seismic design basics - superstructure

Seismic Design Basics – Superstructure

Dr. Ajit Khanse, Ph. D.

Approved by The Practicing Institute of Engineering, Inc., NY State Updated August 2015

Presentation Title

CONTENTS

Basic

Seismic Design

Standards

Seismic Analysis

Procedures

Plate Tectonics

Seismicity

2

Seismic Design

Category

Presentation Title 3

Seismicity refers to the geographic and historical distribution of earthquakes. The dots represent the epicenters of significant earthquakes. It is apparent that the locations of the great majority of earthquakes correspond to the boundaries between plates.

WORLD SEISMICITY: 1900 – 2013

Presentation Title

PLATE TECTONICS

4

Pattern of earthquakes defines the boundaries of tectonic plates. 23 major plates. [USGS]

Presentation Title

RING OF FIRE

5

Volcanic arcs and oceanic trenches partly encircling the Pacific Basin form the so-called Ring of Fire, a zone of frequent earthquakes and volcanic eruptions. The Challenger Deep is the deepest known point in the oceans, with a depth of 35,994 ft (6.82 miles).

[USGS]

Presentation Title

Earthquakes in the Midwestern and Eastern United States?!

6 Intraplate Earthquakes

1) 8/23/2011 – Earthquake in Washington, D. C., Mw = 5.8

2) 8/23/2011 – Earthquake in Colorado, Mw = 5.3

3) 1980 – 5 earthquakes recorded N. of Philadelphia, PA

4) 1979 & 1980 – New York State and the adjacent areas experienced 131 earthquakes of magnitude 1 to 5

5) 1931 – Valentine, Texas, magnitude 6.4 earthquake.

6) 1884 – New York City area

7) 1886, Charleston, South Carolina. Estimated magnitude 6.8. Soil liquefaction.

8) 1811 & 1812 -- New Madrid, Missouri (7.2 ≤ M ≤ 8.3). Soil liquefaction.

[www.geo.mtu.edu]

Presentation Title

INTRAPLATE EARTHQUAKE

7

• An intraplate earthquake is an earthquake that occurs in the interior of a tectonic plate, whereas an interplate earthquake is one that occurs at a plate boundary.

• Intraplate earthquakes are not well understood. the causative fault is deeply buried, and sometimes cannot even be found.

• Examples, the 1811-1812 earthquakes in New Madrid, Missouri; Charleston, South Carolina (1886) and Gujarat, India (2001).

Intraplate Earthquakes

[Wikipedia]

Presentation Title

RESERVOIR – INDUCED SEISMICITY – I

8 Lalwani, Hydrodynamics, 2009

CAUSE & EFFECT: Rapid filling rates, The large annual fluctuations of lake levels, Filling at a later time above the previous

highest water level Elevated ‘rate of change of pore pressure’

(dp/dt) values over a filling cycle, Result in diffusion of pore pressures from the

reservoirs to hypocentral locations along a saturated, critically stressed network of faults and fractures.

Presentation Title

RESERVOIR – INDUCED SEISMICITY – II

9

Monticello Dam, South Carolina

Oroville Earthen Dam, CA. Tallest in US at 770 ft. M = 6.1 in 1975

Zipingpu Dam, the 2008 Wenchuan/Sichuan earthquake, Mw=7.9

The 1967 Koyna, India earthquake, Mw = 6.3

Lake

Ele

vatio

n (m

eter

)

Mw = 6.3

Capacity = 100 BCF

Lalwani, Hydrodynamics, 2009

Presentation Title

Humans Induced Seismicity

Reservoir

Mining

Hot water extraction

Waste water extraction

Oil or gas extraction: Carthage Gas Fields, E. Texas

Enhanced Geothermal System (EGS):

Geysers geothermal field in California

Nuclear tests

10

Presentation Title

BASIN EFFECT – I

11

Epicenter was 220 miles away from

Mexico City. Estimated 35,000 people

died in Mexico City, where 412

multistory (8 to 25 floors) buildings

collapsed completely and another

3,124 were seriously damaged. (USGS)

21-story, steel-frame building 15-story reinforced concrete building

8-story RC building The 1985 Michoacan (Mexico) Earthquake, Mw= 8.3

Presentation Title

Source-averaged basin amplification is period-dependent,

with the highest amplifications occurring for the longest

periods and greatest basin depths.

