The design of prestressed continuous beams of uniform section

Abstract

Summary The economy of continuous prestressed beams is investigated, with particular reference to three-span beams. Maximum moment envelopes and maximum moment coefficients for three-span beams of several span ratios are given, and compared to those for simply supported spans. At the critical section prestressing may be arranged to neutralize the mean moment, so that the maximum stresses are equalized in top and bottom fibres; this results in an equation giving the beam dimensions in terms of the range of moment. At other sections the stresses are smaller, and the position of the cable may be adjusted so as to eliminate secondary moments; the required cable profile depends on the span ratios and the loading, and a method of determining a suitable profile is described. Precast beams erected as simple spans and subsequently made continuous are shown to be less economical in material than cast in situ continuous beams when the preliminary cables required for the erection of the precast spans are left in place; haunched beams are shown to be less economical than beams of uniform section when live load predominates. Three examples illustrate the calculations for the three types of construction.