Page 346 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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FLOOR AND ROOF SYSTEMS

7.30 CHAPTER SEVEN

Since the number of anchorages and connections does not increase linearly with increasing span,
cable structures with longer spans can cost less per square foot of enclosed area than those with
shorter spans. This is contrary to the economics of most other structural systems, which increase in
cost per square foot of enclosed area as the span increases.
Another type of cable structure is the cable-truss dome, or “tensegrity” dome. It consists of a series
of radial cable trusses, concentric cable hoops, a central tension ring, and a perimeter compression
ring. The dome is prestressed during erection and is typically covered with fabric roofing.
Cable spacing depends on type of roof deck. Close spacing up to a maximum of 10 ft is generally
economical.
For watertightness and to avoid potential problems due to roof movements at points where cables
penetrate a roof, it is desirable to place cables either completely below or completely above the roof
surface. If cables must penetrate a roof, the joints should be caulked and sealed with a metal-protected,
rubber-like collar.
In design of cable roofs, special consideration should be given to roof movement, especially if
the roof deck does not offer a significant contribution to rigidity. Care should be taken that joints in
a flexible roof do not open or that a concrete deck does not develop serious cracks, destroying the
watertightness of the roof. Insulation may be necessary to prevent large thermal movements.
Consideration should also be given to fire resistance. Sprinklers may be required or desirable. If the
cables are galvanized, corrosion usually is unlikely, but the possibility should be investigated,
especially for chemically polluted atmospheres.

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