Relative to the very-hard rock reference structure, general

maximum amplification is about a factor of 8. At Mexico

city (1985 Michoacan earthquake), seismic motion was

amplified up to a factor of 60 compared to the bedrock.

[Aviles and Perez-Rocha 1998]

12

BASIN EFFECT – II

The 1985 Michoacan (Mexico) Earthquake, Mw= 8.3

Presentation Title

BASIN EFFECT – III

13

[Aviles and Perez-Rocha 1998] Map of seismic zonation and isoperiod curves (in sec) of Mexico City

Zone Depth

(ft)

Eff. Vs

(ft/s)

Predominant

period (s)

Transition 43 285 0.6 Lake 125 250 2.0

Deep Lake 185 200 3.4

Characteristics of the soil profiles

• Extremely soft, saturated surface clays • At some places Plasticity Index ≈ 300 • Friction angles as low as, ϕ = 5-15o

BASIN EFFECT

The 1985 Michoacan (Mexico) Earthquake, Mw= 8.3

Presentation Title

SAN ANDREAS FAULT, CA

14

[USGS]

The San Andreas (strike-slip) fault zone separates the Pacific and North American Plates, which are slowly grinding past each other in a roughly north-south direction.

The Pacific Plate (western side of the fault) is moving horizontally in a northerly direction relative to the North American Plate (eastern side of the fault)

Presentation Title

A FEW RELEVANT TERMS

• Liquefaction

• Plasticity Index, PI

• Shear Wave Velocity, VS

•Period of Vibration, Tn

• Seismic Design Category (SDC)

• Importance Factor I and Occupancy Category (OC)

• Response Modification Factor, R

15


Niigata, Japan 1964 Alaska 1964: Surface rupture

Geotechnical Failures

[USGS]

Soil liquefaction describes a phenomenon whereby a saturated soil substantially loses strength and stiffness in response to an applied stress, usually earthquake shaking or other sudden change in stress condition, causing it to behave like a liquid.

ANIMATION SLIDE

Presentation Title

CONSEQUENCES OF LIQUEFACTION

17

Liquefaction can lead to damage or failure of structures: (1) Loss in bearing support which causes large vertical

downward movement. (2) Imposition of horizontal forces on the footing from

lateral flow or lateral spreading of the soil. (3) Settlement of the soil as pore water pressures in the

liquefied layers dissipate. C10.5.4.1, AASHTO LRFD Bridge Design 2010

Geotechnical Failures

Presentation Title

ATTERBERG LIMITS → PLASTICITY INDEX

18

The plasticity Index indicates the range of moisture content at which the soil is in the plastic state. Plasticity Index, PI = LL – PL where LL = Liquid Limit and PL = Plastic Limit

Liquid State: Deforms easily; consistency of pea soup to soft butter

Plastic State: Deforms without cracking; consistency of soft butter to stiff putty

Semisolid State: Deforms permanently, but cracks: consistency of cheese

Solid State: Breaks before it will deform; consistency of hard candy

Liquid Limit (LL) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Plastic Limit (PL) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Shrinkage Limit (SL) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Incr

easi

ng M

oist

ure

Con

tent

s, w

ASTM D4318 and D2216

Presentation Title

SHEAR WAVE VELOCITY, VS

19

S-wave (secondary wave, Shear Wave, transverse wave)

velocity is given by: , μ = the shear modulus and ρ = the density of the material.

SV

[Stokoe, et al 2003]

• S-waves cannot travel through a fluid, since a fluid cannot support shear. • Magnitude of shear wave velocity is used in soil classification.

Presentation Title

KOBE, JAPAN 1995, Mw= 6.9 LOMA PRIETA 1989, Mw=6.93

20

‘There is not a fiercer hell than the failure in a great object’ – Keats

Presentation Title

DESIGN STANDARDS

(1) ASCE/SEI 7-10 Minimum Design Loads for Buildings and Other Structures

(2) AASHTO LRFD Bridge Design Specifications, 7th Edition, 2014

(3) AASHTO LRFD Seismic Bridge Design, 1st Edition, 2012

(4) AISC 341-10 Seismic Provisions for Structural Steel Buildings, 2010

(5) International Building Code 2014

(6) https://geohazards.usgs.gov/secure/designmaps/us

(7) CALTRANS Seismic Design Criteria, Version 1.7, April 2013

(8) FEMA P-750 (2009) NEHRP Recommended Seismic Provisions

(9) FEMA P-695 (June 2009) Quantification of Building Seismic Performance

Factors

21

https://geohazards.usgs.gov/secure/designmaps/us



Presentation Title

DESIGN BASICS • Soil Type and Profile

• Seismic Design Category (SDC)

