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8/12/2019 University Seminar

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MEWAR UNIVERSITY

SEMINAR TOPIC:Seismic Designof Foundations

BY

ZUBER AHMEDEnrolment No. MUR-1101408Roll No. MUM-CV-SE-04

M.Tech.(P/T) Structures (5th Semester)


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Earthquake impacton foundations

InertiaBecause of structural and self weightof foundation soils

Does not influence soil behavior Shear strength degradation

Need to know whether the soil is loose

or soft


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Geotechnical design forearthquakes

Conventional designSand Drained approachClay Undrained approach

Foundation design for earthquakesConservatively use undrained shearstrength for contractive soils and drained

shear strength for dilative ones


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Liquefaction-related foundation failure

Courtesy: Yuminamochi (1999)


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Liquefaction-related foundationfailure

Courtesy: Yuminamochi (1999)


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Courtesy: Stewart and Chu (2002)

Liquefaction- related foundation failure


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Liquefaction- related foundation failure


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State of stress

a. Staticcondition

b. Duringearthquake


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Inertial effect

v 0 h 0 =

(1 - )k v 2 , k h 2

(1 - )k v 1 , k h 1


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MEWAR UNIVERSITY

Strength loss: Liquefaction

v 0 h 0 =


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Strength loss consequences

Triggering ofLiquefactionCyclic softening or cyclic mobility

Foundation failure due toPunchingSupport loss


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Bearing capacity loss due to poundingand inclination

N

N E

S

/

k , h k v

1

Loss due to pounding

Loss due to inclination


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Seismic bearing capacity:No strength loss

For Dense / Stiff soilsSmall or ve residual pore water pressureCan use effective stress strength parameters

Use seismic bearing capacity factors (e.g.,Richards et al. 1993) depending on PHGA andPVGA

Allow a factor of safety of 2


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Seismic bearing capacity factors

N

/ N E

S

c

c

0 0.4 0.80

0.2

0.4

0.6

0.8

1.0

k k h v /(1 - )

40 o30 o20 o10 o

Richards et al. 1993


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Seismic bearing capacity:Example

Inputs = 30 , PHGA = 0.3, PVGA = 0.15

Seismic coefficients k h = 0.5 0.3 = 0.15; k v = 0.5 0.15 = 0.075

Seismic bearing capacity calculations N e/ N s 0.5, N qe / N qs 0.65, N ce / N cs 0.65Allowable seismic bearing capacity = allowable

static bearing capacity 0.6 1.5


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Undrained shear strength

S

u /

v

q c 1 (MP a)0 2 4 6 8

0

0.1

0.2

0.3

0.4

Non-Liquefied

Liquefied


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Bearing capacity loss due tostrength degradation

s u 1

Layer 1: Undrained

shear strength =

s u 2

Layer 2: Undrainedshear strength =

D

T

B

s u 1

s u 2 /

N E c

0

1

2

3

4

5 0 0.2 0.4 0.6 0.8 1.0

1 . 5

1

0 . 5

0 . 2 5

T B /

= 0


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Earthquake-induced settlement: Noliquefaction

Settlement often governs seismicstructural design rather than bearingcapacity

Available centrifuge data indicatesettlement could be ~1% of footingwidth per load cycle

Simple pseudo-static procedures(Richards et al. 1993) is also used


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Earthquake-induced settlement:Liquefied sites

Estimate volumetric strain for factor ofsafety against liquefaction using, e.g. ,Ishihara and Yoshimine (1992)

Calculate Settlement assuming no lateral

movement Design for differential settlement 1/2

to 1/3rd of total settlement


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Summary Not much of reduction in seismic bearing

capacity unless

Site is affected by very strong seismicity or nearsource or on liquefiable ground For bearing capacity estimation use

su for loose or soft soils, and c and otherwise Always check whether structure can tolerate

permanent ground displacements


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MEWAR UNIVERSITY

Compliance springs shallow foundationsMode Stiffness G Reduction

Vertical 4Gr 0 /(1 - ); r

0 =

(B D / )Large EQ: 0.5 to 1.Micro EQ: 0.7 to 1

Sliding 8Gr 0 /(2 - ); r 0 =(B D / )1/2

Rocking 8Gr 30 /(3 - 3 ); r 0 =[B D 3 /(3 )]1/4

Large EQ: 0.5 to 1.Micro EQ: 0.33 to 1

Torsion 16 Gr 30 /3;

[B D (B 2 /+D 2)/(6 )]1/4Large EQ: 0.5 to 1.

Micro EQ: 0.7 to 1


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Earthquake-related pile failure causes

Permanent ground movement Exceedance of moment capacity at pile cap

connection Support Loss

Reduction in lateral confinementGap formationBuckling


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Shallow foundation Permanent ground movement Exceedance of moment capacity at pile cap

connection Support Loss

Reduction in lateral confinementGap formationBuckling


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Pile design for earthquakes Design for lateral load and lateral ground

movement as applicable Use the p y, t z, Q z approach allowing

forStrength loss during earthquake if anyGap formationGroup action


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Simplified pile design fornon-liquefied sites

Apply free field displacements consideringthe pile to be a beam on elastic medium

Iteratively modify the input displacementfield until convergence

Use soil springs obtained following, e.g., APIRP2A, NCHRP Rep 461, PEER 2011/04


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Simplified pile design Modify static soil springs depending on

Soil type, frequency of earthquake load Use p-multipliers depending on pile type, pile

position within a group, whether the load isstatic or cyclic and softening of material

behavior Handle gap formation invoking elastic unload

in the soil spring response


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Simplified pile design for liquefied sites

Non-liquefiable

Liquefiable

Non-liquefiable

Mud line

TypicalPile deformation

Profile

Design lateralload

Plastic hingeformation

Piles

Pile CapSuperstructure

Passive

30% Effective

Vertical Stress

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