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Torsional response of multi-storey buildings using 3-D inelastic dynamic analysis Zaghloul, Hassan Mohamed


This thesis investigates the static code provisions as they pertain to torsion of the 1985 edition of the National Building Code Of Canada (NBCC 85) for eccentric multistory buildings. This is done by calculating the displacements and ductility demand of several practical five storey eccentric buildings designed according to the 1985 code, and comparing the response to similar non eccentric buildings. The analysis is carried out using a modified version of the computer program PITSA, which carries out a pseudo elastic dynamic analysis to model the inelastic response. A modification to the program, developed in this thesis, accounts for the effect of gravity forces on the ductility demand. A number of parameters are considered, namely the type of eccentricity, the aspect ratio, the gravity loads, gravity load distribution, and the initial eccentricity ratio. The effect of the design on each parameter is investigated. The following factors are seen to largely affect the reponse, but are not recognized in the code: 1. The static eccentricity specified in the code is not stated whether it is a result of an eccentric center of mass (CM) or an eccentric center of rigidity (CR) building. This study shows that the behavior of the CR buildings are different from CM buildings in that the bigger frames are more damaged in CR buildings but the smaller frames are more damaged in the CM buildings. 2. Gravity loads have a potentially large impact on the response. For beams carrying no gravity loads, the ductility demand in the upper floors is about 15, while ifthe gravity loads are considered to be eccentrically distributed, the ductility demand ranges from 2 to 5 with the bigger frames underdesigned and smaller frames overdesigned. When gravity loads are uniformly distributed, the code provisions are about right. 3. The ±50% increase in the nominal torsion specified in the code can be changed without a significant change in the ductility demand of the longitudinal frames as the torsional moments are essentially carried by the transverse frames. 4. The increase in the building dimension in the direction parallel to the earthquake results in an increase in the dynamic amplification, and the torsional provisions can generally be said to cover the highest possible dynamic amplification, as the design is generally acceptable for these buildings. The result of that is an overdesign in buildings with small aspect ratios, or alternatively, small dynamic amplification. 5. The torsional provisions tend to overdesign the bigger frames in CM buildings and overdesign the smaller frames in CR buildings for large eccentricity ratios. Other findings pertinent to this study show the following: 1. The code-specified period used in the calculation of the design base shear is a conservative estimate. This period should be established using the structural properties and deformation characteristics of the resisting elements in a properly substantiated analysis. 2. The Modified Substitute Structure Method can now model a building with earthquake as well as static loads. 3. PITS A is a reliable tool in the evaluation of the damage in a three-dimensional frame buildings. 4. The torsional moments are essentially carried by the transverse frames, and the longitudinal frames resist lateral loads for an earthquake applied in the longitudinal direction.

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