A comparative study : the dynamic behavior of tall buildings with diagrid and hexagrid structural systems subjected to seismic loads

Abstract

Most advancements in achieving new structural heights for tall buildings are not possible without the innovation in structural systems. One type of tubular shell structural system, diagrids, like the one used in Hearst Tower, have gained much popularity in high-rise constructions over the past decade due to its high efficiency by resolving both gravity and lateral loads with the same elements. A new iteration of such tubular shell system named hexagrid was examined in this paper. The aim of the study is to evaluate the dynamic response and behavior of such systems when they are subjected to seismic loads, and to compare their efficiency to the better understood diagrid system. Three 60-story and three 36-story models using diagrid and hexagrid exoskeleton systems designed to control the governing wind drift requirements were constructed in commercial software ETABS. For each height, one benchmark diagrid structure and two hexagrid structures using horizontal and vertical configurations were modeled. ASCE 7-10 Code based linear dynamic Modal Response Spectrum Analyses and modal analyses were then carried out for two locations, San Francisco and San Diego, to study the seismic performance based on the dynamic response and modal properties. The results from hexagrid architype models were compared against those of benchmark diagrid models to study the difference in dynamic behavior and relative efficiency. The analysis results showed similar mode shapes across different systems, which was attributed to the similar geometry and load-resisting mechanism of tubular shell structures. However, neither of the hexagrid configurations are as stiff as the diagrid system, resulting in larger seismic-induced lateral displacements and acceleration. An efficiency analysis shows, from the perspective of structural weight, that neither one of the two hexagrid configurations are as efficient as diagrids in controlling lateral drifts, but vertical hexagrids are comparable to diagrids in controlling lateral acceleration. It was also concluded that the studied vertical hexagrid configuration is stiffer and more efficient than the studied horizontal hexagrid configuration. The results of the study could be used by design professionals, architects and structural engineers alike, to make a more informed decision in system selection.