Earthquake Hazard Analysis and Dynamic Site Response Evaluation for Design of Tower Buildings in Dubai, UAE

Prof. Azm Al-Homoud, American University of Sharjah

Abstract:

The United Arab Emirates is not considered to be an area of high earthquake risk, but the Northern Emirates are adjacent to an area where moderate risk exists. The Oman Mountains to the east, together with the Zagros Mountains of south-western Iran, form a large mobile belt, with most of the recorded seismic events being concentrated on the Iranian side of the Gulf, with some tremors in the sea north of Dubai and around the Straits of Hormuz.

This study is intended to provide design engineers with Peak Ground Acceleration to be used in the design of a 60 storey tower in Dubai, UAE.
The hazard zoning assessment is based on statistical and average behavior of the regions considered, which are based mainly on historical and instrumental data available on seismic events on the studied regions and takes into consideration geologic, tectonic setting and seismotectonics of the studied area of UAE and surrounding.  

 First, a site-specific study accounting for the regional seismicity and geology of UAE and surroundings; the expected recurrence rates and maximum magnitudes of events on known faults and source zones in and surrounding UAE; location of the site with respect to these, and the characteristics of the subsurface site conditions was conducted. Probabilistic seismic hazard analysis at 5% critical damping was conducted considering seismicity data of near by Iran and the Arabian Gulf, tectonics of the region and a modified attenuation relation for Zagros region (the main sources of earthquakes risk in UAE)  ( Zare, 2000).  The used attenuation relation resulted from calibration of constants to reflect the region characteristics (i.e. soil type, transmission path, source, etc.). An updated catalogue, containing both historical and instrumental events, is used in the study. Seismic source regions are modeled and relationships between earthquake magnitude and earthquake frequency is established.

The output of the seismic hazard analysis was the anticipated peak ground acceleration in with a 10% probability of being exceeded during time spans of 50, 100 and 200. These Peak Ground Acceleration values are at the bedrock level without incorporating the soil condition effect at the site (i.e. amplification/de-amplification effect). The results of the seismic hazard assessment for the project site, gave bedrock Horizontal Peak Ground Acceleration PGA values of 158   (0.158 g) for 50 years time span, 193   (0.193 g) for 100 years time span, and 268   (0.268 g) for 200 years time span.  According to design codes, a 50 years life time of buildings is adopted. Therefore, the recommended design bedrock Horizontal PGA value for the project site is 0.158 g.

Next, site-specific response spectrum is developed incorporating the local soil effect at the project site and the bedrock input motion. The acceleration response analysis took into consideration seismic zonation evaluated in this study and the site classification reached based on the results of the field and laboratory geotechnical investigations conducted at the site by a geotechnical investigation firm. 

Based on analysis of the results of the field and laboratory investigations incorporated in the site geotechnical investigation and soil testing report (i.e. Shear wave velocity from tests in two boreholes, Standard Penetration Resistance N with depths for 9 boreholes, and other reported soil data) at site of the proposed project, and following the procedure of soil data averaging and interpretation detailed in each of the two codes, the soil profile at the site of the project is classified as Class C   (Very dense soil and soft rock) according to UBC97, and similarly class SC (very dense soil and soft rock) according to IBC 2003.

Ground motion accelerations, represented by response spectra and coefficients derived from these spectra, were determined in accordance with the general procedures of the 1997 edition of the Uniform Building Code, hereafter referred to as UBC97, and the general procedures of   the International Building Code, hereafter refereed as IBC 2003.   The developed spectrum are following each of these codes are developed in the study.  

Moreover, a thorough dynamic local site effect analysis is carried out using the computer program ProShake, which incorporate a methodology of analysis well verified based on measured data from several earthquake incidents and adopted world wide by the practitioners.

The results of analysis’s from ProShake and their proximity to the results in Figures 6, of UBC97, supports the adaptation of the developed design response spectra following UBC97. This goes in line with the recommendation of adopting UBC97 design response spectra  rather than IBC2003 design  response spectra based on the fact that the first is more conservative than the second.

Liquefaction Potential analysis is conducted at the site of the project, for the evaluated design PGA and the soil data available from the soil report (mainly SPT with depth). The analysis is carried out using the software LPA. Results showed that, liquefaction Potential at the site during earthquakes is ruled out given the relatively stiff soil profile and expected level of ground shaking.

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