5acee Paper

The 5th Asia Conference on Earthquake Engineering

October 16-18, 2014

THE EFFECT OF GROUND MOTION SELECTION PROCEDURE AND

HAZARD SPECTRA ON DESIGN SEISMIC DEMAND OF NEAR-FAULT

REINFORCED CONCRETE TALL BUILDINGS

Mandeep Pokhrel1, Pennung Warnitchai2 and Teraphan Ornthammarath3

ABSTRACT

Ground motion records and hazard spectra used for non-linear response history analysis are the

important factors influencing the evaluation of seismic demand of tall buildings near active faults. Tall

buildings located near active faults are likely to be subjected to earthquake ground motions containing

high energy pulse, which may impose large seismic demand on the buildings. Design engineers usually

follow the ASCE 7(2005) standard to design such buildings which does not have any special provision

for selecting and scaling ground motions for near-fault buildings. However, Tall Buildings Initiative

(TBI (2010)) has developed a state-of-the-art procedure for selecting and scaling ground motions which

addresses this issue. This study compares seismic demand of a near-fault tall building using these two

different procedures for selecting and scaling ground motions. Two types of hazard spectra (Uniform

hazard spectrum & multiple Conditional mean spectrum) and different sets of ground motions matched

with those spectra have been used. A 40 story reinforced concrete core wall building has been used as

a case study building. Response spectrum analysis (RSA) and non-linear response history analysis

(NLRHA) were carried out to compare elastic and inelastic demand of the building respectively. The

findings of this study suggests that the advance ground motion selection and scaling procedure of

TBI(2010) based on better understanding of near-fault seismic hazard gives less seismic demand for

near-fault tall buildings than the ASCE 7(2005) code based procedure.

Keywords: Pulse-like ground motion, Near-fault tall buildings, Conditional mean spectrum, Uniform

hazard spectrum

INTRODUCTION

Ground motions close to ruptured fault have been found to cause severe damage to the structures located

nearby fault zone (Northridge 1994, Kobe 1995, Chi Chi 1999) than those located far away from such

zone. Near-fault zone typically comprises of the area located within a distance of 20 km from ruptured

fault. Near-fault ground motion contain special type of pulse-like motion in either acceleration, velocity

or displacement time history (Bertero, et al., 1978)(Loh, et al., 2000). The response of structures during

an earthquake depends directly on the level of ground shaking and in case of near-fault structures on the

nature of ground motion time history. Characteristic short duration impulsive nature of near-fault motion

at the beginning of the record imposes high input energy into the buildings causing devastating damage

(Chopra et al., 1976) (Bertero et al., 1978). Near-fault motions (mostly pulse-like) can have devastating

effects to medium and high rise buildings in city like Manila in Asia where the fault-line runs across the

city centre.

The response of near-fault buildings is also dependent upon the spectral shape of ground motion records.

The parameter ε (normalized difference between spectral acceleration of a recorded ground motion with

median predicted spectral acceleration using ground motion prediction equation) which is used in

ground motion prediction equation is an indicator of spectral shape(Baker, 2005).The near-fault records

are usually associated with high positive value of ε(>1). So, the selection of ground motion for the

dynamic analysis of near-fault buildings should not only consider the dominant scenario earthquake

represented by magnitude (Mw) and distance(R) but also records with similar target spectral shape or ε

value. Baker & Cornell (2006) developed a new kind of hazard spectrum known as conditional mean

1 Project Engineer, Asian Institute of Technology, Pathumthani, Thailand, [email protected] 2 Professor, Asian Institute of Technology, Pathumthani, Thailand, [email protected] 3 Lecturer, Mahidol University, Nakhon Pathom, Thailand, [email protected]


October 16-18, 2014

spectrum (CMS) and found that we can alternatively use CMS which is representative of causal M, R,

ε at the site and select ground motions which has similar spectral shape as that of the constructed CMS.

