# Special structures

So far in this book only three structural systems have been mentioned: shear walls, braced frames and moment frames. Although these are by far the most common seismic resisting systems, several architectural forms rely on other systems for seismic resistance. Examples include tension membranes and shell structures.

Tension membranes are usually so light-weight that inertia forces are much smaller than wind forces. If a tension membrane meets the structural requirements for gravity and wind force you can usually assume that it will perform satisfactorily during a quake.

Shell structures resist and transfer forces through tension and compression forces within the thickness of their shells. The strength of shells depends upon a geometry that facilitates axial force transfer and

▲ 6.15 A screen print from the RESIST software. To the left the only input screen to design a reinforced concrete shear wall, and to the right, a model of the building. Analysis results, on another screen, are presented as bar charts.

minimizes bending moments. Shells are constructed as either continuous rigid curved surfaces, fully triangulated members as in a geodesic dome or as a lamella roof, comprising two opposed skewed grids of arch-like elements. Compared to more conventional structural forms, the gravity force paths of shells are complex and necessitate sophisticated computer analyses. The addition of seismic forces further compounds the difficulty of structural design.

There are certainly no rules-of-thumb for the preliminary design of these structures against seismic forces. Structural engineering advice is essential. However, it is possible for designers to draw analogies with more common structural systems. For example, the seismic resisting structure of a dome subject to horizontal force can be considered

Curved diaphragm effective for x and y direction forces

▲ 6.16 A simplified approach to understanding the horizontal resistance of a rigid shell structure.

▲ 6.16 A simplified approach to understanding the horizontal resistance of a rigid shell structure.

▲ 6.17 A simplified approach to a triangulated shell structure.
▲ 6.18 Bracing lengths for an irregular shell structure.

simplistically in plan as two curved shear walls (Fig. 6.16). A similar approach is applicable to triangulated shells. Their surface areas can be considered as doubly-curved braced frames. At the base of the structure where the frames are like conventional vertical frames they need adequate numbers of braced bays to transfer forces to the foundations (Fig. 6.17). Away from the walls roof areas act more like curved diaphragms. Where a shell structure is rather irregular, an appreciation of its seismic resistance can be gained by identifying lengths of ' shear walls' at ground floor level (Fig. 6.18).