Post-Tensioned Beams & Long Spans

What is a post-tensioned beam?

A post-tensioned beam is a linear structural element in concrete reinforced by prestressing cables stressed after casting. Prestressing makes it possible to design beams capable of spanning 15 to 30 metres — or even more for exceptional structures — while maintaining optimized cross-sections and reduced weight compared with conventional reinforced concrete.

The principle relies on the parabolic profile of the cables within the beam: low at mid-span (where bending is maximum) and high over the supports. This geometry creates an internal force system that counteracts the bending stresses generated by permanent and live loads.

Why use post-tensioning for long spans?

In conventional reinforced concrete, spanning 15 metres requires very deep beams (1.2 to 1.5 m), which are heavy and costly in materials. Post-tensioning reduces the beam depth by 25 to 35% for the same span, thereby lowering self-weight, foundation loads, and the volume of concrete needed.

For transfer beams — structural elements that carry the loads of several columns removed at the facade or on lower levels — post-tensioning is often the only technically viable solution. It concentrates a considerable load-bearing capacity in an element of reasonable dimensions, compatible with the building's architectural constraints.

Advantages over conventional solutions

The primary advantage is the clear span: where conventional reinforced concrete reaches its practical limits around 10-12 metres, post-tensioning routinely achieves 20 to 25 metres. This capability transforms the design of large spaces: auditoriums, shopping centres, and sports halls can be built without the forest of intermediate columns that restricts layout and sight lines.

For long-span car parks, post-tensioning enables spans of 16 to 18 metres between column lines, freeing up space for traffic flow and parking. The reduction in column numbers increases the ratio of spaces per square metre and improves vehicle manoeuvrability — a direct benefit for investment returns.

The optimized cross-sections result in a 30 to 40% reduction in passive reinforcement and a 15 to 25% reduction in concrete volume compared with a reinforced concrete solution for the same span. On a project with numerous long-span beams, these material savings amount to hundreds of millions of FCFA.

Typical applications

Post-tensioned beams are essential for shopping centres (16-20 m bays), auditoriums and multipurpose halls (20-30 m spans without columns), exhibition halls, multi-storey car parks (optimized spaces-to-area ratio), and sports halls. BEPCO designs and builds these structural elements across 11 countries in Africa.