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

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 effect of how floor slabs are modeled in nonlinear structural models to compute lateral load capacity of the building is also investigated. 39-story RC building subjected to earthquake ground motions in Bangkok was first designed according to the current building codes. Then, pushover analysis was conducted to compute lateral load capacity of the building with three different specified strengths: design strength (with factor), nominal strength (without factor), and actual strength (with material over-strength). It was found that modeling floor slabs by elastic shell elements in nonlinear structural model should not be used in computing the ultimate lateral load capacity of the building because the contribution from slab-column framing action is unrealistically large at large roof displacement. When floor slabs are modelled by inelastic effective beam width approach, slab-column framing action contributes about 60% of the ultimate 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 design process 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 redundancy due to the redistribution of internal forces is about 1.55 and the contribution from steel strain hardening to the over-strength factor is relatively small.