Load factors for permanent actions and cable preload in cable-stayed bridges

Abstract

Today, cable-stayed bridges are often the most economical solution for spans in the range of 150 m to 500 m and have therefore found widespread application over the past decades. In the design of cable-stayed bridges, the choice of partial factors applied on the permanent loads and the cable preload has a significant effect on economy and may even affect the feasibility of a project. While different design approaches are being used today, most designers consider the uncertainties of the two actions as correlated and hence apply the same partial factor to both actions simultaneously. However, some clients have always been asking designers to consider different partial factors for these actions, and a corresponding code clause has recently been proposed in the frame of the development of the second generation of Eurocodes currently under way. The present paper aims to provide a basis for discussion based on the findings of a case study on three existing cable-stay bridges, where the impact of the newly proposed factors on the design values of the effects of actions governing the ULS design of the deck and cable system was investigated and compared to the corresponding results determined considering permanent load and cable preload as fully correlated, i.e. applying the same partial factor to these actions simultaneously. Finally, recommendations for the load factors are given taking into account aspects of economy and safety.