Best Mech--Vertical Ground Motions and Its Effect On

Kathmandu, Nepal International Seminar on

November 29-30, 2009 Hazard Management for Sustainable Development

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VERTICAL GROUND MOTIONS AND ITS EFFECT ON

ENGINEERING STRUCTURES: A STATE-OF-THE-ART REVIEW

Bipin SHRESTHA

ABSTRACT

During the recent earthquakes, the vertical component of the ground motion found to be

exceeding the horizontal component, which directly contradicts the current codal

provision that assumes the value of the vertical ground motion to be 1/2 to 2/3 of the

horizontal component. Almost after every destructive earthquake engineers postulates that

the structural damage such as elephant foot buckling of large column, fracture of large

diameter reinforced concrete column supporting building and freeway structures was due

to strong vertical ground motion. Therefore, seismic design of the structure without the

consideration of the vertical ground motion component may result in unquantifiable risk

from the collapse, especially those constructed in the close proximity of the fault. However

there seems to be no consensus as to the importance on damage due to vertical motions,

and little that has been learned from the recent earthquake in Loma-Prieta, Northridge, or

Kobe which indicates conclusively that damage to structures was predominantly by

vertical motions. This paper presents the assemblage of the state-of-the-art study on the

vertical ground motion and its effects on the engineering structures.

Keywords: vertical ground motion, Spectral ratio, Fourier spectra, vertical response

period, shear response

1. INTRODUCTION

It is a well Known fact that the civil engineering structures are subjected to the three

dimensional earthquake ground motions. But it is only the horizontal motion which has

been extensively studied and considered in the design Process whereas the vertical

component of the ground motion has generally been neglected in design and hardly

studied from hazard point of view. Also most of the Prevailing building codes including

NBC 105, IS 1893, UBC 97 and many other codes worldwide assume the vertical

component of the ground motion to be ½ to ! of the horizontal component. However, in

recent destructive earthquakes such as the 1989 Loma Prieta, 1994 Northridge, 1995 Kobe

and 1999 Chi-Chi, it was found that vertical ground motion may equal or even

signiÞcantly exceed the local horizontal ground motion. In such situations, most existing

code speciÞcations must be considered un-conservative.



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In recent years many authors has highlighted this fact and done significant researches to

identify and quantify the damaging potential of the vertical component of ground motion.

Many studies reported data showing that the vertical peak acceleration may be even higher

than the horizontal value. Others have attributed the observed failure on the Reinforced

concrete structures to the reduction of shear strength caused by vertical ground motion

effects. As recently shown by Kunnath et al. (2008), vertical motion may magnify and

potentially create reversal of bending moment in longitudinal bridge girders. Widespread

phenomenon of bearing failure and deck unseating, as observed during the recent

earthquakes, was partially attributed to the destructive impact of vertical motions.

However effects on vertical acceleration on response of the long span cable stayed bridge

and its steel tower was found to be slight (Shrestha, 2009; Abdel raheem, Hayashikawa

and Aly, 2002). Based on a large body of available studies, it is possible to conclude that

vertical shaking may escalate the axial column force, cause an increase in the moment and

shear demand, and amplify plastic deformation, extend plastic hinge formation and finally

diminish the ductility capacity of structural component. In order to include the vertical

motion effects in design, recent efforts have considered the development of vertical ground

motion spectra by focusing mostly on near-fault accelerograms (e.g.;Elnashai and

Papazoglou, 1997; Bozorgnia and Campbell, 2004; Kalkan and Gülkan, 2004). These

studies have developed vertical ground motion spectra and concentrated on its parallel use

with the horizontal ground motion spectra. In some existing building codes, Eurocode 8

for example, much more attention is given to the uncertainties of the spectrum ratio in the

near-fault zone.

2. VERTICAL COMPONENT OF GROUND MOTION AND V/H RATIO

A common perception among the Professional engineer is that the vertical component of

the ground motion is lower than the horizontal component, thereby V/H ratio (ratio of

vertical to horizontal peak ground acceleration) is assumed to remain less than the unity.

Many codes suggest scaling of a single spectral shape, originally derived for the horizontal

component using an average V/H ratio of 2/3 as originally proposed by Newmark et al.

