Bridges

This article discusses the 375m long incrementally launched bridges over the Usuthu River using reinforced concrete, with no pre-stressing, to carry the construction stage loads. This bridge was part of a project for upgrading the road between Siphofaneni and St Phillips in Swaziland. The scope included preliminary and detailed design, assistance with tender evaluation and construction supervision. All required work had to meet the requirements of the Swaziland Environmental Agency. The project formed part of a larger agreement, between Swaziland and the European Union, to develop the transportation infrastructure for the sugar industry. The purpose of the infrastructure development was to reduce the transportation cost for the sugar industry. The existing quotas which guarantees the price and quantities of sugar to be exported each year from Swaziland to the European Union should then be reduced over time.

The unusual feature of the 375m long incrementally launched bridges, is the fact that the construction stage is designed as reinforced concrete with no pre-stressing. Incrementally launched bridges are normally designed with post-tensioned concentric cables to resist construction stage loads. The main reasons for using reinforced concrete for the construction stage design, were cost savings and ease of construction. This article will compare the reinforced concrete design with a design using post-tensioned concentric cables, for the construction stage. The comparison will evaluate the differences in design methodology, the implications for the final product, the construction cost implications and the construction methods.