• Determination of Natural Period of Vibration, Tn

• MCER and Design Response Spectra

• Importance Factor I and Occupancy Category (OC)

• Seismic Performance Factors: R, Ω0 & Cd

•Analysis Selection Procedure

• Equivalent Lateral Force (ELF) Procedure

• Conclusions

22


Site Class SOIL TYPE AND PROFILE

A Hard rock with measured shear wave velocity, Vs > 5000 ft/sec B Rock with 2500 ft/sec < Vs < 5000 ft/sec

C Very dense soil & soil rock with 1200 ft/s < Vs < 2500 ft/s, or with either N > 50 blows/ft or Su > 2.0 ksf

D Stiff soil with 600 ft/sec < Vs < 1200 ft/sec, or with either 15 blows/ft < N < 50 blows/ft or 1 ksf < Su < 2 ksf

E Soil profile with Vs < 600 ft/sec, or with either N < 15 blows/ft or Su < 1.0 ksf, or any profile with more than 10 ft of soft clay defined as soil with PI > 20, w > 40%, and Su < 0.5 ksf.

F

Soils requiring site-specific ground motion response evaluation: • Peats or highly organic clays (H > 10 ft of peat or highly

organic clay, where H = thickness of soil) • Very high plasticity clays (H > 25 ft with PI > 75) • Very thick soft/medium stiff clays (H > 120 ft)

Vs = shear wave velocity, Su = undrained shear strength, PI = plasticity index, w = moisture content, N = std penetration test (SPT) blow count

[Table 3.4.2.1-1 of AASHTO LRFD Seismic Bridge Design, 2012]

Presentation Title

SEISMIC DESIGN CATEGORY (SDC)

24

[C11.6, ASCE 7-10]

There are various correlations of the qualitative Modified Mercalli Intensity (MMI) with quantitative characterizations of ground-shaking limits for the various SDCs.

MMI V No real damage SDC A 0< SM1 <0.1g MMI VI Light nonstructural damage SDC B 0.1g<SM1<0.2g MMI VII Hazardous nonstructural damage SDC C 0.2g<SM1<0.3g MMI VIII Hazardous damage to susceptible structures SDC D 0.3<SM1<1.12g MMI IX Hazardous damage to robust structures SDC E SM1 > 1.125g

Presentation Title


25

• SDCs perform one of the functions of the Seismic Zones used in earlier U.S. building codes.

• To step progressively from simple, easy and minimums to more sophisticated, detailed, and costly requirements as both the level of seismic hazard and the consequence of failure escalate.

• SDCs also are dependent on a building’s Occupancy Category and, therefore, its desired performance.

• To simplify building regulation by assigning the same SDC regardless of the structural type.

[C11.6, ASCE 7-10]

Presentation Title


26

[C11.6, ASCE 7-10]

• The ground motions used to define the SDCs include the effects of individual site conditions on probable ground-shaking intensity.

• Structures are assigned to a SDC based on the more severe condition determined from SM1, 1-second acceleration and SMS, short-period acceleration.

• For SDC E, SM1 > 1.125g. This generally occurs in

near-fault area, i.e., less than 15 miles from fault-rupture.

• Most SDC F occur over liquefiable soil (Soil type F).

Presentation Title

MCER 1-second spectral response acceleration parameter, SM1 (%g)

27

Map with associated regions of Seismic Design Category, assuming Site Class D conditions. [FEMA P-750 (2009) NEHRP Recommended Seismic Provisions.]

Presentation Title

MCER 1-second spectral response acceleration parameter, SM1 (%g)

28

Map with associated regions of Seismic Design Category, assuming Site Class D conditions for California sites. [FEMA P-750 (2009) NEHRP]

Presentation Title

NATURAL PERIOD OF VIBRATION, Tn

29

2 1 2 2 stn

n n

mTf k g

The time required for the undamped system to complete one cycle of free vibration is the natural (fundamental) period of vibration of the system.

where, ωn = natural circular frequency of vibration, in radians fn = natural cyclic frequency of vibration, in Hz m = mass of system, in kip-sec2/ft (slug) = W/g k = stiffness of system, kip/ft W = weight, in kip g = gravitational acceleration = 32.2 ft/sec2 = 386 in/sec2

= lateral static displacement of mass due to lateral force mg. 𝛿𝑠𝑡

Presentation Title

PERIOD DETERMINATION

30

The fundamental period of vibration of the structure T, is used to determine the design base shear as well as the exponent k that establishes the distribution of the shear along the height of the structure. (1) Ta = 0.1N for structures not exceeding 12 stories in height. (Eq. 12.8.8)

(2) (sec) (Eq. 12.8-7) where hn is the height in ft and the coefficients (0.016 < Ct < 0.03) and (0.75 < x < 0.9) are determined from Table 12.8-2.

