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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.