STRUCTURAL DESIGN FOR ARCHITECTURE

Structural Design for Architecture

Fig. 3.57 In the framed-tube system the columns on the

perimeter of the building are closely spaced. The building

acts as a vertical cantilever in response to wind loading.

The cross-section of the cantilever is a rectangular tube

formed by the closely spaced perimeter columns. Those on

the windward and leeward faces of the building act as

flanges while the walls which are parallel to the wind

provide a shear connection between these by rigid frame

action. The World Trade Centre buildings in New York are

perhaps the most well-known buildings to be based on

this principle. The lower diagram here gives an indication

of the variation in the level of load which occurs between

columns. It will be seen that the effectiveness of columns

in the windward and leeward walls is affected by their

proximity to the walls which provide the shear connection.

The phenomenon is known as shear lag (see Fig 3.60).

3.6.4 Cable structures

The most efficient forms of structure are

those which are stressed in pure tension.

Fig. 3.58 The braced-tube system. The action of this is

similar to the framed-tube but in this case the shear load

between the windward and leeward walls is carried by

diagonal bracing elements.

The steel cable structure, in which the

primary load carrying elements are flexible

cables acting in pure tension, is an example

of this type. Because these are very efficient

structures they are capable of very long spans

(Fig. 1.13).

The key feature of cable structures is high

flexibility which allows changes in geometry to

occur in response to variations in load and

maintains the state of purely tensile stresses.

The extent to which a geometric change can be

96 permitted in a building is limited, however,