(3) For masonry or concrete shear wall structures, (Eq. 12.8.9)

(4) For a single column bent, (AASHTO 2014, Eq. A-4, A-3, A-5)

where and (ft)

(5) Period of bridge may be determined from Sec. 5.4.2 of AASHTO 2014

[Sec. 12.8 of ASCE 7-10 & AASHTO 2014]

taxnT C h

0.0019

wa nT h

C

122

s COLUMN

r

W WT

gK

rFK

2

2 0.854 aT S

Presentation Title

FOUR WAYS OF ACCESSING RESPONSE SPECTRA

1) Response Spectra provided by a geotechnical

engineering consultant.

2) Construction of Response Spectrum from different

ground motion acceleration time histories.

3) Construction of MCER and Design Response Spectra

as per Sec. 11.4, ASCE 7-10.

4) Accessing Response Spectra from

http://earthquake.usgs.gov/designmaps/us/application.php

31 Response Spectrum

Presentation Title

RESPONSE SPECTRUM – I

32

[2007] This Response Spectrum is used for North Ramp at St. George Ferry Terminal

Response Spectrum – 1

Presentation Title

Linearly-elastic single degree-of-freedom system

33

Single degree-of-freedom (SDF) system, ζ = 5%

Elasto-perfectly plastic system

Linearly-elastic system Seismic Base Shear, V

V = CS.W

k W


2nWTgk

Presentation Title

SEISMIC GROUND MOTIONS OF ChiChi-TCU65 1999 record

34

Time (sec)

Acce

lera

tion

(g)

Velo

city

(in/

sec2

) D

ispl

acem

ent (

in)

a (max) = 0.82g

v (max) = 51 in/sec2

s (max) = 37 in

Three components (x, y & z dir) of accelerations are obtained from accelerometer. Δt = 0.005 s

← In

tegr

atio

n ←

Inte

grat

ion

This is N123E component

SDC - E: Hazardous damage to robust

structures


Presentation Title

EQUATION OF MOTION

35

22 ( )n n gu u u u t

The equation of motion of a linear single degree-of-freedom

system subjected to seismic ground acceleration : 𝑢 𝑔(𝑡

= acceleration of system, in/sec2

= velocity of system, in/sec u = displacement of system, in ζ = damping ratio

2n

nT

Tn = natural period of vibration, sec Δt = 0.005 sec (typ)


uu

Presentation Title

SPECTRAL RESPONSE of SDF SYSTEM to ChiChi-TCU65 record

36

Period (sec)

Pseu

do-a

ccel

erat

ion

(in/s

ec2 )

Sp

ectra

l Dis

plac

emen

t (in

)

22

n

A DT

Tn=1.1 sec

Tn=4.0 sec

Tn=2.1 sec

1.1

2.1

4.0

Time (sec)

Dis

plac

emen

t (in

)

Linearly-elastic single degree-of-freedom system, ζ = 5%

u (max)= 20”

u (max)= 35”

u (max)= 70”

u = 20”

u = 35”

u = 70”


Presentation Title

PSa Response Spectra, 20 ground motions to 84%

37

Period (sec)

Pseu

do-a

ccel

erat

ion

(in/s

ec2 )

1.3g

1.3g

84th percentile curve

MCE curve

22

n

A DT

sof mA

Pseudo-acceleration,

Equiv. Static Force,

SDC – E

g = 386 in/sec2

Hazardous damage to robust

structures


Presentation Title

MCER AND DESIGN RESPONSE SPECTRUM – III

38

[Sec C11.2, ASCE 7-10]

MCER = Risk-targeted Maximum Considered Earthquake Ground Motion.

Design Response Spectrum shall be determined by dividing ordinates of

MCER response spectrum by 1.5.


