Quantification of Different Sources of Over-Strength in Seismic Design of a Reinforced Concrete Tall Building

Abstract

An over-strength factor in seismic design plays an important role in computing actual forces in a structural member designed to remain elastic. However, sources contributing to this over-strength have not yet been systematically quantified for tall buildings. This paper aims to investigate the contribution from different sources of the over-strength factor in a reinforced concrete (RC) tall building. The contribution of slab-column gravity load framing to the lateral load capacity of the building is also investigated. A 39-story RC tall building subjected to earthquake ground motions in Bangkok was first designed according to the current building codes. Then, pushover analysis was conducted to compute the capacity curves of the building with three different specified strengths: design strength (with f factor), nominal strength (without f factor), and actual strength (with material over-strength). It was found that slab-column framing action contributes about 60% of the total lateral load capacity of the building.  The building has an overall lateral over-strength factor of 3.36 to 3.71. The over-strength factor arising from the design is 2.12 to 2.42 in which the contributions from strength reduction factor, material over-strength, and other sources involving the design requirements are about 1.10, 1.17, and 1.77, respectively. The over-strength factor arising from the redundancy due to the redistribution of internal forces is about 1.55 and the contribution from the steel strain hardening to the over-strength factor is relatively small.