Statics: Span and Deflection

Metal props used in structures

(a)                                                                                           (b)
Fig. 7-8: Props used to support a footbridge

The most effective ways to increase the stiffnesses or reduce the deflections of a structure are to reduce spans or to add supports, as shown in the last example obtained from nature.

The Cardiff Millennium Stadium was selected to hold the Eve of the Millennium concert on 31 December 1999. However, the Stadium was designed for sports events rather than for pop concerts. During pop concerts, spectators bounce and jump in time to the music beat and produce dynamic loading on the structure which is larger than the loading due to their static weight. If one of the music beat frequencies occurs at or is close to one of the natural frequencies of the cantilever structure, resonance or excessive vibration may occur, which affects both the safety and the serviceability of the structure.

To enable the Eve of the Millennium concert to take place, the cantilever structure of the Cardiff Millennium Stadium had to be reinforced with temporary metal props, which are similar to the prop roots in Section 7.4.2. In this way, the spans of the cantilevers were effectively reduced and consequently their stiffnesses and natural frequencies were increased above the range where any unacceptable resonance induced by the spectators was possible.

Fig. 7-8 shows a steel prop used to support the deck of a footbridge, which is a critical structural member to the bridge.