Use of External Prestressing to Improve Load Capacity of Continuous Composite Steel Girders

Abstract

Prestressing of steel girders is an attractive technique to increase their load capacity. For new construction, prestressing will result in reduction in the overall steel weight of the structure, which will result in cost saving. Prestressing can also be used to minimize the deflections when the construction depth is restricted or when a high span to depth ratio is mandated. Prestressing technique is also an effective and practical method to strengthen existing structures when an increase in permissible live loads is needed. Numerous experimental and analytical investigations have been conducted to study the behavior of externally prestressed simply supported composite steel girders under positive moment. However, very few studies have been performed to investigate the effect of continuity on the behavior of prestressed steel girders. Such studies mainly used test specimens that only simulated the negative moment region. Therefore, the behavior under all aspects of continuity was not fully investigated. In this research, a comprehensive experimental investigation was conducted to more thoroughly understand the behavior of externally prestressed continuos composite steel girders under negative moment. Three half-scale test specimens consisting of continuous (two-span) composite girders were load tested to their ultimate capacity. The girders were prestressed using 7-wire –1/2 in. low relaxation strands. Among the parameters tested were shored versus unshored construction, and application of prestress to the steel section only or to the composite section. The results showed an increase in the ultimate load capacity by about 15%, a decrease in deflection, as well as an increase in the elastic limit compared to a non-prestressed girder. The results also showed that prestressing continuous composite girders reduces stresses in steel beam and increase the load level where the beams start to buckle. The use of strands to prestress steel girders provided flexibility of performing the required profile to optimize the strength of the steel girder.