Week 9 Challenge
Aim: - To calculate the scour depth and fix span and type of foundation for linear waterway also to calculate the force which is acting on a bridge as per IRC 6. and what is zero shear and definition of liquefaction and its detail.
Given Data:-
Linear waterway = 90 M
Design discharge = 700 cumes
Silt factor = 1
HFL = 3 M (from the ground)
Velocity of water = 1.2 m/sec.
Top road level = 100 M
Ground Level = 93.5 M
Diameter of Pile = 1.2 M
Vertical capacity = 250 tonnes
Horizontal capacity = 35 tonnes
Well Diameter = 8 M
Capacity = 70 tonnes.
Well Diameter = 6 M.
Capacity = 50 tonnes.
Well Diameter = 7 M.
Capacity = 60 tonnes.
Introduction:-
Scouring in Bridge.
Scouring is defined as the lowering or loss of top bed soil (erosion) around the bridge piers, abutments, etc. due to the water flow currents. Scouring is always a potential threat to bridge stability eventually it can lead to foundation failure.
As per the report 60% of bridge failures in the world are because of scouring.Bridge scouring is a dynamic act which depends on various factors like flow depth, flow angles, water currents around the pier or abutments, the shape of pier and abutments, sediments properties etc. Scouring in the bridge is very difficult to monitor and also the foundation failure due to scour is sudden without any warnings.
Scouring Effect
Failure in foundation due to scouring.
Calculation of scouring Depth.
Db = `((1.3 X Q)/L)`
= `((1.3 X 700)/90)`
= 10.11 M
Ksf = 1
Dsm = `1.34 ((Db^2)/(ksF))^(2/3)`
= `1.34(10.11^2/1)^(2/3`
= 6.26 M
Maximum depth of scour for design of foundation,
For Abutment,
= 1.27 X dsm
= 1.27 X 6.26
= 8.41
Depth of foundation below foundation.
= 8.41 - HFL
= 8.41 - 3
= 5.41 M
For Pier,
= 2 X dsm
= 2 X 6.26
= 12.52
Depth of foundation below foundation.
= 12.52 - HFL
= 12.52 - 3
= 9.52 M
Conclusion
The depth of foundation for the Abutment is 5.41 M.
The depth of foundation for the Pier is 9.52 M.
From the above calculation, we have to go pile foundation instead of an open foundation. because depth is too high.
As per the given linear waterway 90 M, we can assume that 6 spans of 15-meter distance.
Calculating the forces as per IRC 6 on the foundation,
General
This design note pertains to the design of abutment . Superstructure consists of Box Girder of 15.00 m c/c of pot cum PTEF bearing. The substructure consists of RCC wall type abutment on pile foundation.
Basic Design Data
Span and Cross section Unit
1 C/c of exp joint 15.00 m
2 C/c of bearing 14.00 m
3 Dist. Of bearing to exp joint 0.50 m
4 Total Deck width 12.50 m
5 Carriage width 8.50 m
6 Footpath width (One Side) 1.50 m
7 Width of Crashbarrier ( Right Side) 1.00 m
8 Width of Crashbarrier ( Left Side) 1.00 m
Superstructure Details
1 No fo girder 1
2 Spacing of girder 0 m
3 Cantilever on both sides 0 m
4 Depth od superstructure 2.700 m
5 Wearing coat thickness 0.065 m
6 Dist. of FRL from bearing of least height 10.9 m
7 Bearing thickness 0.065 m
8 Pedestal thickness 0.25 m
9 Bearing + pedestal thckness 0.315 m
10 Type of superstructure Box Girder type
Level
1 Height of Substructure Assumption 6 m
2 Finish road Level FRL 100.000 m
3 Bearing Level BRL 97.300 m
4 Pier Cap Top level CTL 97.235 m
5 Pier Cap Bottom level CBL 96.235 m
6 Ground Level GL 93.500 m
7 High Flood Level HFL 95.900 m
Material Use
1 Grade of concrete fck M 35
2 Design strength of concrete fcd 15.63
3 Grade of Steel fyk Fe 500
4 Design strength of steel fyd 434.78
5 Density of RC concrete 2.5
6 Density of Wearing coat 2.2
7 Coef. Of thermal expansion of conc. 1.2 E-05/ c
8 Shrinkage strain 2 E-04
9 Modulus of elasticity of Steel Es 200000 Mpa
10 Modulus of elasticity of Concrete Ecm for M35 32000 Mpa
11 Mean axial tensile strength of concrete fctm 2.8 Mpa
Dimension of the abutment
Unit
1 Length of the abutment at top 2.52 m
2 length of the abutment at bottom 1.50 m
3 Width of abutment 12.50 m
4 width of median wall 3.50 m
5 Stem thickness 1.50 m
6 Dritwall thickness 0.30 m
7 Depth of abutment cap varying 0.50 m
8 Depth of abutment cap uniform 0.50 m
9 Return Wall thickness 0.50 m
Soil Parameter
1 Angler of shear resistance 30
2 Density of dry backfill 2 t/m3
3 Density of submerged backfill 1 t/m4
4 Net SBC 25 t/m5
5 Gross SBC (LWL) 33.42 t/m6
6 Gross SBC (HFL) 27 t/m7
7 Live Load surcharge 1.2 m
