Post on 18-Jul-2016
description
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Seminar Design Of Cantilever Retaining Walls
With Seismic Analysis
Retaining Wall To hold back the masses of earth or
loose soil where conditions make it impossible to let those masses assume their natural slopes.
To retain earth or such materials to maintain unequal levels on its two faces.
Types of Retaining Wall
Types of Retaining Wall
Parts of Cantilever Retaining Wall
Application Basement
Wing walls and abutments of bridge
Retain slopes in hilly terrain roads
Side walls in bridge approach roads
Lateral Support to embankment
Forces acting on Retaining Wall Lateral earth pressure Self Weight of Retaining Wall Weight of Soil above the base slab Surcharge Soil reaction below base slab Frictional force at the bottom of base
slab
Earth Pressure
Earth Pressure Active and Passive Earth Pressure
Coefficients :1). Rankine Theory:
2). Coulomb Theory:
Earth Pressure Total earth pressure force acting along
the back of the wall is
The total force acts along the back of the wall at a height of H/3 from the base of the wall.
Stability requirements of RW Following conditions must be satisfied
for stability of wall (IS:456-2000). It should not overturn It should not slide It should not subside, i.e Max.
pressure at the toe should not exceed the safe bearing capacity of the soil under working condition
Check against overturningFactor of safety against overturning = MR / MO 1.55 (=1.4/0.9)Where,
MR =Stabilizing moment or restoring moment MO =overturning moment
As per IS:456-2000,0.9 MR 1.4 MO
Check against overturning
Check against overturning
Check against Sliding FOS against sliding =Resisting force to
sliding/Horizontal force causing sliding
= W/Pa 1.55 (=1.4/0.9)
As per IS:456:20001.4 = ( 0.9W)/Pa
Friction W SLIDING OF WALL
Maximum pressure at the toe
T
x1
x2
W1
W2
W3
W4
b/2b/6e
xb
H/3
Pa
W
Hh
Pmax
Pmin.
R
Pressure below the Retaining Wall
Maximum pressure at the toe Let the resultant R due to W and Pa lie at a
distance x from the toe. X = M/W, M = sum of all moments about toe. Eccentricity of the load = e = (b/2-x) b/6
Minimum pressure at heel= >Zero.
For zero pressure, e=b/6, resultant should cut the base within the middle third.
Maximum pressure at toe
= SBC of soil.
be
bW 61Pmin
be
bW 61Pmax
Depth of foundation
Df
Behaviour or structural action
Behaviour or structural action and design of stem, heel and toe slabs are same as that of any cantilever slab.
Design of Cantilever RW
Stem, toe and heel acts as cantilever slabs Stem design: Mu= (ka H3/6) Determine the depth d from Mu = Mu,
lim=Qbd2
Design as balanced section or URS and find steelMu=0.87 fy Ast[d-fyAst/(fckb)]
Design of Heel and Toe
1. Heel slab and toe slab should also be designed as cantilever. For this stability analysis should be performed as explained and determine the maximum bending moments at the junction.
2. Determine the reinforcement. 3. Also check for shear at the junction. 4. Provide enough development length.5. Provide the distribution steel
Design of Stem
Design of Slab
Dynamic Response of Retaining Walls
The dynamic response of even simplest type of retaining wall is quite complex.
Wall movement and pressure depends on the response of the soil underlying the wall, the response of the backfill, the inertial and flexural response of the wall itself, and the nature of the input motions.
Most of the current understanding of the dynamic response of retaining wall has come from the model test and numerical analyses.
Provision of IS 1893:1984 for Calculation of Dynamic Lateral Pressure• As per the provision of IS: 1893:1984 the general
conditions encountered for the design of retaining wall. The active earth pressure exerted against the wall is given by
where,• Pa = active earth pressure• w = unit weight of soil• h = height of wall
Provision of IS 1893:1984 for Calculation of Dynamic Lateral Pressure
• Two values shall be calculated from above equation, one for 1+αv and the other for 1-αv and
• maximum of the two shall be the design values. The values of the notations shall be taken as:
αv= vertical seismic coefficient its direction being taken consistently throughout
the stability analysis of wall and equal to 2/3 αh Ø= angle of internal friction of soil ƛ=αh/(+-αv) α=angle which earth face of the wall makes with the vertical i=slope of earth fill δ=angle of friction between the wall and earth fill αh= horizontal seismic coefficient
Calculation of Horizontal and Vertical Seismic Coefficient
• Since the relevant code dealing with the provision of seismic design of retaining wall is still under revision the data provided in the IS: 1893:2002 Part I is referred for relevant seismic data.
Where,Z= Zone factorI= Importance factorR= Response reduction factorSa/g= average response acceleration coefficient
Problem DefinitionHeight 6 m
SBC 180 kN/m2
18 kN/m3
μ 0.55φ 30δ 20α 90
Zone V
Result Comparison
Retaining Wall4.60
4.80
5.00
5.20
5.40
5.60
5.80
6.00
6.20
6.40
RCC Volume Compar-ision
RWRW+Seismic+CoulombRW+ Seiesmic
RCC
Vol (
m3)
Retaining Wall0
100
200
300
400
500
600
700Reinforcement Comparision
RWRW+Seismic+CoulombRW+Seismic
Stee
l (kg
)
Result Comparison
Reataining Wall0
10000
20000
30000
40000
50000
60000
70000
80000
Cost Comparision
RWRW+Seismic+CoulombRW+Seiesmic
Cost
(Rs
.)
Conclusion Historically, underground facilities have experienced
a lower rate of damage than surface structures. Some underground structures have experienced
significant damage in large earthquakes, that is why its important to do seismic analysis in underground structure.
But, Inclusion of seismic analysis in design results in
increased dimensions & results in costlier structure.
Conclusion Rankine’s design approach is simpler and
gives a more conservative design.
But Coulomb’s design is more practical one since it involves real life scenario – the friction between the wall and the backfill.
The Coulomb’s design approach gives a cost-effective design as compared to Rankine’s design approach.
References Dr.H.J.shah Reinforced Concrete Vol.II By IS 456:2000 Plain And
Reinforced Concrete Code Of Practice By Charotar Publication
Deepankar Choudhury1,*, T. G. Sitharam2 And K. S. Subba Rao2 Seismic Design Of Earth-retaining Structures And Foundations
Shravya Donkada and Devdas Menon Optimal Design Of Reinforced
Concrete Retaining Walls The Indian Concrete Journal April 2012
S.N.Sinha Second Edition Reinforced Concrete Design Mc Grawhill Co. New Delhi.
References N.Krishna Raju & R.N. Pranesh Reinforced Concrete Design By New
Age International Publisher
IS 1893(Part 1):2002 Criteria For Earthquake Resistant Design Of Structures
IS 1893:1984 Criteria for earthquake resistant design of Structures (Fourth Revision)
IS 1893(Part 3) Criteria For Earthquake Resistant Design of Structures (Part 3) Bridges And Retaining Walls