CMS is a spectrum whose spectral values at different periods are conditioned on a target spectral

acceleration value(Sa) at a structural period of interest(T*) (Baker & Cornell,2006) (Baker,2011). The

Sa(T*) corresponds to a causal earthquake(M,R,ε) whose M,R & conditional mean ε value is used to

compute mean spectral values at other periods. General seismic design code like the ASCE 7(2005)

recommends the use of uniform hazard spectrum(UHS), as a target spectrum, which is an envelope

spectrum computed independently for a range of time periods for the same level of probability of

exceedance. UHS does not represent the spectrum from single scenario earthquake (M, R, ε) and it is

impossible for a real ground motion to have such a spectral shape with very high positive ε value at all

the periods. At low probability of exceedance (say 2% in 50 years) ordinates of UHS are always higher

than median predicted response spectrum for dominant earthquake scenario (M,R). Even if a ground

motion has spectral acceleration as high as UHS, it can only be at single period but not at multiple

periods. So, a ground motion having spectral acceleration same as UHS for a range of periods must have

a lower probability of exceedance than that of UHS itself. Thus, the use of UHS as a target spectrum for

any site will be too conservative (Baker & Cornell, 2006).

In this study, focus has been given to selection process of ground motion records using the conventional

code-based approach of the ASCE 7(2005) with UHS as target spectrum. And, another approach used

is recently proposed state of the art recommendation by TBI (2010) with special provision for selection

of ground motion records for near-fault buildings using of multiple site specific CMS.

SEISMIC HAZARD

Overview of Building and Site

The location of the building used for this study is 409 Shaw, Manila, Philippines. The nearest active

fault posing substantial threat to the building is West Marikina Valley Fault(WMVF) located at a

distance of around 400m.This fault is a strike-slip fault with slip rate of 7 to 10 mm/yr. and is a major

threat to most of the tall buildings in Manila. A 40 story tall reinforced concrete building designed

following Los Angeles Tall Building Structural Design Council (LATBSDC) 2005 has been used as a

case study building for this study (Munir & Warnitchai, 2012). The case-study building is 415 feet tall

with a typical story height of 10 feet and 20 feet lobby height. It has three levels of garage parking in

the basement with a story height of 10 feet each whose foundation rests on a firm bedrock.

Hazard Spectra

The site specific uniform hazard spectrum and de-aggregation results required in this study was used

from the Probabilistic Seismic Hazard Assessment(PSHA) report of Manila,Philippines prepared by

AIT Consulting(AIT Consulting,2012).The earthquake event(M,R,ε) controlling MCE level hazard at

409 Shaw site for first three mean translation natural period of the building are shown in Table 1.It can

be seen that the fault controlling MCE level hazard at all three periods is the same and is located very

near to the site (400 m only).There are no distant sources(faults) controlling hazard at any periods at the

site. Thus, the building at the site will be highly affected by ground motions coming from the nearby

fault(some may be pulse-like motions).Two types of hazard spectrum has been used in this study, viz.

uniform hazard spectrum at MCE level and multiple conditional mean spectra.The uniform hazard

spectrum at MCE level corresponds to pseudo-acceleration response spectrum having 5% equivalent

viscous damping with 2% probability of exceedance in 50 years (ASCE 7, 2005).And,CMS has been

used as another hazard spectrum for multiple periods of the case study building following TBI(2010)

recommendation. Inaddition to CMS, Conditional Spectrum(CS) has also been used to account for

variability in spectral response at other periods.As the case study building is a tall MDOF inelastic

structure, it will be sensitive at multiple periods (especially to 1st, 2nd and 3rd mean mode periods) due

to higher mode effect and lengthened effective first mode period. So, multiple CMS spectra and CS were

constructed for selecting ground motion records to perform NLRHA as shown in Fig. 3(a).


October 16-18, 2014

Ground Motion Selection and Scaling

TBI approach using CMS

Based on recent state of the art recommendation by TBI Guidelines Working Group (2010) ground

motion selection and scaling using CMS as target spectrum has been done in this approach. CMS for

first three mean natural translation period of the building were constructed. Ground motions were

selected and linearly scaled using PEER(PEER Ground Motion Database, 2010) web-based tool to

match with target CMS spectra at specific mean natural periods (scaling to single Sa value). As building

site is located less than 5 km from an active fault, records containing pulse like motion were selected

for NLRHA. Proportion of records containing pulse-like motion to be used in analysis depends on

epsilon (>1) and site-to-source distance (<5km) (NEHRP Consultants Joint Venture, 2011). This

proportion was calculated using

Proportion of Pulse Motions = 𝑒0.905−0.188𝑅+1.337𝜀

1+𝑒0.905−0.188𝑅+1.337𝜀 (1)

Where, R is in km. In this study, proportion of pulse-like motion to be used were found out to be 0.91,

