Load Balancing as a Design Method for Concrete Prestressed Bridge Girders

Abstract

Abstract: Load balancing is a design method for prestressed structural members, primarily beams and slabs, that provides efficient use of materials. When using load balancing, the prestressing tendon is designed for a prestressing force and eccentricity in order to ‘balance’ all or a portion of the dead load. Commonly the procedure is used for post-tensioned cast in place structures. However, it is not used typically for precast, pre-tensioned members. Instead, current design methods focus on the ultimate design strength and allowable stresses at service loads, which can provide inefficient use of prestressing force. This “inefficient” prestress force can cause many complications for prestressed members, such as excessive cracking in the end regions, uncontrolled amounts of camber after release, and inefficient use of materials. Although ultimate strength and allowable stress design methods are important for safety, they provide a design that rarely experiences the full extent of the loads applied. Designing a prestressed member by balancing dead load will provide a more efficient design that is also safe. Creating a uniform compression force of the concrete member under dead load is the goal for the load balancing method. Uniform prestressing forces necessary for the load balancing will greatly reduce the amounts of end cracking, control camber and deflection, and limit the waste of material. Mild steel can be added to the member to meet modern design codes, ensure strength of the member and control cracking for un-balanced load. Load balancing as a design method should be considered for an efficient pretensioned concrete member design to reduce many problematic issues associated with modern bridge girders.