(1973). As a result, all components of motion have the same frequency content in almost

all design codes. The frequency content, however, is demonstrably different (figure 2).

Also, the 2/3 rule for V/H is unconservative in the near-field and over-conservative at

large epicentral distances. Table- shows some of the landmark earthquakes with significant

V/H ratio. The V/H ratio was confirmed to be > 1.0 within a 5 km radius of earthquake

source, > 2/3 within 25 km radius and dependent on earthquake magnitude from studies by

Collier and Elnashai(2001).



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Event Station(Mw) Hor1(g) Hor2(g) Ver(g) V/H

Gazli, Uzbeksitan 1976 Karakyr(6.8) 0.71 0.63 1.34 1.89

Imperial valley, USA 1979 El cenro array 6 (6.5) 0.41 0.44 1.66 3.77

Nahhani, Canada 1985 Site1(6.8) 0.98 1.10 2.09 1.90

Morgan hill, USA 1984 Gilroy array#7(6.2) 0.11 0.19 0.43 2.25

Loma-prieta, USA 1989 LGPC(6.9) 0.56 0.61 0.89 1.47

Northridge, USA 1994 Arleta fire station(6.7) 0.34 0.31 0.55 1.61

Kobe, Japan 1995 Port Island (6.9) 0.31 0.28 0.56 1.79

Chi Chi, Taiwan 1999 TCU 076 (6.3) 0.11 0.12 0.26 2.07

Table 1 Ground motion database

3. FREQUENCY CONTENT

The vertical component of the ground motion is associated with vertically propagating the

P-waves, whereas the horizontal components are is more associated with S-waves. The

wave-length of P- waves are shorter than the S-wave, i.e. frequency content of the vertical

component of the ground motion is higher than the horizontal component. The figure 1

show horizontal and vertical component of the most frequently quoted ground motion of

1940 el-centro earthquake. Figure shows Fourier spectra, acceleration response spectrum

and Arias intensity curve. This figure confirms the higher frequency content of the vertical

component of the ground motion thus result in higher ratio of vertical to horizontal spectral

acceleration at the short period range. Although the content over the frequency range of the

vertical ground motion is lower than that of the horizontal component, it has tendency to

concentrate all its energy in narrow high frequency band. Therefore such high frequency

content leads to largest response in short period range, which often coincides with the

vertical period of the RC structure, thus causing significant response amplification.



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Fig 1 Ground motion timehistory of El-centro 1940 vertical(0.21g) and

horizontal(0.32g) respectively from top

Fig 2 Comparison of Fourier spectra and Arias intensity of vertical and

horizontal ground motion

Time [sec]

20191817161514131211109876543210

Accele

ratio

n [g]

0.3

0.2

0.1

0

-0.1

-0.2

-0.3

Time [sec]

20191817161514131211109876543210

Accele

ratio

n [g]

0.3

0.2

0.1

0

-0.1

-0.2

-0.3



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Fig 3 Response spectrum and comparison of spectral ratio at short

period for El-centro with code value

4. TIME LAG BETWEEN PEAK VERTICAL AND PEAK HORIZONTAL MOTION

One of the important features of the ground motion is the relationship between the arrival

times of peak vertical motion with the peak horizontal motion. In general peak vertical

ground motion occurs earlier than peak horizontal motion as shown in figure 1 (Peak

vertical acceleration occurs 1 sec earlier than the peak horizontal ground motion), whereas

in other cases the near coincidence occurs in the time domain. In case of peak vertical

motion occurring significantly before the peak horizontal motion, then it may be valid to

design the structure separately for the effects of vertical and horizontal ground motion but

when these two components are nearly coinciding then the consideration of combined

effect in the design in necessary. Elnashai and Collier (2001) investigated the time interval

by using records from Imperial Valley (1979) and Morgan Hill (1984) earthquakes. They

considered 32 records at various distance with similar site conditions. The study concluded

that the time interval increases with distance from source and should be taken as zero for a

distance of 5 km from the source. However the wide variety of ground motion exhibit

diverse result thus concluding the local site effect, travel path and source depth as other

significant contributors to the arrival time of peak of two components.



