Design Guidelines For Steel Trapezoidal Box Girder Systems

The frequency of use of steel box girders has increased in the state of Texas and throughout the United States over the past 10 years. Some of the advantages of the structural shape that have led to the increased utilization include improved aesthetic, maintenance, and structural benefits. Geometric continuity is achieved since the trapezoidal shape of the steel girders match the prestressed concrete U-beams that are frequently used in Texas. The smooth shapes of the girders also provide a sleek appearance as shown in Figure 1.1. In addition, since the girders are closed they tend to remain dry and there are fewer places for debris and other corrosion causing agents to collect. However, the primary advantage of box girders is the large torsional stiffness that makes the girders ideal for use in curved interchanges for which the bridge geometry can lead to large torques. The torsional stiffness of a box section is generally in the range of 100 to more than 1000 times larger than that of a comparable I-shaped section. While the large torsional stiffness has led to increased use in curved girder applications, there also have been a number of applications in which the girders have been used in straight bridge applications to match adjacent prestressed concrete U-beams. In these cases, straight steel box girders are used in regions where the clear span requirements preclude the use of the concrete U-beams.