Interstate Highway Bridge Case Study

1.0 Introduction
On August 1, 2007 at about 6:05 p.m., interstate highway bridge I-35W, which was an eight lane, 1 907 foot-long bridge, collapsed over the Mississippi River in Minneapolis, Minnesota. A total of 1 000 feet of the bridge had collapsed and fallen 108 feet into the Mississippi River triggering 111 vehicles to fall, causing 13 lives to be lost and another 145 injured.

The I-35W Bridge was designed as a deck-truss bridge in 1964 and opened in 1967 with six traffic lanes and two auxiliary lanes. The bridge had undergone two major upgrades, with one in 1977 and the other in 1998, adding another two inches of concrete deck, while also widening the bridge to eight lanes. On the day of the collapse, construction was underway on the deck-truss portion of the bridge and as a result, four of the eight lanes were closed due to parked machinery and paving materials in the way. During the

The design error was in the thickness in the gusset plates. Shown in Fig. 2 are the gusset plate thickness comparison and can see U10 is only a half inch thick. When deciding how thick gusset plates are to be the designer must calculate the truss force of the upper chord (top section on bridge), lower chord (lower section on bridge) and also the force on the diagonal truss. Since the bigger something is the more expensive it gets. The least amount of thick gusset plates Sverdrup & Parcel and Associates, Inc. needed to withstand the calculated stress, the more money they could save. The problem was, Sverdrup & Parcel and Associates, Inc. failed to calculate the force on the diagonal truss and so, when deciding gusset thickness had inaccurate data. As it turns out the peak of the diagonal force is near the U10 gusset which is only a half inch thick as shown in Fig. 3. This miss calculation, did not immediately prove fatal as the bridge stood for 40