8 Type of soil median soil
9 Coefficient of friction 0.5
Substructure
Abutment
Length ( Longitudinal ) = 12.5 m
Length ( Transverse ) = 2.52 m
Height = 0.5 m
Volume = 15.75 m3
Weight = 39.375 t
Trapezoidal Part
Top Length = 12.5 m
Bottom Length = 12.5 m
Top width = 2.52 m
Bottom width = 1.5 m
Height = 0.5 m
Volume = 12.5625 m3
Weight = 31.40625 t
Total weight of pier cap = 70.78125 t
Abutment Shaft
Length ( Longitudinal ) = 12.5 m
Length ( Transverse ) = 1.5 m
Height = 6 m
Volume = 112.5 m3
Weight = 281.25 t
For Material
Material Basic Combination Seismic Combination
Concrete 1.5 1.5
Steel 1.15 1.15
Load On superstructure
Dead Load of Superstructure
Total weight of superstructure = 228.210 t
Reaction = 114.105 t
Total reaction on abutment = 114.105 t
Distance betwwen bearing & CL of abutment shaft = 0 m
Longitudinal movement = 0 t-m
Transverser ecentricity of Dl from C/l of abutment = 0 m
Transverser Movement = 0 t-m
SIDL (Excluding wearing course)
Crash barrier weight = 84.375 t
Railing weight = 0 t
Foothpath / service weight = 0 t
Total reaction on abutment = 42.1875 t
SIDL ( Wearing course)
Load intensity due to wearing course = 0.22 t
DL of wearing course = 18.24 t
Total reaction on abutment = 9.12 t
Summary force at footing Level
Load Item P ML MT HL HT
t t-m t-m t-m t-m
Dead Load 114.105 0 0 0 0
SIDL 1 42.185 0 0 0 0
SIDL 2 9.12 0 0 0 0
Live Load
1 Lane 70 R Wheel
Live Load reaction'
Total live load = 92 t
No of lanes = 2 nos
% reduction in live load = 0 % reduction
impact factor - = 1.22 As per IRC 6
Total load with imact = 112.24
Ra = 80.94 t
Rb = 31.29 t
vertical reaction on the abutment = 84.55 t
Breaking force = 16.91 t 20 % of Ra
Total horizontal force at abutment = 8.455 t Fh/2
Breaking Type = POT CUM PTFE
Transverse eccentricity = 3.155 m
Transverse movement = 266.75525 t-m
1 Lane Class A
Live Load reaction'
Total live load = 38.8 t
No of lanes = 2 nos
% reduction in live load = 0 % reduction
impact factor - = 1.22 As per IRC 6
Total load with imact = 47.336
Ra = 25.16 t
Rb = 22.34 t
vertical reaction on the abutment = 28.45 t
Breaking force = 5.69 t 20 % of Ra
Total horizontal force at abutment = 2.845 t Fh/2
Breaking Type = POT CUM PTFE
Transverse eccentricity = 4.5 m
Transverse movement = 128.025 t-m
Summary force at footing Level
Load Item P Load (Min) ML MT HL HT
t t t-m t-m t-m t-m
Class 70 R vehical 84.55 31.29 84.55 266.76 8.445 0
Class A vehical 28.45 22.34 28.45 128.025 2.845 0
Earth Pressure Load and Live Load surcharge
Horizontal force due to earth pressure on abutment acts in L-L direction
Width of abutment (Skew) b = 12 m
Horizontal Earth Pressure Coefficient Kah = 0.279
Horizontal Earth Pressure Coefficient Kav = 0.102
Earth pressure
Normal Case -LWL
Foundation Bottom Level
H = Dirtwall top level - Founding level
= 9.015
Horizontal force in long. Dirn. = 0.5*kah*γ*H2*b
= 272.09
Lever arm from foundation bottom = 0.42*H
= 3.7863
ML = 1030.214367
Abutment portion
Vertical Force = 99.47
Liver arm from toe = 6.5
ML = 646.555
Summary of forces at Foundation bottom level
Normal Case
p HL ML due to HL ML due to P
t t t-m t-m
LWL 99.47 272.09 1030.22 646.55
Live Load Surcharge
Normal Case: Abutment portion
Foundation Bottom Level
H = Dirtwall top level - Founding level
= 9.015
Horizontal force = kah*γ*1.2*H*b
= 72.44
Lever arm = H/2
= 4.5075
ML = 326.5233
Summary of force at foundation bottom level
p HL ML due to HL
t t t-m
LWL 0 72.44 326.5233
Forces on Abutment
Width of abutment b = 12.5 m
Return Wall is on one side =
Horizontal distance of CG of abutment shaft from vertical face = 0.5 m
Dirt Wall Bracket
Weight = 4.05 t
Horizontal Lever Arm from bearing c/l = 0.953 m
Dirt Wall
Weight = 28.59 t
Horizontal Lever Arm from bearing c/l = 0.67 m
Abutment Cap
Weight = 70.78 t
Horizontal Lever Arm from bearing c/l = 0 m
Abutment Shaft
Weight = 281.25 t
Horizontal Lever Arm from bearing c/l = 0 m
Summary of force at foundation bottom level
Description Weight Lever arm ML
t m tm
Abutment shaft 281.25 0.95 268.03
Abutment cap 70.78 0.67 47.42
Drit wall bracket 28.59 0.00 0.00
Drit wall 4.09 0.00 0.00
Total 384.71 315.45
Calculation for Return Wall
Portion a
Weight = 53.9 t
Hor.Lever Arm from toe in L-L dirn. = 5.26 t-m
Portion b
Weight = 3 t
Hor.Lever Arm from toe in L-L dirn. = 2.86 t-m
Portion c
Weight = 0.06 t
