Seismic Overstrength of Shear Walls in Parking Structures with Flexible Diaphragms

The objective in current design practice for parking structures is that energy is dissipated through the formation of plastic hinges at the base of shear walls while floor diaphragms remain elastic and are vertically supported by a combination of shear walls and gravity resisting columns. Unfortunately, this objective is not always achieved due to inaccuracies in current methods for calculating demands on shear walls and in calculating the capacity of shear walls (IBC 2003 International Building Code. International Conference of Building Officials. Whittier, CA.  [Google Scholar], ACI code). When demands are overestimated and capacity underestimated, then diaphragm can fail prior to flexural yield of shear walls as was observed in several parking structures in the 1994 Northridge earthquake.

Eigenvalue and inelastic dynamic response analyses were performed in order to investigate the effects of diaphragm flexibility on wall responses and of wall overstrength on diaphragm responses. The elongated periods of parking structures due to diaphragm flexibility were found to significantly decrease seismic force demand on shear walls relative to what is calculated using codes of practice in which diaphragms are assumed to be rigid. This leads to the over design of shear walls, which further compounds the problem by preventing the flexural yielding of these walls and thereby driving inelastic response to diaphragms. Various degrees of diaphragm flexibility, shear wall layout, seismic zone, and the number of stories were considered in these analyses.

Inelastic static pushover analyses were preformed to investigate the design and capacity evaluation of shear walls. The results illustrate that the shear capacity of walls may be close to twice that calculated by codes of practice. The largest overstrengths were observed in shear walls with low height-to-length ratios in which a significant portion of the lateral load was taken by direct strut action to the foundation and without placing demands on the longitudinal tension reinforcement in the shear walls. The article concludes that methods in codes of practice for calculating shear wall demands and capacities need to be improved if good seismic performance of parking structures is to be achieved.     

Influence of Diaphragm Flexibility on Seismic Response of Unreinforced Masonry Buildings

The effects of diaphragm flexibility on the seismic response of low-rise unreinforced masonry buildings are examined using one-way stiffness- and strength-eccentric single-story systems subjected to unidirectional ground excitation. A wide range of diaphragm stiffnesses are considered. Results show that diaphragm flexibility can induce different effects depending on the configuration of the system and the level of diaphragm flexibility. When diaphragm is relatively stiff, amplified displacement demands can be imposed on the flexible side of the structure. When diaphragm is relatively flexible, peak displacements of in-plane loaded walls generally reduce. A diaphragm classification is developed to capture these salient effects.     

Cyclic Performance of an Elliptical-Shaped Damper with Shear Diaphragms in Chevron Braced Steel Frames

Several advantages of yielding dampers in controlling seismic energy have attracted the attention of many researchers in designing new buildings and retrofitting existing structures. In recent decades, various shapes and substances of such dampers have been used in engineering structures and their behavioral features, including the energy dissipating capacities, have been assessed. In this article, a novel method is presented to obtain the design relationship of two types of yielding elliptical dampers in terms of their selected geometric properties, i.e. distance between the shear diaphragms or virtual diameter and thickness. In addition, two different elliptical-shaped steel dampers equipped with the shear diaphragms are proposed and modeled using the finite element software ABAQUS and their performances are investigated. Then, 30 and 25 models, respectively, of the first and second types are studied using pushover analysis. The designed dampers considering the proposed relationships are used in two chevron braced steel frames placed between the bracing and the beam. Due to their desirable efficiency in energy dissipation and increase in the equivalent viscous damping of the frame, better efficiency is achieved in the modified damper with easier fabrication.