Gusset plate used on the I-35W Mississippi River Bridge
|Material||Steel, copper, or aluminum|
|Uses||Bridges, buildings, and other structures|
|Connection Methods||Welding, bolts, or rivets|
Gusset plates are thick sheets of steel that are used to connect beams and girders to columns or to connect truss members. A gusset plate can be fastened to a permanent member either by bolts, rivets or welding or a combination of the three. Gusset plates not only serve as a method of joining steel members together but they also strengthen the joint. They can be used in bridges and buildings along with other structures.
On August 1, 2007, the I-35W Mississippi River Bridge failed due to the improper design of the gusset plates. The plates that failed were found to be only a 1/2 inch thick rather than a larger size appropriate to the application; with an increase in loading due to alterations over the years, the gusset plates became even more inadequate. The National Transportation Safety Board attributed most of the cause of the failure of the bridge to this flaw.
Gusset plates are usually either made from cold rolled or galvanized steel based upon its use. Galvanized steel offers more protection from rust, so when the gusset plate is outside, galvanized steel is usually used. The gusset plate is usually painted to match nearby steel and fixtures and to give it an extra layer of protection.
Occasionally gusset plates will be made from copper or aluminum, but only with small structures that don't require much support. The copper and aluminum gusset plates also provide a more attractive finish for exposed structures.
Gusset plates are used for various structures. Gusset plates are used to connect beams and columns together or to connect truss members. They can be either the only way of connecting the beam and columns or they can be used with bolts and welds. Gusset plates are therefore used in most structures, but the material and size of the gusset plate varies based on the structure. Bridges usually require thick sheets of steel for their gusset plates, but trusses sometimes only require small sheets of aluminum for their gusset plate. The size and strength of the gusset plate depends on size and the function of the structure. The larger the force on the connecting members, the larger the size of the gusset plate. Gusset plates provide an easy way to retrofit structures that can no longer safely support the applied loads.
Gusset plates can be made into a variety of shapes and sizes from a range of materials. These calculations are made based upon the forces and loads applied to the gusset plate through the nearby steel components. A gusset plate can be either the only connection method or it can be used in conjunction with bolts or welds.
Gusset plates are usually square or rectangular, but can be triangular or made into a customized shape to fit the joint. The shape of each plate is designed so that welding or bolts can be applied to different edges of the plate.
There are several different prominent connection types that include gusset plates which include KT gusset plate, uniform force bracing connection, and bolt group.
A KT gusset plate connects several members together through one gusset plate. The gusset plate is welded to a beam and then two or three columns, beams, or truss chord are connect to the other side of the gusset plate through bolts or welds.
A uniform force bracing connection connects a beam, column, and one other member. The gusset plate is bolted to the column and welded to the beam. The connection of the last remaining member can be through either bolts or welds.
Notable bridge failures
The most notable bridge failure due to gusset plates is the collapse of I-35W Mississippi River bridge in Minneapolis, Minnesota on August 1, 2007. Investigators found that the bridge had 16 under-designed gusset plates that all fractured and ripped like paper, and that the remaining gusset plates were properly designed and remained intact. The 16 under-designed plates were all about a half inch thick when they should have been larger sizes to be in accordance with the American Association of State Highway and Transportation Officials (AASHtO) “Standard Specifications for Highway Bridges,” 1961.
In addition to the gusset plates being under-designed for their original loading, there were other factors that led to the demise
- At the time of the collapse of the bridge, approximately 300 tons of construction equipment was located near several of the under-designed gusset plates.
- The bridge was completed in 1967, but in 1977 and 1998, a median barrier, larger outside walls, and a thicker concrete deck were added to the bridge, causing additional loading on the already under-designed gusset plates.
- The temperature on the day of the collapse also could have introduced additional stresses on the gusset plates since the bearings on the bridge were partially frozen limiting their effectiveness 
- Astaneh-Asl, Abolhassan (December 1998). "Seismic Behavior and Design of Gusset Plates". Retrieved April 26, 2011.
- Turner, B. "What is a Gusset Plate?". Retrieved April 26, 2011.
- Roberts, Charles C. "NTSB UPDATE ON THE MINNEAPOLIS BRIDGE FAILURE ANALYSIS". Retrieved April 26, 2011.
- Stachura, Sea (November 13, 2008). "Despite final NTSB report, some still have questions". Retrieved May 3, 2011.
- "Connection Types". Retrieved April 26, 2011.
- University of Minnesota (November 20, 2008). "Independent Study Of The I-35W Bridge Collapse Results Parallel NTSB Report". Science Daily. Retrieved April 26, 2011.