose an efficient strutting layout plan
All units in m