0.92, and 0.93 for first, second and third mean natural periods respectively. Thus, all the records selected

contains pulse-like motion. Due to proximity of the building site and active fault, geometric mean

spectral acceleration of fault-normal and fault-parallel rotated ground motion has been used instead of

as-recorded ground motions. The selected ground motion records were linearly scaled to match with the

target spectral acceleration at different time periods. Three sets of scaled ground motions (each

containing two horizontal components & a vertical component) were matched with one target CMS. The

sets were selected such that geometric mean Sa of the horizontal components were within the bounds of

CS. This type of selection considers variability in the spectral acceleration. Three CMS has been used

as target spectrum. Therefore, a total of 9 sets of scaled ground motions were selected for NLRHA as

shown in Figs. 1(a), 1(b) and 2(a).Vertical components of records were scaled using same scaling factor

as that for horizontal components. Ground motion records coming from strike-slip faulting mechanisms

were only selected because WMVF is a strike-slip fault. Velocity-time history of all ground motion sets

were checked to see whether record contained pulse-like motion or not. PEER uses wavelet analysis to

identify records with pulse-like motion before including in its ground motion database. The detail

information of 9 sets of ground motion records selected are given in Table 2, 3 and 4.

ASCE 7 approach using UHS

In the conventional ASCE 7(2005) code-based approach, ground motion selection and scaling for

NLRHA was done using conservative UHS at MCE level. In this case also, PEER web-based tool was

used for selecting ground motion records to match the target UHS spectrum over a period range of

0.2T1min to 1.5T1max to account for higher mode effects and inelastic response of the building. Uniform

weight (equal to 1) was used for scaling over the specified period range. ASCE 7(2005) requirement

was followed and as-recorded seed pair of ground motions were linearly scaled such that the SRSS Sa

response spectrum of each ground motion was not less than 1.17 times target UHS Sa over specified

period range. Total 7 sets of scaled ground motions (each containing two horizontal components) having

least MSE were selected among many ground motion records coming from same controlling earthquake

scenario. Records were selected strictly from strike-slip faulting mechanisms. SRSS response spectral

acceleration of the as-recorded ground motions were computed and scaled linearly by factors less than

8 in the period range of 0.64s - 5.94s to match target 1.17UHS as shown in Fig. 2(b).Records have been

constrained to come from strike-slip mechanisms with magnitude within range of 6.5 Mw - 7.7 Mw,

distance range of 0 km – 15 km and soil site category C. The detail information of 7 seed pair of ground

motion records selected are given in Table 5.

After the selection of records using two separate approaches, geometric mean Sa of as-recorded

horizontal components of 7 sets of records matched to UHS was computed. Similarly, geometric mean

Sa of 3 sets of records matched to three different CMS were computed. The geometric mean Sa of the


October 16-18, 2014

selected records using different hazard spectra are compared in Fig. 3(b) to see which response spectra

has higher hazard level than UHS. It can be clearly seen that the hazard at lower periods is higher for

records matched to UHS rather than records matched to CMS. At higher periods, the hazard due to

records matched to UHS and CMS is similar.

COMPARISON OF SEISMIC DEMAND

Two separate models of the 40 story reinforced core wall building were used for elastic and inelastic

analysis of the building. A three dimensional linear elastic finite element model of the building was

made in ETABS version 9.7.4 software(ETABS (2008)) for RSA. And, a three dimensional non-linear

inelastic model of the structure system made by Munir & Warnitchai (2012) in Perform 3D version 4

software (Perform-3D(2006)) was used for NLRHA. In the non-linear model, bottom portion (level 0 to

level 5) of the core wall is expected to behave as a plastic region and the remaining portion (level 6 to

roof) is expected to remain elastic. So, the bottom portion was modeled using inelastic shear wall (fiber

model) elements and upper portion was modeled using elastic shear wall elements. Coupling beams are

modeled using elastic beam elements with plastic moment hinges at the two ends. Columns and slabs

are modeled as elastic column and shell elements respectively. The geometric non-linearity (P-∆) effects

has been considered. In the non-linear model, damping ratios in first six translation modes has been set

to 1%, 1.4%, 2%, 2.7%, 3.8% and 5.3% using formula ξi = 1.4ξi-1,where ξi is damping ratio of ith mode

(Satake, et al., 2003).In Perform 3D,these damping ratios have been applied using modal damping model.

Additionally, 0.01% of Rayleigh damping has been used to dampen the response of higher modes in the

structure.

