Nineteen forty-nine was an exciting year for the concrete industry. It was the year that structural concrete literally leaped into full competition with structural steel as a result of the construction of the 155-foot-span prestressed concrete Walnut Lane Bridge in Philadelphia. It was the technique of prestressing that made long spans in concrete possible members so shallow and elegant that not only the architects, but also the engineers became excited advocates.
HOW IT DEVELOPED
In 1886 P. H. Jackson patented a process for tightening steel tie rods in artificial stone and concrete arches for floor slabs. Unfortunately, with the steel then available, the tension in the tightened rods dissipated rapidly due to creep and shrinkage of the concrete. Then in 1925, another American, R. E. Dill, tried tensioning high-strength steel bars coated to prevent bonding to the concrete. It was Eugene Freyssinet of France who made prestressed concrete practical through the use of high-strength wire for the prestressing force.
Freyssinet's concept of prestressed concrete developed from his experience as a designer and builder of reinforced concrete multiple-arch bridges prior to World War I. He developed a flat jack to be placed in the crown of arch bridges to permit rapid removal of the supporting formwork. When accelerating deflections due to creep unexpectedly occurred in one of his early concrete arch bridges, the decentering jack flats were reinserted and expanded to raise the crown of the arch. Thus, the principles of prestressing were developed from the need to control deflections.
This article also discusses the bridge market, the buildings market, and post-tensioned structures.