Cold-formed steel structural members can provide an extremely lightweight floor framing system. These members, usually C or Z shapes, are normally spaced 24 in center to center (c to c) and can span up to about 30 ft between supports. Because of their light weight, these members can be handled and installed easily and quickly. Connections of cold-formed members are usually accomplished by welding or by the use of self-drilling screws.
This type of floor-framing system is frequently used in conjunction with cold-formed steel load-bearing wall studs for low-rise construction. Spans are usually short to keep depth of floor system small. This depth has a direct bearing on the overall height of structure to which costs of several building components are proportional.
Space in apartment buildings often is so arranged that beams and columns can be confined, hidden from view, within walls and partitions. Since parallel walls or partitions usually??are spaced about 12 ft apart, joists that span between beams located in those dividers can be short-span.
In Fig. 8.18, the joists span in the short direction of the panel to obtain the least floor depth. They are supported on beams of greater depth hidden from view in the walls. With moment connections to the columns, these beams are designed to resist lateral forces on the building as well as vertical loading. (Depth of the beams may be dictated by lateral-force design criteria.) As part of moment-resisting frames, the beams usually are oriented to span parallel to the narrow dimension of the structure. In that case, the joists are set parallel to the long axis of the building. When beam and joist spans are nearly equal, framing costs generally will be lower if the joists are oriented to span between wind girders, regardless of their orientation (Fig. 8.19). This arrangement takes advantage of the substantial members required for lateral-force resistance without appreciably increasing their sizes to carry the joists.
The service core of a high-rise residential building, containing stairs, elevators, and shafts for ducts and pipes, usually is framed with lightweight, shallow beams. These are placed around openings to provide substantial support for point loading.
Because of lighter dead and live loads, columns in apartment buildings are much smaller than columns in office buildings and usually are less visible. Orientation of columns usually is determined by wind criteria and often is oriented as indicated in Fig. 8.20. However, seismic loads (if applicable) and/or P- effects may control in the longitudinal direction, and in that case, additional lateral-load resisting elements such as frame bracing or shear walls can be added.