Elastic demand of the core walls were computed using response spectrum analysis. UHS at MCE level

was used as one of the response spectrum loading and multiple CMS were used as another type of

loading. Demands were computed in both the directions without any reduction by response modification

factor. Elastic base shear demand obtained from RSA using ASCE 7(2005) approach was found to be

19% and 16.5% more than the base shear demand obtained using TBI (2010) approach along the two

principle directions of the building as shown in Fig. 4. Elastic moment demand from RSA using the

ASCE 7(2005) approach were found to be similar to that computed using TBI (2010) approach in both

the principle directions as shown in Fig. 5.

Inelastic demand of the core walls have been computed by NLRHA using ground motions selected using

two different approaches. Since, the same model has been used, ground motions selected will be the

major factor influencing the seismic demand.The inelastic base shear demand obtained from NLRHA

using ASCE 7(2005) approach was found to be 35% and 23.5% more than the base shear demand

obtained using TBI (2010) approach along the two principle directions of the building as shown in Fig.

6. Since, there is flexural yielding at the bottom portion of the core wall, inelastic story moment demand

along the mid-height of the building was checked. It was found that inelastic story moment obtained

from NLRHA using ASCE 7(2005) approach was found to be nearly equal to that obtained using TBI

(2010) approach in both principle directions of the building as shown in Fig. 7.

CONCLUSIONS

In this study, seismic demand of high rise reinforced concrete building located near an active fault has

been computed using two different approaches for selecting ground motions and hazard spectra. It was

found that base shear demand of the core wall computed using TBI(2010) approach for ground motion

selection and scaling was less than that using ASCE 7(2005) code-based approach. The base shear

demand from CMS at T2 was found to be maximum among the three CMS used. This means that the

spectral demand of near-fault tall buildings at second mode (not first mode as is typically assumed)

governs the elastic/inelastic base shear design demand. One of the drawback of using TBI(2010)

approach is additional amount of computational time and effort required to analyze the building.

However, TBI (2010) approach considers most of the parameters affecting near-fault tall buildings

explicitly unlike the ASCE 7(2005) code based approach which has no special provision for such

buildings. Hence, the results obtained by following ground motion selection procedure of TBI (2010)


October 16-18, 2014

can be used for the design of near-fault tall buildings with more reliability than the ASCE 7(2005) code

based approach used for general type of building.

Table 1. De-aggregation at natural time periods of the case study building from uniform hazard

spectrum at MCE level

T(s) M R (km) ɛ

3.58 7.28 0.40 1.12

0.77 7.26 0.40 1.21

0.34 7.25 0.40 1.40

(a) (b)

Figure 1. Ground motion records linearly scaled to CMS at (a)T1 and (b)T2

(a) (b)

Figure 2. (a) Ground motion records linearly scaled to CMS at T3 (b) Scaled SRSS Sa of selected

ground motion records with average SRSS Sa and UHS at MCE level scaled by 1.17.

Table 2. Rotated ground motion records selected using CMS (T1) as target spectrum

NGA

#

Earthquake

Event Year

Scale

Factor Tp (s) Mw

Rrup

(km)

Vs30

(m/s)

Duration

(s)

1605 Duzce-Turkey 1999 1.7695 -- 5.6 7.14 6.6 276 25.885

1176 Kocaeli-Turkey 1999 1.413 4.5 4.6 7.51 4.8 297 35.000

821 Erzican-Turkey 1992 2.6845 2.7 2.2 6.69 4.4 274.5 20.775

0.01

0.1

1

10

0.01 0.1 1 10

Sp

ectr

al A

ccel

erat

ion

(g

)

Period (s)

1605 1176

821 CMS (3.58s)

GM CS

0.01

0.1

1

10

0.01 0.1 1 10

Sp

ectr

al A

ccel

erat

ion

(g

)

Period (s)

182 170

1602 CMS (0.77S)

GM CS

0.01

0.1

1

10

0.01 0.1 1 10

Spec

tral

Acc

eler

atio

n (

g)

Period (s)

1119

174

1605

CMS (0.34s)

GM

CS

0.01

0.1

1

10

0.01 0.1 1 10

Sp

ectr

al A

ccel

erat

ion

(g

)

Period (s)

1.17UHS

A.M. of SRSS Sa

727

1148

1617

864

879

1111

1787


October 16-18, 2014


NGA

#

Earthquake

Event Year

Scale

Factor Tp (s) Mw

Rrup

(km)

Vs30

(m/s)

Duration

(s)

170 Imperial Valley-06 1979 4.1906 4.5 -- 6.53 7.3 192.1 39.985

1602 Duzce-Turkey 1999 1.6843 -- 0.91 7.14 12 326 55.900

182 Imperial Valley-06 1979 1.7481 4.2 4.5 6.53 0.6 210.5 36.820


NGA

#

Earthquake

Event Year

Scale

Factor Tp (s) Mw

Rrup

(km)