The MCER ground motions are based on the 2008

USGS seismic hazard maps and also incorporate

three technical changes to previous ASCE/SEI 7-05:

1) Use of risk-targeted ground motions,

2) Use of maximum direction ground motions, and

3) Use of near-source 84th percentile ground motions.

Presentation Title

MCER AND DESIGN RESPONSE SPECTRUM – III

39

for TS ≤ T ≤ TL (Eq. 11.4-10)

for T > TL (Eq. 11.4-11)

SDS = ⅔.Fa.CRS.SSUH or ⅔.Fa.SSD, lesser. (Eq. 11.4-1, 11.4-2, Table 11.4-1)

00

0.4 0.6 ( .11.4 9) DSaTS for T T EqST

SD1 = ⅔.Fv.CR1.S1UH or ⅔.Fv.S1D, lesser. (Eq. 11.4-3, 11.4-4, Table 11.4-2)

[Sec. 11.4, ASCE 7-10] Response Spectrum – 3

Presentation Title

RESPONSE SPECTRUM – IV

40 Response Spectrum – 4

http://earthquake.usgs.gov/designmaps/us/application.php

Presentation Title

IMPORTANCE FACTOR & OCCUPANCY CATEGORY

41

The Occupancy Category (OC) is used as one of two components in determining the Seismic Design Category (SDC) and is a primary factor in setting drift limits. In the quantitative criteria for strength, the Importance Factor I is shown as a divisor on the Response Modification Factor R in order to send a message to designers that the objective is to reduce damage for important structures in addition to preventing collapse in larger ground motions. [C11-5, ASCE 7-10]

Presentation Title

SEISMIC PERFORMANCE FACTORS, SPFs

42

• Values of the Response Modification Factor R, the system Overstrength Factor, Ω0, and the Deflection Amplification Factor, Cd, for currently approved seismic-force-resisting systems are specified in Table 12.2-1 of ASCE 7-10.

• R factors are also given in Sec. 3.10.7 of AASHTO LRFD Bridge Design 2014.

[Fig. 1.1, FEMA P-695, 2009]

Presentation Title

ANALYSIS SELECTION PROCEDURE (ASCE 7-10)

43

SDC Structural Characteristics ELF Sec. 12.8

MRS Sec. 12.9

RHA Ch. 16

B, C All structures P P P D, F Reg structures < 160 ft height P P P

Reg structures with T < 3.5 TS P P P Structures with a few irregularity P P P All other structures NP P P

E All structures NA P P

• Nonlinear static (pushover) analysis is not addressed in the standard. • The value of TS (= SD1/SDS) depends on the site class because SDS and SD1 include such effects. • Refer Table C12.6-1 for values of 3.5TS for various cities & site classes. • MRS = Modal Response Spectrum Analysis. RHA = Response History Analysis

Table 12.6-1

Presentation Title

ANALYSIS SELECTION PROCEDURE (AASHT0 2014)

44

SDC Regular Bridges with 2 thro’ 6 Spans

Not Regular Bridges with 2 or More Spans

A Not required Not required B, C, or D Use ESA or EDA Use EDA

• ESA = Equivalent Static Analysis, Sec. 5.4.2 (SDF model) • EDA = Elastic Dynamic Analysis, Sec. 5.4.2 • Non-Linear Time History (NLTH) Analysis (Sec. 5.4.4) is not

required, unless P-Δ effects are large, damping provided by base-isolation system is large or requested by Owner. NLTH should be used for Critical or Essential bridges in SDC D, E and F category.

• Inelastic Static (pushover) Analysis (ISA) is used to establish displacement capacities for normal bridges in SDC D.

Table 4.2-1

Presentation Title

EQUIV. LATERAL FORCE (ELF) PROCEDURE – I

45

This procedure is useful in preliminary design of all structures and is allowed for final design of the vast majority of structures. Three basic steps: 1) Determine the seismic base shear, V =CS.W (Sec. 12.8.1) 2) Distribute the shear vertically along the height of the structure. (Sec. 12.8.3) 3) Distribute the shear horizontally across the width and breadth of the structure. (Sec. 12.8.4)

[Sec. 12.8, ASCE 7-10]

Presentation Title

EQUIV. LATERAL FORCE (ELF) PROCEDURE – II

46

[Sec. 12.8, ASCE 7-10]