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Fig.4. Minimum time interval between peak vertical and horizontal accelerations plotted as

a function of magnitude and distance for the 1979 imperial valley earthquake (Mw = 6.5)

and the 1984 Morgan Hill earthquake ( Mw = 6.3)

5. VERTICAL RESPONSE PERIOD

It is quite clear from the figure 3 that the vertical to horizontal spectral ratio significantly exceeds

the code recommended one at initial period (in the case of El-centro ground motion it was for

0.05- 0.15 seconds) and codal provision is too conservative for the later periods. Then the

question in the mind will be, does this higher spectral ratio at initial period of the response spectra

will have any effect on structure? Yes, it does. The period of the RC building does lies within this

effective range (0.05sec-0.15sec) according to finding from Papadopoulou (1989).

Number of

floors

Horizontal

period (s)

Vertical

period (s)

1 0.1 0.040

2 0.2 0.0643 0.3 0.0824 0.4 0.0915 0.5 0.0996 0.6 0.1067 0.7 0.1148 0.8 0.120

Table 2: Fundamental natural period of RC building (Papadopoulou)



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His finding suggest that the natural period of the RC structure lies within the constant

amplification range for vertical strong-motion records which was found to lie between periods of

0.05 s and 0.15 s . This with the confirmed severity of near-field vertical strong-motion in terms

of peak ground acceleration suggests that large dynamic axial forces, acting both upwards and

downwards, should be expected in the near-field. Kim and Elanshai (2008) performed the

parametric study on RC bridges (2spans and single pier) with different geometric configuration.

Fundamental period of the bridge was calculated are listed on table 2, which again clearly

indicates for most of the cases the fundamental period of vibration is near about 0.15sec.

Figure 5. Layout of Simple bridge(Kim and Elnashai)

Table 3: Geometric configuration and fundamental period of the bridges (Kim and Elnashai)



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It is stressed again that the most worrying aspect of vertical response is the

correspondence of building frame and short span bridge periods with the

predominant periods of vertical strong-motion records. This correspondence of

fundamental period of the structure with the Predominant periods of vertical strong

ground motion leads to the significant amplification of forces particularly on vertical

load carrying members.

7. EFFECT ON BUILDING STRUCTURES

The major effect of the vertical ground motion on the building is to increase the axial

demand on the vertical load carrying member. It is observed that the axial force caused by

the vertical motion, having comparable amplitude to the horizontal motion, are larger than

the corresponding transverse loading only in most of the cases. This pattern is significant

for upper floor rather than for lower floors. Nonlinear dynamic analysis of an 8-storey, 3-

bay moment resisting RC frame designed according to UBC, has lead to the confirmation

of the occurrence of net tensile forces and displacements, thus dispelling the question of

high frequency excitation often used to support the insignificance of the vertical

component(Koukleri). The results for the Imperial Valley Centro-6 motion are shown in

Table 4.

!

Table 4 Effect of vertical motion on column compressive forces for a RC frame (Koulkeri)

It is a well known fact that the shear capacity of the column depends upon the axial

demand. An increase in the axial force demand in the column such as the one

imposed by the vertical components with significant amplitude results in an increase

in the shear capacity of the column. This is beneficial to the seismic behavior of the



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column. However, a decrease in the axial force demand on the column results in a

decrease in the shear capacity of the column. Vertical ground motion can put a

column into tension for short durations of time, thus reducing the column’s shear

capacity to just the shear strength of the transverse reinforcement. This may lead to

the failure of the structure.

Figure 6 Effect of vertical motion on shear response of RC columns. Shear capacity Vs

demand time history of critical 7th storey column of dual frame at attainment of 3 percent

inter-storey drift ot ground motion of 1971 San Fernando (Georgantzis)



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Similar results were also observed for steel frame buildings during study undertaken

by Broderick et al. following the report of damaged steel building during Northridge

earthquake. Although vertical ground motion didn’t significantly influenced the

transverse response of the building, significant increase in column rotational ductility

demand was found and attributed to occurrence of lower yield point. Beams of the

steel frames were found to be more affected by the vertical motion. Problem was

more concentrated particularly at the connections where the amplitude of the

obtained vibrations was such as to imply the imposition of a large number of cycles

close to and exceeding yield. More than 100 cycles were counted in certain cases,

enough for low-cycle fatigue to become significant and cyclic deterioration to occur

in typical connection elements. Such response only takes place if the vertical

component is included in the analysis.

