Tension structures

7.1 Tension-tie structure used to support a membrane roof at the Schlumberger Research Centre, Cambridge (architect: Michael Hopkins & Partners)

The principal advantages of tension structures are:

• they are a simple and efficient structural form

• their ability to create long-span enclosures

• they can be erected relatively easily

• their ability to accommodate flexible cladding materials or membranes

• they have discrete supports, leading to concentrated foundation forces.

Their disadvantages are partly related also to these advantages:

• heavy foundation forces both in compression (under the masts) and in tension (at the tie holding down points)

• additional space is required around the structure for the holding down arrangement

• the structural elements or ties often perforate the enclosure.

The supporting compression members in tension structures are commonly tubular sections. The attachment of the ties at the top of the masts is important structurally and architecturally. Often these compression members or masts are designed to resist multiple attachments to ties along their length.

Tension elements can be readily introduced into other forms of construction, which are not strictly 'tent-type' enclosures. These are:

• arch structures, with ties at their base or at intermediate locations

• portal frames, with ties at or close to eaves level (see Figure 3.12)

• the bottom chord of roof trusses, which is subject to tension (and also to compression due to wind uplift)

• tension elements in bracing systems.

7.1 Design opportunities for tension structures

The clear distinction between tension, compression and bending elements help to create structures with intrinsic visual interest. Tension structures can be very light visually since the sizes of the individual components are minimised.

The tension elements used in the primary structure are almost always external to the building and are 'expressed' visually. This is both from a desire to emphasise the structure, and also because it would be difficult to provide the depth or height required for the tension structure without an excessive increase in the building volume. Furthermore, an external structure reduces the visual bulk of the building by minimising the clad volume, and reduces the apparent bulk by breaking up the external surfaces. In auditoria, and sports stadia, tension structures lead to less visual obstruction for spectators.

Tension structures present a strong image that is appropriate for some projects and preferred by some clients, whilst at a small scale, the elements and details which form a tension structure provide interest and enrich the design. The aesthetics of the detailing of the tension system, and the principle of suspension and tension, can provide a motif which helps drive the whole design. The 1972 Munich Olympic Stadium by Frei Otto is a classic early example of this principle.

If a building is to be extended later, a largely external structure can be designed to reduce the disruption at the interface by making it possible to make the structural connections without perforating the enclosure.

Tension components are equally used in secondary structures, such as glass façades, where their lightness, delicacy and refinement of detail emphasise the transparency of the glass (see Chapter 9). Similarly, atrium roofs, staircases and footbridges are other examples in which a tension structure can be appropriate.

The reduction in component sizes can also help in transportation and erection. They can also be useful if heavy point loads are to be supported, as they may be connected to the suspension structure at designated points without penalising the whole structure. In the Fleetguard project, for example, the entrance bridge, stairs and roof-top mechanical plant are all suspended from the primary structure (Figure 7.2).

A tension structure may also provide a solution to problems related to particular site conditions. The Oxford Ice Rink is built on poor ground with a very low bearing capacity. The masts concentrated loads at two points and relieved loads elsewhere, which meant that expensive piled foundations were required in only two places (see Figure 7.3).

The tension structure of the Lord's Mound Stand is a response to the particular problem imposed by the site with the existing seating and roadway, and the need to minimise the obstruction to the spectators' view (see Colour Plate 25).

As noted earlier, a disadvantage of tension structures is often the need to provide heavy holding down positions at foundation level, and to protect the tension cables against vandalism, access, fire and corrosion. Penetrations through the external envelope should also be adequately weatherproofed (refer to Chapter 10).

Richard Rogers Steel
7.2 Fleetguard, Quimper, in which heavy loads were supported by the primary structure (architect: Richard Rogers Partnership)
Fleetguard Quimper
7.3 Oxford Ice Rink with discrete foundation supports (architect: Nicholas Grimshaw & Partners)

7.4 Forks at column head, Fleetguard, Quimper (architect: Richard Rogers Partnership)

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