Staggered Truss System

In this system, story-high trusses span in the transverse direction between the columns at the exterior of the building. Flexibility in architectural planning is achieved by arranging the trusses in a staggered pattern at alternate floors, as shown in Fig. 3.10. The floor system acts as a diaphragm transferring lateral loads in the short direction to the trusses. Lateral loads are thereby resisted by truss diagonals and are transferred into direct loads in the columns. The columns therefore receive no bending moments. The truss diagonals are eliminated at the corridor locations to allow for openings. Since the diagonal is eliminated, the shear is carried by the bending action of the top and bottom chord members at these locations.

Because the staggered truss system resists a majority of gravity and lateral loads in direct stresses, it is quite stiff. In general, additional steel tonnage required for controlling drift is quite small. Therefore, high-strength steels may be used throughout the entire frame. In regions of low seismicity, the system has been used for buildings in the 35- to 40-story range. Transverse spans must be long enough to make the trusses efficient, with 45ft (13.72 m) considered the minimum practical limit. Since the trusses are supported only at the peri-meter, the need for interior columns and associated foundations is eliminated, contributing to the economy of the system. The system is not limited to simple rectangular plans, and can be used for curvilinear plan shapes, as shown in Fig. 3.11.

The essential structural action in a staggered truss system is the transfer of lateral loads across the floor to the truss on the adjacent column line. This action continues down on the truss line across the next floor down the next truss, etc., as shown schematically in Figs. 3.12 and 3.13. Thus, between the floors, lateral forces are resisted by the truss diagonals, and at each floor, these forces are transferred to the truss below with the floor system acting as a diaphragm. The columns between the floors receive no bending moments, resulting in a very efficient and stiff structure. Since the trusses are placed at alternate levels on adjacent column lines, a two-bay-wide column-free interior floor space is created in the longitudinal direction.

Figure 3.9h. Empire State Building bracing system; riveted structural steel frame encased in cinder concrete.
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