SHELLS, TRUSSES AND SPACE FRAMES

А shell is a spanning and space-enclosing element of domed or other vault-like form but with a thickness less than was usual for the masonry and mass-concrete forms. Like the latter, a shell may be curved in two directions or in one only; but the two curvatures of the doubly-curved form may be of opposite sense and the singly curved form may be taken to include barrel-shaped and folded or corrugated forms that span along the length of the barrel or the folds, and act as deep beams. To achieve the reduction in thickness, tensile strength must be provided in the shell itself, or at the level of support, or in both places, in accordance with the requirements of the surface geometry, the pattern of loading, and the type of support.

The shell, together with the doubly-curved tensile membrane or cable net, will continue to play an important role where economy is not the main consideration. Whitney roof of the Terminal Building at Kennedy Airport demonstrates its versatility at the limits of practicality; Jorn Utzon's original proposal of sharply-ridged shells for the Sydney Opera House went beyond these limits and called for a different arched type of construction.

Trusses and space frames are assemblies of linear members that act primarily in axial tension or compression. The usual role in a building is carrying a roof — in place of the dome, vault, beam, or slab.

A truss is a structural member usually fabricated from straight pieces of metal or timber to form a series of triangles lying in a single plane. (A triangle cannot be distorted by stress.) A truss gives a stable form capable of supporting considerable external load over a large span with the components stressed in axial tension or compression. The individual pieces intersect at truss joints. The connected pieces forming the top and bottom of the truss are referred to as the top and bottom chords. The sloping and vertical pieces connecting the chords are referred to as struts and posts of the truss. The two systems most commonly used in trusses are the Pratt and the Warren; in the former, the sloping web members are parallel to each other, while, in the latter, they alternate in direction of slope.

The term space frame describes a three-dimensional assembly in which the interconnections are such that a load at any point is distributed in all directions through the assembly. The joints need not be rigid and, ideally, should allow free relative rotations of the members.

The most important space frames are lighter framed equivalents of domes and vaults, or of slabs spanning in two or more directions simultaneously. The framed dome is a very early form. But even in fully developed timber-framing systems, the ribs were aligned radially and circumferentially, and the system was then braced by the outer covering. Early iron-framed domes merely reproduced this timber form.

Further development of the framed dome or vault has taken place almost entirely in the 20^th century and has lagged somewhat behind parallel developments in airframe structures, where there was a greater motive to seek the most efficient use of material to save weight.