CFRP bridge

To take advantage of any arising technology requires skill, commitment and not a little imagination. In some cases the engineer must be bold in concept and. audacious in intent. Even if the proposal does not come to fruition, discussion of its basis and feasibility can prompt more imaginative thinking in what may be more mundane projects. The last design case in this chapter is one such example - the use of CFRP potentially to double, or triple, the span of suspension bridges. Bridges from conventional materials have a theoretical maximum span of around 5 000 m. The relationship between load and span is given by the following expression:

where w_s, w_d, and vv_e are the dead load of the superstructure, deck load and heave load respectively, L is the span, r is the density, a is a design coefficient and s_all is the allowable load for the hangers and cables. The limiting span L_lim is given by:

The figure below shows the specific design load as a function of span (a = 1.66) using different materials of construction. Use of GRP doubles the potential span whereas CFRP would triple it.

Specific design load versus the main span for a suspension bridge

In a typical, superstructure the cables represent about 70% of the superstructure weight, so even limiting the concept to using unidirectional materials for composite ropes will achieve the benefit to a significant degree. Where could such a bridge be constructed? The Strait of Gibraltar has been suggested (see below).

Proposal for a CFRP bridge across the Strait of Gibraltar

Owing to water depth a span of at least 8 400 m is necessary; well beyond the scope of a steel structure. Will it be built ? Certainly not in the short or medium term. Difficulties in securing the necessary financing arrangements for such a venture would surely be prohibitive without the experience of smaller similar projects. Whether it is ever built or not; whether it is outrageous or not; it is an example whereby engineers and their colleagues - materials scientists, chemists and designers - are using composites to push the limits.

POST SCRIPT: the opening of the world's first structural GRP bridge has been announced. It has an overall length of 10 m suspended by Kevlar fibres from two structural GRP towers of height 17.5 m. Weight of the deck structure is quoted as a mere 150 kg/ m. Clearly there is a long way to go to match the grandest of ambitions, but the opportunities are there for the taking.