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FLOOR AND ROOF SYSTEMS

7.28 CHAPTER SEVEN

a compression ring is installed around the opening to resist the thrusts. Also, if desired, portions of
a dome may be made movable, to expose the building interior.
Designers have a choice of a wide variety of structural systems for domes. In general, dome
construction may be categorized as single-layer framing (Fig. 7.30a and 7.30b); double-layer (truss)
framing, or space frame, for greater resistance to buckling; and stressed skin, with the roof deck
acting integrally with structural framing. Greater stiffness can be obtained by dimpling, pleating
(Fig. 7.30c), or undulating the surface.
Figure 7.30a shows a ribbed dome. Its principal components are half arches. They are shown con-
nected at the crown, but usually, to avoid a cramped joint with numerous members converging there,
the ribs are terminated at a small-diameter compression ring circumscribing the crown. The opening
may be used for light and ventilation. If the connections at the top and bottom of the ribs permit rota-
tion in the plane of each rib, the system is statically determinate for all loads.
Figure 7.30b shows a Schwedler dome, which offers more even distribution of the dead load and
reduces the unbraced length of the ribs. Principal members are the arch ribs and a series of horizon-
tal rings with diameter increasing with distance from the crown. The ribs transmit loads to the base
mainly by axial compression, and the rings resist hoop stresses. With simplifying assumptions, this
system can also be considered statically determinate. For spherical domes of this type, an economical
rise-span ratio is 0.13, achieved by making the radius of the dome equal to the diameter of its base.

7.23 CABLE STRUCTURES

High-strength steel cables are very efficient for long-span roof construction. They resist loads
solely by axial tension. While the cables are relatively low cost for the load-carrying capacity
provided, other necessary components of the system must be considered in making cost compar-
isons. Costs of these components increase slowly with increasing span. Consequently, the larger
the column-free area required, the greater the likelihood that a cable roof will be the lowest-cost
system for spanning the area.
Components other than cables that are needed are vertical supports and anchorages. Vertical supports
are needed to provide required vertical clearances within the structure, because cables sag below
their supports. Usually the cables are supported on posts, or towers, or on walls.
Anchorages are required to resist the tension in the cables. Means employed for the purpose
include heavy foundations, pile foundations, part of the building (Fig. 7.31a), perimeter compression
rings, and interior tension rings (Fig. 7.31b). For attachment to the anchorages, each cable usually
comes equipped with end fittings, often threaded to permit a jack to grip and tension the cable and
to allow use of a nut for holding the tensioned cable in place. In addition, bearing plates generally
are needed for distributing the cable reaction.
Cable roofs may be classified as cable-stayed or cable-suspended. In a cable-stayed roof, the deck
is carried by girders or trusses, which, in turn, are supported at one or more points by cables. This
type of construction is advantageous where long-span cantilevers are needed, for example, for
hangars (Fig. 7.31a). In a cable-suspended roof, the roof deck and other loads are carried directly by
the cables (Fig. 7.31b).
The single-layer cable roof structure in Fig. 7.31b is composed of radial cables, a central tension
ring, and a perimeter compression ring. Since this system is extremely lightweight, it is susceptible
to wind uplift and wind-induced oscillations unless a heavy roof deck, such as precast-concrete panels,
is utilized. Uplift and oscillation can be eliminated with the use of a double-layer cable roof (Fig. 7.31c)
in which the primary and secondary cables are pretensioned during erection.
For a double-layer system with diagonal struts between the primary and secondary cables,
truss action can be developed. If pretension is sufficiently high in the compression chord, com-
pression induced by increasing load only decreases the tension in that chord but cannot cause
stress reversal.
For both single- and double-layer systems, circular or elliptical layouts minimize bending in the
perimeter compression ring and are thus more efficient than square or rectangular layouts.

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