SDS is obtained from Eq. 11.4-7, 11.4-5 & Table 11.4-1 SD1 is obtained from Eq. 11.4-8, 11.4-6 & Table 11.4-2 TS varies from 0.2 sec to 0.9 sec. (Table C12.6.1) TL varies from 4 sec to 16 sec. (Fig. 22-15 through 22-20)

DSS

SCRI

1 DS L

SC for T TRTI

1

2 D L

S LS TC for T T

RTI

Seismic Base Shear: V = CS.W

Presentation Title

OVERVIEW TO FEW DESIGN THEMES 1) Seismic Force Resisting Systems

2) P-Δ Effects

3) Vertical Ground Motions for Design

4) Design Requirements for Bridges in SDC B 5) Seismic Analysis Procedures – Inelastic

6) Seismic Analysis Procedures

7) Equivalent Lateral Force (ELF) Procedure

8) Modal Response Spectrum Analysis (MRS)

9) Response History Analysis (RHA) Procedures

10)Base Isolated Structures

11)Structures with Damping Systems

12)Soil-Structure Interaction (SSI)

13)Fluid-Structure Interaction (FSI)

47

Presentation Title

SEISMIC FORCE RESISTING SYSTEMS (SFRS) A. Bearing wall systems

B. Building frame systems

C. Moment-resisting frame systems

D. Dual systems with special moment frames capable of resisting at least 25% of seismic forces

E. Dual systems with intermediate moment frames capable of resisting at least 25% of seismic forces

F. Shear wall frame interactive system with ordinary RC moment frames and ordinary RC shear walls

G. Cantilevered column systems

H. Steel systems not specifically detailed for seismic resistance, excluding cantilever column systems

48

For bridges, SFRS are given in Sec. 3.3 of AASHTO LRFD Seismic Bridge Design, 2009

REF

: Tab

le 1

2.2-

1, A

SCE

7-10

Presentation Title

P-Δ EFFECT ON COLUMNS

49

Fig. 4.2 of CALTRANS SDC 2010

P-Δ effects adversely influence both the stiffness and strength of structures. Figures show idealized static force-displacement responses for a simple, one-story structure (such as a cantilevered column).

Fig. C12.8-7, ASCE 7-10

Presentation Title

BRIDGE COLUMN DETAILING

50 Fig. 2.4 of CALTRANS 2010

Plastic hinge forms below ground in the shaft

Plastic hinge forms at or above the shaft/column interface, thereby, containing the majority of inelastic action to ductile column element

Presentation Title

VERTICAL GROUND MOTIONS for Seismic Design

51

Required where a more explicit consideration of vertical ground motion effects is advised: • Certain tanks, • Material storage facilities, bins, silos, etc. • Electric power generation facilities, etc.

New method for construction of design vertical response spectrum is proposed.

[Chapter 15 & 23, ASCE 7-10]

Presentation Title

VERTICAL GROUND MOTIONS for Seismic Design

52

[New Chapter 23, ASCE 7-10]

Design Vertical Response Spectrum

Values of Vertical Coefficient CV are obtained from Table 23.1-1 SDS = the design spectral response acceleration parameter at short periods TV = the vertical period of vibration

Presentation Title

DESIGN REQUIREMENTS FOR BRIDGES IN SDC B

1) Identification of Earthquake Resisting Systems (ERS), A-3.3

2) Demand Analysis

3) Implicit capacity check required (displacement, P-Δ, support length)

4) Capacity design should be considered for column shear; capacity checks should be considered to avoid weak links in ERS

5) SDC B level of detailing

6) Liquefaction check should be considered for certain conditions

53

D/C ≤ 1 Capacity Design

SDC B detailing

Liquefaction SDC B Identify ERS

Demand Analysis

Implicit Capacity

[AASHTO LRFD Seismic Bridge Design, 2009]

Presentation Title

INELASTIC SEISMIC ANALYSIS PROCEDURES

54

Matrix depicting possible inelastic seismic analysis procedures for various structural models & ground motion characterizations along with trends of uncertainty in the result

[FEMA 440, 2005]

Presentation Title

SEISMIC ANALYSIS PROCEDURES

1) Equivalent Lateral Force (ELF) Analysis or

Nonlinear Static Procedure (NSP)

2) Modal Response Spectrum (MRS) Analysis or

Elastic Dynamic Analysis (EDA)

3) Linear Response History (LRH) Analysis

4) Nonlinear Response History (NRH) Analysis

55

[ASCE 7-10; AASHTO LRFD Seismic Bridge Design, 2009]

