Page 343 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch07.qxd 9/29/05 5:17 PM Page 7.27

FLOOR AND ROOF SYSTEMS

FLOOR AND ROOF SYSTEMS 7.27

or triple-hinged (statically determinate). Much lighter members can be employed for a diagonal grid, or
lamella, system (Fig. 7.29b), but many more members must be handled.
With stressed-skin construction, the roof deck acts integrally with the framing in carrying the load.
As in folded-plate construction, the stiffness can be increased by pleating or undulating the
surface (Fig. 7.29c).
Regardless of the type of structural system selected, provision must be made for resisting the arch
thrust. If ground conditions permit, the thrust may be resisted entirely by the foundations. Otherwise,
ties must be used. Arches supported above grade may be buttressed or tied.

7.22 DOME ROOFS

Dome roofs are preferable to arches where the large column-free area to be covered is circular, elliptical,
or approximately an equal-sided polygon. They often have been used for the roofs of exhibition
buildings, arenas, planetariums, water reservoirs, and gas tanks. Also, the feasibility of covering large
stadiums with domes has been demonstrated. Domes are relatively lightweight, despite long spans,
because they can be shaped so that loads induce mainly axial stresses.
Domes may be readily supported on columns, without ties or buttresses, because they can be
shaped to produce little or no thrust. For a shallow dome, a tension ring usually is provided around
the base to resist thrusts. If desired, however, domes may be extended to grade, thus eliminating the
need for walls (Fig. 7.30). If an opening is left at the crown, for example, for a lantern (Fig. 7.30b),

FIGURE 7.30 Steel-framed domes. (a) Arch rib; (b) Schwedler; (c) pleated rib.

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