Crown Point Bridge over Lake Champlain, N.Y. Courtesy Michael Bills, Fort Edward, N.Y.

On August 1, 2007, the I-35W Bridge over the Mississippi River in Minneapolis collapsed, resulting in the loss of 13 lives. The bridge consisted of 14 spans, and the failure of the bridge 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 the gusset plates had been made.

The NTSB report on I-35W had some important findings and recommendations on bridge inspections. These findings suggest that visual inspections may not detect or accurately quantify corrosion on gusset plates, and that visual inspection may grossly underestimate section loss in gusset plates. The NTSB recommends the use of non-destructive testing (NDT) methods to determine section loss. Additional guidance is needed in evaluating distorted (bowed) gusset plates.

In 2009, the AASHTO Manual for Bridge Evaluation, Section 4 on Bridge Inspection, was revised to include the following on truss inspections:

• Gusset plate member connections should be inspected closely according to the provisions of Article 4.8.3.10.
• Confirm, if appropriate or as necessary, that gusset plate dimensions and connection details match those shown in the bridge record plans and shop drawings.
• Record any differences found, and record all gusset plate dimensions and connection information if record plans or shop drawings are not available.
• Field inspections of gusset plates need to focus on corrosion, distortion, and connections. Section losses can occur along gusset plate areas that trap debris or hold water, usually along the top of the bottom chord.
• Distortion in the gusset plate can be from original construction or can be caused by overstressing of the plate due to overloads, inadequate thickness/bracing, forces associated with pack rust between plates, or traffic impact.

To that end, the following measures were put in place by HNTB specifically for the inspection of gusset plates for load-path-non-redundant steel truss bridges. These measures include:

• use D-meter readings and other NDT methods to assist in determining the section loss in gusset plates;
• inspect 100-percent of gusset plates by hand, including dye penetrant testing of suspected cracks in welded and bolted connections;
• check field-measured sizes of gusset plates against design drawings; and
• include the as-inspected conditions of gusset plates when determining load ratings based on controlling members.

It should be noted that gusset plates connecting main members are always considered to be primary members and subject to the same inspection intensity as the members they connect. In the past it was thought that gusset plates were over designed. Bridge inspectors are now looking at gusset plates as potential controlling elements and more accurately measuring section loss and checking the gusset plate’s load rating. An example of this in practice involves a bridge that made headlines in late 2009, the Crown Point Bridge.

New York’s Crown Point Bridge
Also known as the Lake Champlain Bridge, this 80-year-old, 14-span structure carried two lanes of traffic (one in each direction) over Lake Champlain, connecting NY 109L (Bridge Road) in Crown Point, N.Y., with VT 17 in Chimney Point, Vt., until October 16, 2009, when it was closed to traffic after being deemed unsafe due to problems with the unreinforced concrete piers. It was demolished on December 28, 2009.

The bridge design was a combination of thru truss, deck truss, and deck plate girders. Spans 1, 2 and 3, as well as spans 10, 11, 12, 13, and 14, were two-deck girder spans with transverse floorbeams. Spans 4, 5, 6, 8, and 9 were two-deck trusses, floorbeam, stringer spans. Span 7, over the main channel of Lake Champlain, was a thru truss. The overall length of the bridge was 2,191 feet. At 434 feet, Span 7 was the longest span.

In 2005, the New York State Department of Transportation’s (NYSDOT) biennial inspection identified one yellow structural flag condition for a cracked gusset plate. In 2007, the biennial inspection found no structural flags. However, in 2009, the biennial inspection by others found two red structural flags, and 11 yellow structural flags due to deteriorated gusset plates.

In New York, a red structural flag is used to report a failure or potentially imminent failure of a critical primary structure component. The term “potentially imminent” means that a failure is likely before the next scheduled inspection. A yellow structural flag is used to report a potentially hazardous condition, which, if left unattended beyond the next anticipated inspection, would likely become a clear and present danger. This flag also is used to report the actual or imminent failure of a non-critical structural component, where such failure may reduce the reserve capacity or redundancy of the bridge, but would not result in a structural collapse presenting a clear and present danger.

