BRIDGE GIRDER

Nearly 50 years have elapsed since precast, prestressed concrete was the first introduced in the construction industry. During this period, precast, prestressed members have been utilized mainly in bridge system. The high quality of precast, prestressed concrete makes it the material choice for short to medium span bridges. Of the system described here, only post tensioned I girders are used with structural "half-slab system" to produce a composite deck system.

PRODUCTION

Bridge girders can be produced segmentally or unsegmentally. The segmental precast girder, or shortened as segmental bridge girder, is a development of the precast system by exploitation of the prestressed implementation system in the manner of the post tensioning. The main purpose of this segmental system is to facilitate the transportation of the members from the plant/ production location to the jobsite. The segmental part length is limited to the maximum of 6 m and the maximum weight of 8 tons so that it can be handled and transported by using light trucks to reach the remote location.

MATERIALS

Selection of the concrete strength and type of prestressing steel is influenced by the type of girder and construction procedure adopted. A concrete compressive strength of 50 MPa is used for the girders since a high degree of quality control is maintained at the precasting plant. Precast concrete slab and diaprragm have a concrete strength in the 30 MPa. Prestressing steel used here is 12.7-mm diameter low relaxation strand with an ultimate strength of 1900 MPa.

DESIGN CONCEPT

Flexural design of prestressed concrete girders is primarily based on the working stress method with the flexural strength checked, similar to the approach taken by American Association of State Highway and Transportation Officials (AASHTO) Specification. Design for shear and diagonal tension involves both working stress and strength design calculation. The formulas are somewhat similar to those used in the United States. If the section requires shear reinforcement, it may provide in the form of mild steel reinforcement. Some codes are adopted as the basic design, such as: SKBI-1.3.28.87, Indonesian BMS 1992, Indonesian Concrete Code, ACI 343-77, AASHTO, CP 110-1972.