Presentation Title

EQUIV. LATERAL FORCE (ELF) ANALYSIS

56

This procedure is useful in preliminary design of all structures and is allowed for final design of the vast majority of structures. Three basic steps: 1) Determine the seismic base shear, V =CS.W (Sec. 12.8.1) 2) Distribute the shear vertically along the height of the structure. (Sec. 12.8.3) 3) Distribute the shear horizontally across the width and breadth of the structure. (Sec. 12.8.4)

[Sec 12.8, ASCE 7-10; Sec 5.4.2, AASHTO LRFD Seismic Bridge Design, 2009]

Presentation Title

MODAL RESPONSE SPECTRUM (MRS) ANALYSIS – I

57

[Sec. 12.9, ASCE 7-10] Fig. 10.1.2 & 10.1.3, Anil K. Chopra 2012

1) Structure is decomposed into a number of single-degree-of-freedom (SDF) systems, each having its own mode shape and natural period of vibration.

2) Natural period of vibration of an multiple-degree-of-freedom (MDF) system is the time required for one cycle of the simple harmonic motion in one of these natural modes. Natural periods & corresponding modes of structure are computed.

System in first natural mode of vibration

System in second natural mode of vibration

Presentation Title

MODAL RESPONSE SPECTRUM (MRS) ANALYSIS – II

58

Fig. 13.2.5 & 13.2.3, Dr. A.K. Chopra 2012

Effective Modal Masses and Modal Heights

Conversion from multiple-degree-of-freedom system to

equivalent single-degree-of-freedom system

Period

Presentation Title

MODAL RESPONSE SPECTRUM (MRS) ANALYSIS – III

59

1) Displacement in each mode is determined from

corresponding spectral acceleration (obtained from Response Spectrum), modal participation & mode shape.

2) Where at least 90% of the model mass participates in the response, the distribution of forces and displacements is sufficient for design.

3) Each mode will have different peak responses. The resultant response is calculated by Modal Combination Rules like SRSS or CQC method.


Elastic Dynamic Analysis (EDA) is required for “irregular bridges” in SDC B, C & D.


Main Characteristics:

Excitation by suitable earthquake acceleration time histories

Finite Element multiple-degree of freedom model is created

Model may be two- or three-dimensional

Computer with suitable software is required

The responses derived from the Linear Response History

Analysis are multiplied by I to provide enhanced strength

and stiffness for more important facilities, and are divided by

R to account for inelastic behavior. [Sec 16.1, ASCE 7-10]

RESPONSE HISTORY (LRH) ANALYSIS – LINEAR


Procedure is NOT required unless:

P-Δ effects are too large to be neglected

Damping provided by base isolation system is large

Requested by Owner per Article 4.2.2

For complex systems with friction-based passive

energy dissipation devices, nonlinear viscous

dampers, seismically isolated systems, self-centering

systems, or systems that have components with highly

irregular force-deformation relationships

RESPONSE HISTORY (NRH) ANALYSIS – NONLINEAR


Presentation Title

Seismically Base Isolated Structures – I

62

Figure C17.5-2 Isolation system terminology

(a) Fixed base structure, (b) Isolated Structure

Generally used for short-period structures. Since period of structure is lengthened, the displacements may increase.

Fig. 20.2.1 Dr. A.K. Chopra 2012

Presentation Title

Seismically Base Isolated Structures – II

63

The Benicia-Martinez Bridge in the San Francisco Bay Area is 6,156 feet long with 10 steel truss spans supported by concrete piers. The Friction Pendulum Isolation bearings were installed at the tops of the concrete piers, under the roadway trusses. Each seismic isolation bearing measures 13 ft in diameter and weighs 40,000 lb. Each has a lateral displacement capacity of 53 in, a 5000 kip design (dead plus live) load, and a 5 second period.

www.earthquakeprotection.com/

http://www.earthquakeprotection.com/

Presentation Title

SOIL STRUCTURE INTERACTION (SSI) – I

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[C19.1, ASCE 7-10]

The response of a structure to earthquake shaking is affected by interactions between three linked systems: 1) the structure, 2) the foundation, and 3) the geologic media underlying and surrounding the foundation. SSI effects reflect the differences between the actual response of the structure and the response for the theoretical, rigid base condition.

