Date of Award

9-2012

Document Type

Thesis

Degree Name

Master of Science in Civil Engineering (MSCE)

Department

Civil and Environmental Engineering

First Advisor

Dr. Oh-Sung Kwon

Second Advisor

Dr. Said Elkhouly

Third Advisor

Dr. Aman Mwafy

Abstract

High-rise buildings have a priority when considering the potential consequences from natural hazards since they represent concentrated human and monetary assets. The use of high strength materials is promoted in this class of structures to effectively use floor areas, control vibrations and expedite construction. There is a pressing need to investigate the relationship between structural performance and economics of high-rise buildings with different material strengths under lateral loads, particularly seismic loads that control the design of a wide range of buildings.

To realistically represent the modern high-rise construction, five 60-story reinforced concrete (RC) buildings with varying material strengths, ranging from 45 to 110 MPa, are designed and detailed to fine accuracy keeping almost equal periods of vibration. The construction cost in terms of steel, concrete and formwork is calculated and compared with the increase in useable area. Detailed multi-degrees of freedom fiber-based simulation models are developed and used to assess the seismic response of the reference structures under the effect of 20 natural and artificially generated ground motions. Over 1600 inelastic pushover analyses, (IPAs) and incremental dynamic analyses (IDAs) are carried out and the results are processed to assess the structural performance of the reference structures.

It is concluded that the cost effectiveness depends to a large extent on the cost of steel reinforcement. With increasing concrete strength, the capacity of the concrete cross-section to resist axial loads increases, and hence reduces the need for reinforcing steel. Although the unit cost of concrete increases with increasing strength, the reductions in reinforcement ratios and section sizes as well as the increase in the saleable area result in the most effective and profitable design.

Pushover analysis results indicate that the over strength factors, distributions of plastic hinges and lateral stiffness at different strength levels are comparable for buildings designed to different material strengths. The seismic behavior of high-rise buildings with different material characteristics is assessed by verifying the seismic design response factors, namely the over strength, force reduction and deflection amplification factors. The relative structural performance and damage states of the five reference structures are also evaluated through the use of fragility relationships. High safety margins are generally observed for the reference shear wall structures, which frequently increase with increasing material strength. The probability oif moderate damage at twice the design PGA and the probability of reaching collapse decrease with increasing material strength. The derived fragility relationship and damage states confirm that the behavior of high strength concrete structures is not inferior, but may be safer at high ground motion intensity levels, than that of normal strength material buildings. For the reference 60-story buildings, the total profit gained from using high-strength concrete reaches $4.77 million, which is 4.95% higher than the lowest strength concrete building. The overall improvement in profit-performance exceeeds10percentage for the highest concrete strength building compared with the lowest material counterpart. The presented systemic performance and cost assessment study provide provides practical insights into the economics and lateral response of high strength tall building in earth-quake prone regions at different performance levels.

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