Hor.Lever Arm from toe in L-L dirn. = 2.91 t-m
Portion d
Weight = 2.1 t
Hor.Lever Arm from toe in L-L dirn. = 5.29 t-m
Summary Force
Portion Weight Leaver arm ML
t m t-m
a 53.9 5.26 283.514
b 3 2.86 8.58
c 0.06 2.91 0.1746
d 2.1 5.29 11.109
Calculation for Backfill Weight
Width of backfill in transverse direction = 12 m
Portion a
Weight = 991.8 t
Hor.Lever Arm from toe in L-L dirn. = 5.26 t-m
Portion b
Weight = 52.6 t
Hor.Lever Arm from toe in L-L dirn. = 2.86 t-m
Portion c
Weight = 1.1 t
Hor.Lever Arm from toe in L-L dirn. = 2.91 t-m
Portion d
Weight = 38.64 t
Hor.Lever Arm from toe in L-L dirn. = 5.9 t-m
Summary force
Portion Weight Leaver arm ML
t m t-m
a 991.8 5.26 5216.868
b 52.6 2.86 150.436
c 1.1 2.91 3.201
d 38.64 5.9 227.976
Total summary force on foundation
Loads P MAX P MIN ML MT HL
Dead load 114.11 114.11 0 0 0
SIDL 42.19 42.19 0 0 0
SIDL (Wearing Coat) 9.12 9.12 0 0 0
Class 70 R vehical 84.55 31.29 84.55 266.76 8.445
Class A vehical 28.45 22.34 28.45 128.025 2.845
Earth pressure 99.47 99.47 1030.22 0 272.09
Live load surcharge 0 0 326.5300 0 72.44
Weight of Substructure 384.71 384.71 315.45 0 0
Weight of soil above tie 1084.14 1084.14 5598.481 3604 16.93
Total 1846.74 1787.37 7383.68485 3998.785 372.75
For Continous bridge
Longitudinal force clause no 211.1 from IRC 6 we need to consider for the bridge in which free bearing and fix bearing concept is taken for design.
Que) What is zero shear point and if there is more scour at region what will be the effect of that on zero shear point.
The points other than the extreme ends of a beam in a beam at which B.M. is zero are called points of contraflexure or inflexion. The point at which we get zero shear force we get the maximum bending moment of that section/beam at that point.
If Scouring is more, point from where passive earth pressure shall be considered will be different and hence point of zero shear will be lower than normal.
Que) Define the Liquefaction and its effects, how it can affect foundation. Also list the remedial measures for the same. How it is different from scouring?
Liquefaction is a phenomenon in which the strength and stiffness of a soil is reduced by earthquake shaking or other rapid loading. Liquefaction and related phenomena have been responsible for tremendous amounts of damage in historical earthquakes around the world.
Liquefaction occurs in saturated soils, that is, soils in which the space between individual particles is completely filled with water. This water exerts a pressure on the soil particles that influences how tightly the particles themselves are pressed together. Prior to an earthquake, the water pressure is relatively low. However, earthquake shaking can cause the water pressure to increase to the point where the soil particles can readily move with respect to each other.ct
Effects on the foundation.
The soil liquefication has a disastorus effect on bridges and different structures. The structure built on liquefiable soil may face foundation settlement, crack, tilt, and overturning for the open foundation
For the deep foundation like pile foundation piles may buckle due to seismic force in liquefication soil.
Remedial measurement for liquefication soil.
Geological studies identify the landfill sediments, and water table in a seismic region. Soil composed of a mix of small and big grains, can safely endure liquefaction, since the smaller grains fill the pores between the larger grains. Thus, the soil strength is maintained. Furthermore, modern buildings are designed to be liquefaction resistant. In a shallow foundation, the foundation constituents are joined to ensure uniform settlement of the foundation. Thus, the induced shear forces are decreased. The soil characteristics may also be enhanced by improving the soil density, strength, and drainage characteristics. Popular mitigation methods to reduce effects of soil liquefaction are:
Mitigation by deep soil mixing method
Compaction, Permeation, and Jet Grouting
Drain Pile technique
Dynamic compaction and construction of stone columns
How it is different from scouring?
In the scouring surrounding soil of structure (i.e. pier or abutment) is washed away due to the flow of water and create big hole surrounded by structure and in liquefication soil behave like water due to seismic load. it is very different phenomina
