MISSISSIPPI BRIDGE RIVER

Mississippi Bridge River

Mississippi Bridge River

Introduction

On August 1, 2007, the 135W bridge over the Mississippi River in Minneapolis, Minnesota collapsed resulting in the loss of 13 lives. The bridge consisted of 14 spans and the failure originated in one of the deck truss spans over the river. On January 15, 2008, the National Transportation Safety Board (NTSB) released a preliminary finding that a serious error in the original sizing of some of the truss gusset plates had been made.

The thickness of the gusset plates at two truss panel points was significantly less than that required by the 1961 "AASHO Standard Specifications for Highway Bridges," which was the applicable design code for the bridge. On January 15, 2008, FHWA issued a Technical Advisory that made the following three recommendations:

New or replacement non-load-path-redundant steel truss bridges. Bridge owners are strongly encouraged to check the capacity of gusset plates as part of the initial load ratings.

Future recalculations of load capacity on existing non-load-path-redundant steel truss bridges. Bridge owners are strongly encouraged to check the capacity of gusset plates as part of the load rating calculations conducted to reflect changes in condition or dead load, to make permit or posting decisions, or to account for structural modifications or other alterations that result in significant changes in stress levels.

Previous load ratings for non-Ioad-path-redundant steel truss bridges.

Bridge owners are recommended to review past load rating calculations of bridges which have been subjected to significant changes in stress levels, either temporary or permanent, to ensure that the capacities of gusset plates were adequately considered

Truss Gusset Plate Analysis and Load Rating

Truss gusset plates and connections of truss members to the gusset plates are ordinarily stronger than the truss members to which they are connected. For this reason, load ratings of trusses have not usually included a check of the gusset plate capacity. However, there are circumstances that make this check necessary. These circumstances include addition of dead load (concrete barriers, increased slab thickness, etc.) to the structure, increased live load (added lanes or deck width; increased traffic count would not be included) and deterioration or loss of section of the gusset plates. The memorandum on "Identifying Truss Bridges That Require Gusset Plate Capacity Checks" has more details on this aspect.

Truss gusset plates are subject to a complex set of axial and shearing forces. Typical design methodology uses general beam theory and decouples the axial and shearing stresses for analysis. This methodology is used in this procedure and can be considered to yield sufficiently accurate results. All new Level 1 load rating of trusses shall include a check of gusset plate capacity.

Live Load

Level 1 inventory and operating rating of gusset plates shall be computed. The analysis and level 1 load rating should be made using HS 20 loading with impact. When the HS 20 Inventory Rating is not met, the H 20 Inventory and Operating Rating should be calculated. Gusset plate capacities shall be considered in all Level I and ...