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The Steel Staggered Truss Frame (STF) system is a highly economical and efficient steel framing system that accommodates generic architectural and structural requirements for mid- to high-rise buildings. Due to the attributes of enabling small floor-to-floor height, large column-free spaces, reduced number of columns, efficient use of material, light weight, reduced size and load requirements for the foundation, high lateral stiffness, rapid construction, and low overall cost, STFs have become a popular system in regions of low seismicity. Although STFs were originally developed for low seismic regions, the high lateral stiffness and light weight make this system attractive for use in high seismic regions. However, very limited studies have been conducted on the behavior of STFs under strong earthquake ground motions. A key to the lateral-load resisting mechanism of STFs is the active participation of the floor diaphragms to transfer the inertial forces cumulating in a staggered manner across the height of the structure. The increasingly large diaphragm shear force in the lower stories brings concerns regarding the cyclic behavior of diaphragm-to-truss connections, local stress demand in the diaphragms under in-plane force and out-of-plane displacement. Special attention is required for the large openings in the diaphragms that accommodate stair cases and elevators. The stability of the system needs to be investigated, if the trusses are designed to take inelastic action, since the trusses serve as both the gravity and lateral-load resisting systems. In this paper, limitations of the conventional STF for use in seismically active areas are discussed. Subsequently, alternative structural layouts are investigated in which the conventional diaphragms composed of precast concrete panels are replaced by horizontal steel trusses to transmit the large diaphragm shears in the lower stories. Additional braces are added in the story levels without floor-height trusses in order to minimize the column deformations that occur due to secondary bending. A prototype mid-rise STF building was designed according to the spectrum specified in ASCE-7 and its seismic performance was evaluated by nonlinear time-history analyses. |
