A Computational Study of the I-35W Bridge Collapse

Principal Investigator(s):

Taichiro Okazaki, Former Assistant Professor, Civil, Environmental and Geo-Engineering

Project summary:

Reported evidence suggests that failure of gusset plates initiated the collapse of the I-35W Bridge in Minneapolis, Minnesota. The particular gusset plates were at a panel point designated as U10. Therefore, a computational study was conducted on the condition of the U10 gusset plates at the time of bridge collapse. The primary objectives of this study were: (1) to evaluate previous research and existing design methods for gusset plate connections; (2) to examine the mechanical condition of the U10 gusset plates at the time of the collapse; (3) to examine possible scenarios that led to the collapse of the entire bridge; and (4) to identify research needs to improve the design methods for gusset plate connections. The forces delivered to panel point U10 were reproduced using available information of the bridge. The truss forces were introduced to detailed nonlinear, three-dimensional finite element models to calculate stress and strain states of the gusset plates. The results indicate that substantial portions of the U10 gusset plates were yielded at the time of collapse, confirming earlier findings from federal and state investigations. Insufficient strength of the gusset plate, along with weight increase due to past deck reconstruction and construction material and equipment staged on the day of collapse, were identified as the main contributing factors to the substantial yielding. It is important to note that, even with the weight increase, the gusset plates would not have yielded substantially if the gusset plates had adequate thickness. The results also suggest that the interaction of compression and shear played an important role in the gusset plate failure and should be addressed in gusset plate design. This interaction is not well understood based on available research.

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