Yesterday marked the 5 year anniversary of the collapse of the 35W bridge in Minneapolis, close to the University of Minnesota. My sister-in-law was on her way home and had just crossed the bridge 15 minutes before the failure. If you are from the Twin Cities, you likely remember where you were on that day, I know I do.
There have been many reports and analyses about why the bridge failed. Basically a gusset plate is a plate that holds the different structures of the underside of the bridge together. The members come to the plate at different angles and are riveted to the ½” plate of steel. Originally the gusset plate in question was never checked for shear failure. The design of the plate was meant to be a preliminary design, but ended up being the final design without the shear calculations ever being performed.
In addition to the original mistake of insufficient calculations, the total weight of the bridge that day was 4 millions pounds more than the original weight on the bridge. This is due to the approximately 2” concrete overlay that was added in 1977, as well as the 40 years of upgrades to median barriers, railings, etc. Construction was happening on the bridge and the materials for this construction were being stored directly above the gusset plate that failed. Also, when the bridge was constructed in 1967, traffic across the bridge was not even close to the 144,000 cars that passed over it in 2007 (Earliest records show 60,600 cars/day in 1976).
I teach a class in Statics & Strength of Materials (NTI School of Technology at Globe University) to Engineering and Architectural Drafting Students. Although they may not be signing off on major bridge construction projects, there are still lessons they can learn from the 35W bridge failure.
Two important statements for my students:
- Loads are always higher than expected. Even if you think you design for a large load, add another Factor of Safety. Original designers of the 35W bridge did not plan for 4 million additional pounds in the weight of the bridge due to “improvements”.
- Check all forms of failure, even in preliminary designs. Failure can come in due to many types of stresses. Parts can fail in tension, compression, bending, shear and combined stresses. When designing you must check each type of stress for failure before continuing to finalize the design.
Many lives were lost and many more were affected on that fateful day in 2007. With new forms of stress analysis, solid modeling and new inspection techniques, hopefully bridges will be safer in years to come.