Vs30

(m/s)

Duration

(s)

1119 Kobe-Japan 1995 1.8322 1.4 -- 6.9 0.3 312 40.960

174 Imperial Valley-06 1979 2.2765 7.4 -- 6.53 12.4 196.2 39.035

1605 Duzce-Turkey 1999 1.8316 -- 5.6 7.14 6.6 276 25.885

Table 5. As-recorded Ground Motion Records selected using UHS as target spectrum

NGA

# Earthquake Event Year

Scale

Factor Tp (s) Mw

Rrup

(km)

Vs30

(m/s)

Duration

(s)

879 Landers 1992 3.3614 5.1 -- 7.28 2.2 684.9 48.125

1787 Hector Mine 1999 5 -- -- 7.13 11.7 684.9 45.310

1111 Kobe- Japan 1995 5 -- -- 6.9 7.1 609 40.960

864 Landers 1992 4.4477 -- -- 7.28 11 379.3 44.000

1148 Kocaeli-Turkey 1999 8 -- 6.7 7.51 13.5 523 30.000

727 Superstition Hill-02 1987 8 -- -- 6.54 5.6 362.4 22.200

1617 Duzce-Turkey 1999 7 -- -- 7.14 3.9 424.8 41.500

(a) (b)

Figure 3. (a) Conditional mean spectra at T1, T2 and T3 along with UHS and median spectrum for 409

Shaw site all having 2475 years mean return period (b) Comparison of geometric mean

spectral acceleration response spectrum of ground motions scaled to UHS and CMS (T1, T2,

T3) at MCE level.

0.01

0.1

1

0.01 0.1 1 10

Sp

ectr

al A

ccel

erat

ion

(g

)

Period (s)

UHS

CMS (3.58s)

CMS (0.77s)

CMS (0.34s)

Median Spectrum

0.01

0.1

1

10

0.01 0.1 1 10

Spec

tral

Acc

eler

atio

n (

g)

Period (s)

Target Spectrum

UHS

CMS1

CMS2

CMS3

T1 (3.58s)

T2 (0.77s)

T3 (0.34s)


October 16-18, 2014

(a) (b)

Figure 4. Comparison of elastic story (a) shear demand of core walls using UHS and multiple CMS

along (a) 1-1 axis (b) 2-2 axis

(a) (b)

Figure 5. Comparison of elastic story moment of core walls using UHS and multiple CMS along

(a) 1-1 axis (b) 2-2 axis

(a) (b)

Figure 6. Comparison of inelastic story shear demand by NLRHA using UHS and multiple CMS (a)

along 1-1 axis (b) along 2-2 axis

0

5

10

15

20

25

30

35

40

0 20 40 60

Lev

el N

o.

Shear (Kips x 103)

CMS1

CMS2

CMS3

UHS

0

5

10

15

20

25

30

35

40

0 20 40 60 80

Lev

el N

o.

Shear (Kips x 103)

CMS1

CMS2

CMS3

UHS

0

5

10

15

20

25

30

35

40

0 2 4 6 8 10

Lev

el N

o.

Moment (Kip-in x 107)

CMS1

CMS2

CMS3

UHS

0

5

10

15

20

25

30

35

40

0 2 4 6 8

Lev

el N

o.


CMS1

CMS2

CMS3

UHS

0

5

10

15

20

25

30

35

40

0 10 20 30

Lev

el N

o.

Shear (Kips x 103)

CMS1

CMS2

CMS3

UHS

0

5

10

15

20

25

30

35

40

0 5 10 15 20

Lev

el

No.

Shear (Kips x 103)

CMS1

CMS2

CMS3

UHS


October 16-18, 2014

(a) (b)

Figure 7. Comparison of inelastic story moment demand by NLRHA using UHS and multiple CMS (a)

along 1-1 axis (b) along 2-2 axis.

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NEHRP Consultants Joint Venture. (2011). Selecting and Scaling Earthquake Ground Motions for Performing

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0

5

10

15

20

25

30

35

40

0 0.5 1 1.5 2

Sto

ry (

N)


CMS1

CMS2

CMS3

UHS

0

5

10

15

20

25

30

35

40

0 0.5 1 1.5 2 2.5

Sto

ry (

N)


CMS1

CMS2

CMS3

UHS

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