8. EFFECT ON BRIDGE STRUCTURE

One of the early studies on the effect of vertical component of the ground motion

was carried out by saadeghvaziri and Foutch (1991), whom reported the variation

on the axial force due to the vertical motion, reduced the energy dissipation

capacity of the bridge column and also affected the shear capacity of the column.

This finding was further supported by the Papazoglou and Elanshai (1996) on their

paper which included both the field evidence and the damaging effect of the

vertical motion on the building and highway structure as well. Recently, Kunnath

et al. (2008a) examined a two-span highway bridge with double-column bent

considering six different structural configurations. They found that the vertical

component of ground motion causes significant amplification in the axial force

demand in the columns and moment demands in the girder at both the mid-span and

at the face of the bent cap. The increase in girder moment due to vertical motion

caused the demand to exceed the capacity, hence failure would be expected.



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Fig 7 Time history response of selected parameter under

horizontal and combined horizontal and vertical ground motion (Kunnath, Abrahamson,

Chai, Erduran and Yilmaz 2008)



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9. CONCLUDING REMARK

This paper is meant to disseminate the state-of-the-art works on the importance of vertical

ground motion to the Nepali audience. The author particularly likes to raise these concerns

at moment when National building code of the Nepal is in the verge of revision. From the

collection of works worldwide, it is concluded that neglecting vertical component of the

ground motion may lead to serious underestimation of the demand, over-estimation of the

capacity and thus jeopardize overall structural safety.

At this sensitive period of transition the author from his capacity, as a keen student of the

vertical motion and its effects on the structure likes to make few recommendations to the

reviewer of the codes. The present codal provision on the vertical components of ground

motion is not conservative and often this codal provision itself in not implemented during

the design and analysis of earthquake resistant structure. Hence the author highly

recommends the reviewer of the code to go through Eurocode-8, which has most

satisfactory dealt the vertical spectrum among the prevailing seismic codes. Also the

author would like to highlight to fact that present design synthesis could lead to the

catastrophic consequences. It is highly recommended that the sites located within 20 Km

from the major active fault should be designed to the combined effect of horizontal and

vertical ground motion.

REFERENCES

Kim, S.J. and Elnashai, A.S., (2008). “Seismic assessment of RC structure considering

Vertical Ground Motion”, MAE centre report no 08-03, Mid American earthquake center

Papazoglou, A.J. and Elnashai, A.S., (1996). “Analytical and Field Evidence of the

Damaging Effect of Vertical earthquake Ground Motion”, Earthquake Engineering and

Structural Dynamics, Vol. 25, 1109-1137.

Kunnath, S.K., Abrahamson, N., Chai, Y.H., Erduran, E., and Yilmaz, Z., (2008).

“Development of Guidelines for Incorporation of Vertical Ground Motion Effects in

Seismic Design of Highway Bridges”, Technical report CA/UCD-SESM-08-01,

University of California at Davis.

O. Papadopoulou,(1996) “The effect of vertical excitation on reinforced concrete multi-

storey structures”, M.Sc. Dissertation, Imperial College, August 1989 from “Analytical

and Field Evidence of the Damaging Effect of Vertical earthquake Ground Motion”,

Earthquake Engineering and Structural Dynamics, Vol. 25, 1109-1137.

M. Georgantzis, (1996) “Effect of vertical motion on behaviour factors”, M.Sc

Dissertation, Imperial College, August 1995. from “Analytical and Field Evidence of the

Damaging Effect of Vertical earthquake Ground Motion”, Earthquake Engineering and

Structural Dynamics, Vol. 25, 1109-1137.



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S. N. Koukleri,(1996)“The effect of vertical ground excitation on the response of RC

structures”, M.Sc. Dissertation, Imperial College, August 1992. from “Analytical and

Field Evidence of the Damaging Effect of Vertical earthquake Ground Motion”,

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B. Shrestha, (2009)”Effects of near field vertical acceleration on seismic response of the

long span cable stayed bridge”, Msc, Dissertation, Pulchok campus IOE, Tribhuwan

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