SITE EFFECTS AND ARMENIAN SEISMIC CODE

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1 13 th World Conference on Earthquake Engineering Vancouver, B.C., Canada August 1-6, 2004 Paper No SITE EFFECTS AND ARMENIAN SEISMIC CODE Emma GEVORGYAN 1 SUMMARY Seismic Codes of many countries give attention to ground conditions and change earthquake intensity on unfavorable ground conditions. Seismic Code of the Republic Armenia as the country which is located in a zone of increased seismic risk, also consider examine this question. In this paper calculations for 4 categories of soil accepted in the Seismic Code of RA carried out, and the results confirming validity of accepted soil conditions factors in Seismic Code of RA are received. Calculations have been made by program SHAKE-91. INTRODUCTION It is well known, that the quantitative estimation of the local ground condition effect has a great importance for seismic hazard assessment, engineering seismology and earthquake engineering as well. This fact is reflected in seismic codes of many countries which are located in the seismic risk zones. Soil conditions render essential influence on seismic intensity. However quantitative values reflecting the influence of type of soils on the maximal horizontal peak accelerations on earth surface in Seismic Code of the various countries are a little different. In the given paper the soil influence on site seismicity with use of real horizontal accelerations record (accelerations time series) received in Ghoukassian during 1988 Spitak earthquake is considered. SOIL PROPERTIES ACCORDING TO THE SEISMIC CODE OF THE REPUBLIC OF ARMENIA According to the Seismic Code the RA [1], there are four main categories of soils in Armenia. Each category is defined by shear waves velocity V s and predominating period T o, which values are provided in Table 1. In the given paper for calculations the bedrock is accepted as layered one-dimensional halfspace in which the shear waves are propagating only. For each layer its thickness, shear wave velocity, unit weight of soil and damping factor are known. For calculations the SHAKE-91[2] program was used. 1 Research Associate, Engineering Research Center of the American University of Armenia. Yerevan, Republic Armenia. egevorgy@aua.am

2 Categories of soils Table 1. Ground properties according to the Seismic Code of RA V s, m/s T o, s Soil conditions factor I V s > 800 T o < II 00 < V s < < T o < III 200 < V s < < T o < IV V s < 200 T o > INITIAL ACCELEROGRAM AND CALCULATION STEPS The thickness of the layer, m For the analysis the Ghoukassian acceleration time series of the Spitak Earthquake of December 7, 1988, which was a strongest seismic event on the territory of Armenia in the instrumental period, is used. This accelerogram was recorded not far from the epicenter and the soil conditions of the place of recording are know. On the first step using the Ghoukassian acceleration time series the accelerations for the bedrock were obtained by solving the direct task for incident wave propagation (Fig. 1). Here the Spitak earthquake accelerogram is applied to the upper layer, with peak ground acceleration m/s 2, and its Fourier spectra with predominant frequency f o on outcrop surface and at the bedrock level is shown on Fig. 2. In the Table 2 physic-mechanical and dynamic properties of soils, as well as the received peak acceleration s values at contact levels of layers are given. On the second step (inverse task for reflected wave propagation), using obtained accelerations at the bedrock level with the peak acceleration of 112cm/s 2, acceleration time series along the height of the four various geological sections are calculated. Each section consists of 8 layer. Layers from 2 to 8 taken the same as in the section of the place of initial accelerogram recording, but the upper layer for each of four sections was different and corresponded to the one of the 4 categories of ground. Thus, in the first case the top layer consists of I category ground, has thickness h=30 m, shear wave velocity V s =900 m/s and unit weight ρ=200 kg/m 3. In the second, third and fourth cases accordingly: a II category ground - h=20 m, V s =600 m/s, ρ=2300 kg/m 3 ; a III category ground - h=1 m, V s =400 m/s, ρ=20 kg/m 3 ; a IV category ground - h= m, V s =200 m/s, ρ=2000 kg/m 3. The thickness of the each upper layer in the listed four cases is taken according to the Armenian Seismic Code. On the Fig. 2 the calculated accelerograms received at the outcrop surface level of each ground category are given. In Tab. 3 the calculated peak ground acceleration values at the outcrop surface level for 4 geological sections as well as calculated values of the soil conditions factor K o are given. For comparison the values of K o by Seismic Code of RA, are also given.

3 0.187 m/s m/s m/s m/s m/s m/s m/s m/s m/s m/s m/s m/s m/s m/s m/s m/s Figure 1. Accelerograms obtained at each layer for the incident wave propagation

4 a) b) Hz f o =2.49Hz Hz f o =0.02Hz Figure 2. Fourier spectra of initial (Ghoukassian) accelerogram at the outcrop surface level (a) and of the obtained accelerogram at the bedrock level (b) Table 2. Physic-mechanical and dynamic properties of soils and the received peak accelerations values at contact levels of layers N of layers Name of soils Category of soils according to the Seismic Code of RA II Layer s thickness, m Unit weight, kg/m 3 Peak ground accelerations, cm/s 2 1 Loams with inclusion II of rubble and gravel 2 Andesit basalts II porous 3 Dense clay, poorly IV sandy, waterproof 4 Andesit basalts II porous, strongly cracked Boulders, pebbles of II the cast out breeds with a sandyargillaceous filler 6 Dense andesit basalts, I waterproof 7 Boulder- pebbles I adjournment of the cast out breeds with a clay-sandy filler 8 Andesit basalts I

5 a) I type ground (h=30m) m/s m/s b) II type ground (h=20m) m/sc m/sc 2 c) III type ground (h=1m) m/s m/s d) IV type ground (h=m) m/s m/s Figure 3. The calculated accelerograms at the outcrop surface level of I category ground (a), II category ground (b), III category ground (c) and IV category ground (d) in the case of reflected wave propagation Table 3. Calculated PGA values and values of soil conditions factors Ground categories according to Seismic Code [1]. Peak ground accelerations cm/s 2 Calculated values K 0 Value K 0 according to Seismic Code RA II I II III IV

6 CONCLUSION From Table 3, is obvious that values K 0, received by carried out calculations and given in the Seismic Code, are close. Thus, the values of factor K 0, accepted in Seismic Code of RA, well correspond to the results of calculations that proves their sufficient validity. REFERENCES 1. Earthquake Resistant Construction Design Code Seismic Code of the Republic Armenia II A computer Program for Earthquake Response Analysis of Horizontally Layered Sites. B. Schnabel, Y. Lysmer, H.B. Seed. Report No EERC Dec College of Engineering University of California, Berkeley, California.