In February 2008, NYSDOT issued a memorandum entitled, “NYSDOT Implementation of FHWA Technical Advisory T 5140.29: Identifying Truss Bridges that Require Gusset Plate Capacity Checks.” This memo provided specific guidance and direction for reviewing the adequacy of existing truss gusset plates in accordance with the FHWA Technical Advisory. This memo also assisted in the identification of truss bridges that had gusset plates that were subject to significant stress increases due to increased live loads and/or dead loads, and/or truss bridges in which gusset plate capacities had been reduced due to impact, deterioration, and section loss.

The NTSB determined that the thickness of the gusset plates at two truss panel points on the I-35W Bridge was significantly less than that required by the 1961 AASHTO Standard Specifications for Highway Bridges, which was the applicable design code for the Crown Point Bridge.

On January 15, 2008, the Federal Highway Administration issued a Technical Advisory that made the following three recommendations:

1. 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.

2. 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 a significant change in stress levels.

3. Previous load ratings for non-load-path-redundant steel truss bridges — Bridge owners are recommended to review past load rating calculations of bridges that have been subjected to significant changes in stress levels, either temporary or permanent, to ensure that the capacities of the gusset plates were adequately considered.

Gusset plate of trussed floor beam. Note severe corrosion losses. (A suspension bridge in New York City.) Gusset plate repair on Robert Moses Causeway over Five Island Inlet. Cracked gusset plate connecting truss vertical to bottom chord, Robert Moses Causeway over Fire Island Inlet, N.Y.

In general, truss gusset plates and connections of truss members to the gusset plates were assumed to be stronger than the truss members themselves. However, this assumption may no longer be valid as refined analysis, such as the Finite Element Method (FEM), is allowing heavier loads on truss members, which could make end connections the limiting capacity. Load ratings of trusses in the past have not usually included a check of gusset plate capacity.

However, there are circumstances that make this check necessary. These circumstances include addition of dead load (such as concrete barriers, or increased deck slab thickness) to the structure, increased live load (such as trucks using the structure that are heavier than designed), and deterioration or loss of section of the gusset plates. FHWA guidelines for gusset ratings have served as an acceptable , simplified approach, until the ongoing National Cooperative Highway Research Program research project on gusset evaluation is completed.

Truss members are designed by placing live loads in positions and combinations that will maximize the force in the individual member under design. The individual truss members at a joint under investigation will not all have maximum load simultaneously. Because of this, the maximum truss member forces obtained for the design plans from load rating analysis will usually not result in static equilibrium at a truss joint. However, the use of the maximum truss member forces is considered to be sufficiently accurate for this simplified method. Two load cases need to be considered if the diagonals at a truss joint experience stress reversals: case 1 uses the maximum tensile forces; case 2 uses the maximum compressive forces.

Typically, only the thickness of the gusset plate is shown on the design plans, and it would be necessary to have the shop drawings or field verification to obtain the actual length and width of the plates. The bridge inspection report should be reviewed to determine if there has been significant gusset plate section loss, and if so, this should be accounted for in the analysis.

Conclusion
HNTB has updated its truss inspection procedures to comply with national guidelines. Specifically, gusset plate inspections have been intensified and we have instituted better documentation of gusset deterioration and distortion. Gusset plates are load rated along with other members of the truss. Overall, bridge inspectors and load rating engineers should now have a better awareness and understanding of the role gusset plates have in determining the load capacity of a steel truss bridge.

Note: HNTB has performed gusset plate evaluations of several major truss bridges using FHWA Guidelines.

Seth Medwick, P.E., leads the bridge inspection group in the New York office of HNTB Corporation. Medwick has 25 years experience in the industry. He can be contacted at smedwick@hntb.com.