[Fig. from H. Allison Smith & Wen-Hwa Wu, 1997]

Presentation Title

SOIL STRUCTURE INTERACTION (SSI) – II

65

[FEMA 440, 2005]

Rigid Base Model

Flexible base, Kinematic Interaction & Foundation Damping MODEL

Three primary categories of soil-structure interaction (SSI) effects: 1) Introduction of flexibility to the

soil-foundation system (flexible foundation effects),

2) Filtering of the character of ground shaking transmitted to the structure (kinematic effects)

3) Dissipation of energy from the soil-structure system through radiation and hysteretic soil damping (foundation damping effects).

Presentation Title

SOIL STRUCTURE INTERACTION (SSI) – III

66

[Fig. C19-1, ASCE 7-10]

Effects of period lengthening and foundation damping on design spectral accelerations

Period lengthening causes higher displacements. Inertial interaction effects are important for stiff

structural systems on Site Classes C to F

Presentation Title

SEISMIC FLUID STRUCTURE INTERACTION (FSI)

67

• Off-shore structures

• Sea Breakwater walls

• Shore Retaining Walls

• Reservoir and Dams

• Large Fluid Containers

Presentation Title

OFFSHORE STRUCTURES

68

[email protected]

Presentation Title

CONCLUSIONS – I

•Determination of soil type and Seismic Design Category (SDC) at construction location.

• Determination of natural period of vibration, Tn of simple systems.

• Construction and application of MCER &/or Design Response Spectra

(soil & SDC specific).

• Determination of owner-specific Importance Factor I and Occupancy Category (OC).

• Determination of Response Modification Factor R for assigned seismic-force-resisting-system from Design Standards.

• Application of Equivalent Lateral Force (ELF) Procedure to calculate seismic base shear and bending moments.

69

During Presentation, we have reviewed the following:

Presentation Title

CONCLUSIONS – II

1) Determine soil type & SDC

2)

For fixed bent,

3) Find R and I

4) From Fig. C12.8.1 of ASCE 7-10

or slide # 50, find Seismic

Response Coefficient, CS.

5) Seismic Base Shear: V = CS.W

70

2nT Wgk

3

3 ck EIL

Presentation Title

CONCLUSIONS - III

71

This is just the beginning.

It’s a long way to seismic design expertise!

Complexities involved in the analysis of MDF system are tangentially demonstrated.

Distinction between linearly-elastic and inelastic analyses is explained.

Comparison between different inelastic seismic analyses procedures is made with pros and cons.

Methodology behind simple seismically base-isolated structures is explained.

Existence of more complex themes like Seismic Soil-Structure Interaction & Fluid-Structure Interaction is shown.

Presentation Title

THANK YOU

72

“Earthquake effects on structures systematically bring out

the mistakes made in design and construction, even the

minutest mistakes” – Newmark and Rosenblueth

Q?

Presentation Title

FEW NOTATIONS AND DEFINITIONS

73

[Sec. 11.2, 11.3, ASCE 7-10]

MCER = Risk-targeted Maximum Considered Earthquake Ground Motion. Design response spectrum shall be determined by dividing ordinates of MCER response spectrum by 1.5. CR = risk coefficient; see Section 21.2.1.1 CRS = mapped value of the risk coefficient at short periods as defined by Figure 22-3 CR1 = mapped value of the risk coefficient at a period of 1 second as defined by Figure 22-4 SSD = mapped deterministic, 5 percent damped, spectral response acceleration parameter at short periods as defined in Section 11.4.1 SSUH = mapped uniform-hazard, 5 percent damped, spectral response acceleration parameter at short periods as defined in Section 11.4.1 S1D = mapped deterministic, 5 percent damped, spectral response acceleration parameter at a period of 1 second as defined in Section 11.4.1 S1UH = mapped uniform-hazard, 5 percent damped, spectral response acceleration parameter at a period of 1 second as defined in Section 11.4.1

SS = 5 percent damped, spectral response acceleration parameter at short periods as defined in Sec. 11.4.3 S1 = spectral response acceleration parameter at a period of 1 second as defined in Section 11.4.3 SaM = the site-specific MCER spectral response acceleration at any period SMS = the MCER, 5 percent damped, spectral response acceleration parameter at short periods adjusted for target risk and site-class effects as defined in Section 11.4.3 SM1 = the MCER, 5 percent damped, spectral response acceleration parameter at a period of 1 second adjusted for target risk and site-class effects as defined in Section 11.4.3

seismic design basics - superstructure

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