Vertical Ground Motions And Its Effect On Engineering Structures.doc

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. After almost every destructive earthquake some engineers postulates that the structural damage 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 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 ⅔ 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 significantly exceed the local horizontal ground motion. In such situations, most existing code specifications must be considered unconservative. 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. Similar findings on the eroded shear

capacity of columns due to vertical excitation influences were also highlighted. 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; 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.

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 overconservative at large epicentral distances. Table1. 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).

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 Event

Station(Mw)

Hor1(g)

Hor2(g)

Ver(g)

V/H

Gazli, Uzbeksitan 1976 Imperial valley, USA 1979 Nahhani, Canada 1985 Morgan hill, USA 1984 Loma-prieta, USA 1989 Northridge, USA 1994 Kobe, Japan 1995 Chi Chi, Taiwan 1999

Karakyr(6.8)

0.71

0.63

1.34

1.89

El cenro array 6 (6.5)

0.41

0.44

1.66

3.77

Site1(6.8)

0.98

1.10

2.09

1.90

Gilroy array#7(6.2)

0.11

0.19

0.43

2.25

LGPC(6.9)

0.56

0.61

0.89

1.47

Arleta fire station(6.7)

0.34

0.31

0.55

1.61

Port Island (6.9)

0.31

0.28

0.56

1.79

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 P-waves, whereas the horizontal components are is more associated with S-waves. The wave-length of Pwaves 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 2 and 3 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.

Acceleration [g]

0.3 0.2 0.1 0 -0.1 -0.2 -0.3 0

1

2

3

4

5

6

7

8

9

10 11 Time [sec]

12

13

14

15

16

17

18

19

20

0

1

2

3

4

5

6

7

8

9

10 11 Time [sec]

12

13

14

15

16

17

18

19

20

Acceleration [g]

0.3 0.2 0.1 0 -0.1 -0.2 -0.3

1.

a. Ground motion timehistory of El-centro 1940 vertical(0.21g) and horizontal(0.32g) respectively from top

1. b. Comparison of Fourier spectra and Arias intensity of vertical and horizontal ground motion

1. c. 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

4.

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.

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)

0.1 1 0.040 0.2 2 0.064 0.3 3 0.082 0..4 4 0.091 05 5 0.099 6 0.6 0.106 0.7 7 0.114 0.8 8 0.120 Table 2: Fundamental natural period of RC building (Papadopoulou)

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 3, which again clearly indicates for most of the cases the fundamental period of vibration is near about 0.15sec.

Figure 3. Layout of Simple bridge(Kim and Elnashai)

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

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.

5.

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 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 to ground motion of 1971 San Fernando (Georgantzis)

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

6.

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.

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. (2008) 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.

Figure 7 Time history response of selected parameter under horizontal and combined horizontal and vertical ground motion (Kunnath, Abrahamson, Chai, Erduran and Yilmaz 2008)

7.

Concluding Remarks

This paper is meant to disseminate the state-ofthe-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 is 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 satisfactorily dealt the vertical spectrum, among the prevailing seismic codes. Also the author would like to

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highlight to the 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, 11091137. 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. 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”, Earthquake Engineering and Structural Dynamics, Vol. 25, 11091137. Collier, C.J. and Elnashai, A.S., (2001). “A Procedure for Combining Vertical and Horizontal Seismic Action Effects”, Journal of Earthquake Engineering, Vol. 5 (4), 521-539. Eurocode 8 (1994). “Design Provisions for Earthquake Resistance of Structures - Part 5: Foundations, Retaining Structures and Geotechnical Aspects”, ENV 1998-5, CEN European Committee for Standardisation, Brussels Elnashai, A.S. and Papazoglou, A.J., (1997).“Procedure and Spectra for Analysis of RC Structures Subjected to

Strong Vertical Earthquake Loads”, Journal of Earthquake Engineering, Vol. 1 (1), 121-156. 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 university. Saadeghvaziri, M A; Foutch, D A (1991); “Dynamic behavior of R/C highway bridges under the combined effect of vertical and horizontal earthquake motions”, Earthquake engrg. and strut, dyn. Vol20,535-549,1991 Raheem, S A; Hayashikawa, T; Aly, G A (2001); “Effect of vertical ground motion on seismic response of steel tower of cable-stayed bridge”, proceedings of Hokkaido chapter of Japan society of civil engineers, JSCE, No 58(A), pp.112-115 Kalkan ,E; Graizer,V (2007). “Multi component ground motion response spectra for coupled horizontal, vertical, angular acceleration and tilt”, ISET Journal of Earthquake Technology, Paper No. 485, Vol. 44, No. 1, March 2007, pp. 259–284 Pamuk, A., Kalkan, E. and Ling, H.I. (2005). “Structural and Geotechnical Impacts of Surface Rupture on Highway Structures during Recent Earthquakes in Turkey”, Soil Dynamics and Earthquake Engineering, Vol. 25, No. 7-10, pp. 581–589. Kalkan, E. and Gülkan, P. (2004b). “Empirical attenuation Equations for Vertical Ground Motion in Turkey”, Earthquake Spectra, Vol. 20, No. 3, pp. 853– 882. Newmark, N.M., Blume, J.A. and Kapur, K.K. (1973). “Seismic Design Spectra for Nuclear Power Plants”, Journal of the Power Division, Proceedings of ASCE, Vol. 99, No. PO2, pp. 287–303.