The term shell is used to denote a spanning and space-enclosing element of domed or other vault like form, but with a thickness and order of magnitude less than was usual for these 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 like those of a saddle – a possibility almost restricted to the fan vault in masonry – 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. Tî 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 òóðå of support.
The shell, together with the doubly curved tensile membrane or cable net, has so enlarged the formal vocabulary of architecture that it will continue to play an important role where economy is not the overriding consideration. The Saarinen/Ammann and Whitney roof of the TWA Terminal Building in Kennedy Airport demonstrates its versatility at the limits of practicality; Jorn Utzon's original impracticable 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 as ties or struts. The term truss denotes an assembly in one plane, and 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. But they must be capable of transmitting tension as well as compression. The usual role in a building is of carrying a roof – in place of the arch, dome, vault, beam, or slab.
In the early 19th century, the true timber truss, necessarily somewhat separated and with the bottom tie made from shorter lengths of timber lapped joints, was stretched to span about 150 ft. (45m); but the first-span iron roofs (of basically arched form) had then been built, and the development was in iron and steel. With the introduction of wrought iron for the ties, there was a clearer differentiation between these and the struts that were carried over into steel construction. Because there was no limit in the ties buckling, they were made appreciably more slender. The most important space frames are lighter framed equivalents of ties and vaults, or of slabs spanning in two or more directions simultaneously. The framed dome is a very early form, particularly if we include primitive dome-shaped huts. But even in fully developed timber-framing systems, the ribs were invariably aligned radially and circumferentially, and the system was then braced by additional diagonals or by the outer covering. Early iron-framed domes merely reproduced this timber form, and it was only in the second half of the I9th century when inherently stiff triangulated pattern of framing was substituted. It might be regarded as the first true space frame.
Further development of the framed dome or vault has taken place first entirely n the 20th century and has lagged somewhat behind parallel developments in airframe structures, where there was a greater incentive for the most efficient use of material to save weight.