Seismic vulnerability and pounding hazard of asymmetric buildings with transfer system : experimental and analytical modeling

Abstract

Geographically, Hong Kong is not located in a region with frequent attacks from destructive earthquakes, and historically, there is no provision for seismic design of building structures. However, several previous studies and publications suggest that Hong Kong should be regarded as a region with moderate seismic risk. The main objective of the present study is to investigate the seismic vulnerability of existing wind-designed tall buildings in Hong Kong, for which very often transfer systems and asymmetric structural plans are used. It remains a big issue on how such non-seismic-designed high-rise buildings will perform under the strike of earthquakes. In this study, the seismic vulnerability of an asymmetric 21-story building in Hong Kong with transfer plate is assessed through conducting shaking table tests. A 1:25 scaled model was designed according to the "additional-mass-similarity-law" and fabricated using micro-concrete, steel wires and meshes. The completed model was tested on the MTS seismic shaking table at The Hong Kong Polytechnic University, subjected to compressed waves of five past earthquakes with scaled peak accelerations of 0.05, 0.1, 0.15, 0.2 and 0.3g. The test results reveal that the transfer plate and stories above are most vulnerable and susceptible to severe damages under the attack of earthquakes. An asymmetric rocking motion and failure pattern of the upper structure above the transfer plate are observed for the first time in our tests. The damages of the model were evaluated quantitatively through various seismic damage indices, including the ductility, inter-story drift ratio, frequency ratio, final softening, and Park and Ang damage index. A new but simple algorithm was developed to estimate the overall Park and Ang damage index of the whole structure from measured acceleration and displacement data by idealizing one story as one equivalent element. Utilizing the Park and Ang damage index as a benchmark, the other damage indices are correlated explicitly with various damage states (i.e. slight damage, minor damage, moderate damage, severe damage and collapse) for the first time. This correlation provides a practical approach to assess seismic damages rapidly and is expected to be applicable to other similar buildings in Hong Kong. In addition to direct damages to individual buildings, earthquakes may also induce damages to buildings through seismic pounding of adjacent structures, especially in metropolitan regions such as Hong Kong. In this thesis, seismic torsional pounding is studied from both theoretical and experimental aspects. Theoretically, seismic pounding is modeled using the nonlinear Hertz contact. Numerical simulations are conducted to study the torsional pounding between two flexible single-story towers as well as between a flexible tower and a neighboring barrier. An analytical solution is also obtained for the latter case. The results show torsional pounding is much more complex than translational pounding. Possible chaotic impacts make torsional pounding more difficult to be predicted. The more complex torsional pounding between adjacent multi-story buildings is studied through conducting shaking table tests. Two 1:45 steel models were fabricated to simulate two adjacent 21-story buildings with both transfer plates and asymmetric plans. Pounding tests were conducted between the two models as well as between a flexible model and a nearly rigid wall. The observed pounding can be periodic, group periodic (i.e. a group of non-periodic impacts repeating themselves periodically) or chaotic. Energy may be transferred through pounding from the more massive and rigid structure to the lighter and more flexible one, which causes abnormal large responses and damages to the lighter structure. When the separation distance is zero, the two models respond like a new system, which may have a different dynamic characteristic from those of the individual structures. Thus, pounding may induce an unplanned period shift to existing buildings, which makes their seismic responses more unpredictable than a stand-alone-building.