EARTHQUAKES

2.1 INTRODUCTION

2.3.2 N EAR FIELD E ARTHQUAKE S UITE

When aiming for a broad representation of the structural seismic response and its

corresponding demands, a near-field suite is also required in case the location of the considered

structure is close to a major fault line. This suite should also adhere to the selection criteria of

the V&C suite and per the implemented procedure of the far-field suite. Again, adopting the

Shome and Cornell (1999) principle that 20 records should suffice in estimating seismic demands, 20 earthquakes were sought such that their R ≤ 8km to ascertain near-field effects, PGA’s ≥ 0.4g and Sa(T1, 5%) ≥ 0.3g to ensure robust seismic response. Due to the limited availability of near-field

earthquake records in general and California in particular who meet the previously-outlined

criteria, several non-Californian events were included. It is noteworthy to mention that

Northridge 1994 motions were absent from the original V&C suite, as most records are generally

regarded as near-field. Thus, it was included liberally in this near-field suite, in contrast with the

far-field suite where records only with R ≥ 22.8 km were included. Upon implementing the outlined procedure employed to the selection of the far-field suite and the R, PGA and Sa(T1, 5%)

newly set limits, 14 Californian earthquakes were selected plus eight non-Californian ones. All

accelerograms were recorded on firm soils with USGS soil classification ranges between B–C,

A–D for Geomatrix and 1 for CWB (Central Weather Bureau, Taiwan). A list of these 20

Generally, the non-Californian earthquakes are broadly of higher moment magnitude and

peak ground accelerations (PGA) than the Californian ones, as listed in Tables 2-9 and 2-10,

which provide their statistical data. Table 2-11 shows the statistical data the combined suite.

From Tables 2-11, one can envisage that the median (50th percentile) value of the peak

ground acceleration of the combined near-field suite is more than two and a half time (265%)

that of the original V&C suite. Consequently, the comparison of the unscaled spectral

accelerations should be trivial. Indeed the median values of both the Californian and non-

Californian suites are always greater than the maximum bound of the original V&C suite, as

displayed in Figures 2-21 and 2-22. Note also that median values were close or exceeded the 1/T

curve in both figures, as expected for near-field events. Therefore, the confirmation of

earthquake selection was confined to the comparison between the scaled 5% damped spectral

accelerations, where PGA, Sa(T=1s, 5%) and Sa(T=2s, 5%) were scaled to 1g, and plotted amidst

the scaled maximum and minimum bounds of the original V&C suite. These plots for the

Californian earthquakes are provided in Figures 2-23, 2-25 and 2-27. While, the plots of the

non-Californian earthquakes are displayed in Figures 2-24, 2-26 and 2-28.

On inspecting Figures 2-23 and 2-24, a small number of earthquake records of both the

Californian and non-Californian 5% damped spectral accelerations, with PGA scaled to 1g,

exceeded the maximum bound of the scaled original V&C suite. However, one can dismiss such

exceedance on reviewing their high median values in comparison with that of the original suite.

Figures 2-25 to 2-28 confirm the selection of earthquake records as per Table 2-8, where

their 5% scaled spectral accelerations were largely bound between both the scaled maximum and

minimum bounds of the original V&C suite. On inspecting Figure 2-25, three earthquake

records of the Californian 5% damped spectral accelerations, with Sa(T=1s) scaled to 1g,

insignificantly exceeded the maximum bound of the scaled original V&C suite. On the other

hand and from Figure 2-26, only one earthquake record of the non-Californian 5% damped

bound. On further investigation of Figures 2-27 and 2-28, both Californian and non-Californian

5% damped spectral accelerations, with Sa(T=2s) scaled to 1g, were fully confined by the scaled

maximum and minimum bounds of the original V&C suite.

Further studying of the dispersion of both the Californian and non-Californian 5%

damped spectral accelerations, as shown in Figures 2-29 and 2-30; indicate low values that

confirm the selection, as they indicate low randomness (βEQ). From Figure 2-29, shows the

Californian βEQ values to be less than 0.5 for periods of two-second or less, except for Sa(T=2s)

scaled to 1g where values peak to 0.6 g. Figure 2-30 exhibits βEQ values largely under 0.5 for

two-second periods and less, again except for Sa(T=2s) scaled to 1g where βEQ > 0.7 for T ≤ 1.0s.

When recalling Figure 2-16, largely βEQ > 0.5 for T ≤ 2.0s, which highlight quite high

randomness of the original V&C suite in contrast with those lower values of the selected near-

field earthquake suite. Furthermore, Figure 2-31 of the combined near-field suite exhibits βEQ

values that are less fluctuating and of reduced randomness. The βEQ values for the combined

suite have an overall average value βEQ ≈ 0.4 for T ≤ 2.0s, with βEQ < 0.5, except for Sa(T=2s)

scaled to 1g where βEQ < 0.7 for T ≤ 0.5s. Moreover, Figure 2-31 shows an overall smaller

randomness and lower variability than that depicted in Figure 2-16 of the original suite.

Median (50th percentile) and median+βEQ (84th percentile) values of the combined near-

field suite plus the original V&C suite are plotted in Figure 2-32, which shows the marked

difference in magnitude and reduced randomness. Figure 2-32 exhibits clearly the lower

randomness of the combined near-field suite, when inspecting the two curves of median+βEQ.

The values of βEQ for the produced near-field suite were always less than those of the

original V&C suite. Figure 2-33 illustrates the reduction in randomness in seismic input

demands for values of T. For 0 ≥ T ≥ 3.0s, values of βEQ of the near-field and V&C suites were

0.388 < 0.742, 0.439 < 0.695, 0.449 < 0.845, 0.392 < 0.685, 0.408 < 0.704 (˜ 42% drop) and

0.486 < 0.848 at PGA, T = 0.6, 1.0, 1.5, 2.0 (IM of considered structures) and 3.0 seconds

assure that the suite captures a viable range of credible near-field earthquake scenarios and

consequently provide credible results albeit the targeted high acceleration magnitudes of the

suite’s records.

In document Behaviour of three dimensional concrete structures under concurrent orthogonal seismic excitations (Page 31-34)