FHWA Research And Technology Evaluation: Gusset Plates Final Report

2. Evaluation Design

This evaluation sought to understand how FHWA’s investment in gusset plate research improved the design and rating of gusset plate bridges. The evaluation team’s primary hypothesis was that FHWA’s R&T activities led to the development, adoption, and application of improved design standards and guidelines for rating gusset plate bridges. A secondary related hypothesis was that FHWA’s R&T activities led to new knowledge of the design and load rating of gusset plates diffused throughout the industry. These hypotheses are discussed in greater length in the Evaluation Findings chapter.

Figure 2 on the next page shows the logic model for the Gusset Plate Project and illustrates the links between key inputs, activities, outputs, outcomes, and impacts.^[g] The main outcome was the adoption of revised AASHTO design specifications as part of the LRFD BDS and MBE. A secondary outcome was that new knowledge on the design and load rating of gusset plates was diffused to pertinent stakeholders. These outcomes were directly informed by three key FHWA outputs: NCHRP WOD 197, TA 5140, and Load Rating Guidance and Examples for Bolted and Riveted Gusset Plates in Truss Bridges (FHWA-IF-09-014). NCHRP Project 12-84 was the main effort that led to NCHRP WOD 197. Other key activities included developing technical advisories and guidelines, giving presentations at important conferences and AASHTO meetings to encourage buy-in, and FHWA’s ongoing coordination and collaboration with NTSB.^[h]

Figure 3. High-level logic model.

2.1 Evaluation Scope and Feasibility

The evaluation focused on FHWA R&T gusset plate investigations, including testing and research done in response to NTSB Recommendation H-08-001, which culminated in 2013 in the adoption of AASHTO standards on gusset plate design and load rating.

NTSB’s Recommendation H-08-001 for FHWA, issued in January 2008, reads:

For all non-load-path-redundant steel truss bridges within the National Bridge Inventory, require that bridge owners conduct load capacity calculations to verify that the stress levels in all structural elements, including gusset plates, remain within applicable requirements whenever planned modifications or operational changes may significantly increase stresses.⁽⁵⁾

NTSB’s November 2008 recommendations for FHWA—H-08-017, H-08-018, and H-08-019—address separate issues, including the development of a bridge design quality assurance and quality control program, requiring that bridge owners assess truss bridges using nondestructive evaluation technologies where appropriate, and updating bridge inspector training courses and materials. ^(7,8,9);[i]

Initiated in 2009, FHWA conducted research on proper methods to measure corrosion and section loss in inspections of single- and multi-plate gusset plate designs. That research is currently in publication. However, that work is not part of the scope of this evaluation, as preliminary interviews with State department of transportation (DOT) bridge personnel and AASHTO Subcommittee on Bridges and Structures (SCOBS) members indicated it is too early to measure the meaningful effects of this research.^[j] State DOTs have varying schedules for bridge inspections and data was unavailable for the evaluation team to gather on recent State bridge inspections and changes resulting from updates to the MBE.

The scope of this evaluation is limited to the efforts that led to the eventual adoption of AASHTO’s revised standards for the design and rating of gusset plate bridges. This scope includes a description of how FHWA research helped NTSB identify the cause of the bridge collapse and allowed swift responses from FHWA, NCHRP, and AASHTO to the NTSB recommendations, as shown in figure 4 on the next page.

Figure 4. Flow diagram of project outputs by organization. ^[k]

2.2 Evaluation Methodology

To collect evaluation data, the Volpe team conducted in-depth interviews with key stakeholders and reviewed and analyzed documents from FHWA, NTSB, and a selection of States.

Evaluation Participants and Key Questions

Volpe interviewed NTSB investigators on the I–35W Bridge investigation, FHWA engineers, and members and leaders of various Technical Committees of the AASHTO Subcommittee on Bridges and Structures (SCOBS). A list of major stakeholders is included in table 1 below.

Table 1. Stakeholders

The Volpe team interviewed:

• Three NTSB staff who worked on the I–35W Bridge investigation.
• Two FHWA staff who worked on the I–35W Bridge investigation and NCHRP Project 12-84.
• Four current and former members of the AASHTO Committee on Structural Steel Design.

See appendix B for a complete list of stakeholder interviews and their relevance.^[l]

These interviews sought to document:

• How FHWA assisted NTSB with the I–35W Bridge investigation.
• The extent to which FHWA’s forensic engineering on gusset plates informed NTSB’s recommendations to FHWA and AASHTO in November 2008.
• How FHWA’s research and calculations led AASHTO to adopt revisions to LRFD BDS on gusset plate designs and MBE specifications for the load rating of gusset plates.

Table 2 provides general questions Volpe asked during interviews with FHWA and NTSB stakeholders.

Table 2. General Questions for Stakeholder Interviews

Table 3. Most Important Documents from Review

Document Review

Table 3 highlights documents that informed the Volpe’s evaluation. The Volpe team obtained these documents through web searches and correspondence with stakeholders. See appendix C for a complete list of documents the team reviewed.

^g A logic model is a visual depiction of program components-including inputs, activities, outputs, outcomes, and impacts–that are linked in a chain of causality. It describes the relationship between program resources, planned activities, and expected results. It is not meant to be a comprehensive or linear description of all program processes and activities, but rather it explains the theories of change that drive the design of a program and provides relevant hypotheses that can be tested in an evaluation.

^h These activities were led by different offices within FHWA.

ⁱ See appendix D for a full description of NTSB's written recommendations for FHWA.

^j A list of stakeholder interviews, including these preliminary discussions with State DOT bridge personnel and AASHTO SCOBS members, is in appendix E.

^k Figure generated by research team.

^l Stakeholders at four State DOTs were also interviewed prior to a change in the scope of the evaluation. These stakeholders are listed in appendix B.