FHWA Research And Technology Evaluation: Gusset Plates Final Report

1. Introduction

The Gusset Plate Project was selected for review as part of the FHWA R&T program evaluation. In 2014, FHWA initiated an effort to evaluate its R&T Program, which performs long-term, high-risk research to develop innovative technologies and solutions in areas such as environmental impacts, highway infrastructure, operations, planning, policy, and safety. The R&T program evaluation helps FHWA assess how effectively it is meeting its goals and objectives, and provides data to inform future project selections.

1.1 Evaluation Purpose

The R&T program evaluation seeks to answer questions about the R&T Program’s demonstrated benefits, including the following:

1. What research best supports the organization’s objectives?
   
   
2. Are the organization’s research efforts effective?
   
   
3. Is the organization’s research having the desired impacts, and is the organization effectively disseminating research results?
   
   
4. What is the public benefit for research funds being used?

The Gusset Plate Project evaluation aligns with at least three of six R&T Infrastructure Agenda objectives:^[a]

• Improve the security of highway infrastructure and reduce the number of fatalities attributable to infrastructure and design characteristics of work zones.
• Improve the management of infrastructure assets and advance the implementation of a performance-based program for the National Highway System (NHS).
• Improve highway condition and performance through increased use of design, materials, construction, and maintenance innovations.⁽³⁾

This evaluation seeks to understand how FHWA’s investment in gusset plate research contributed to improvements in the design, inspection, and rating of gusset plate bridges. This report documents FHWA’s response to the NTSB recommendations described below—particularly Recommendation H‑08-001—by conducting and disseminating key research to improve specifications for gusset plate design and load rating. This evaluation also examines how FHWA research informed AASHTO’s revision of its Load and Resistance Factor Design (LRFD) Bridge Design Specifications (BDS) and the Manual for Bridge Evaluation (MBE).^[b]

1.2 Project Background

The main span of the I–35W Bridge deck truss in Minneapolis collapsed during the early evening of August 1, 2007. There were approximately 110 vehicles on the collapsed portion; 17 vehicles fell into the water. Thirteen people were killed and 145 were injured.⁽¹⁾

Soon after the collapse, investigators from NTSB arrived on the scene to determine what caused the bridge to fail. NTSB investigates significant transportation accidents across all modes of transportation—civil aviation, railroad, highway, marine, and pipelines.⁽⁴⁾ Seeking technical expertise in bridge design and performance, NTSB turned to FHWA, whose engineers and researchers were on the scene within a day. Before the I–35W Bridge collapse, NTSB and FHWA worked together on numerous investigations helping to forge a strong relationship between the two agencies. This relationship was instrumental in assembling the appropriate staff to efficiently respond to the I–35W Bridge collapse. With technical assistance and guidance from FHWA, the NTSB investigation examined the role of gusset plates in the I–35W Bridge deck truss. A gusset plate is a plate made of steel that is used to join structural components in steel bridges and buildings. Gusset plates are joined to other structural elements using fasteners such as bolts or rivets, or through welding. In this report, a gusset plate refers to the plate element that joins together the main members—the chords, diagonals, and verticals—of a truss bridge.

Before the I–35W Bridge collapse, bridge designers were given considerable discretion in designing gusset plates. There was also no guidance for gusset plate load ratings, as it was commonly presumed that connections such as gusset plates would have been properly designed to be stronger than the members they connect.^[c] However, gusset plates should have been load rated for a change in their condition, such as section loss due to corrosion.

Within months of the collapse, NTSB issued its first recommendation—H-08-001—to FHWA, requiring that bridge owners conduct load-capacity calculations to verify stress levels in all structural elements, including gusset plates.^{(5) , [d]} FHWA responded with a technical advisory (TA), TA 5140.29, to formally respond to the NTSB recommendation. ⁽⁶⁾ This TA provided AASHTO with recommendations to supplement procedures for load rating steel truss bridges to ensure consideration of gusset plate capacity.

A second round of recommendations concerning the design, construction, inspection, and rating of gusset plate bridges followed in November 2008, addressed to FHWA and AASHTO.

A number of FHWA projects sought to address those recommendations. They included projects that investigated:

• Novel ways to inspect gusset plate connections that did not rely on strictly visual inspection (H-08-018).⁽⁷⁾
• Develop quality assurance and quality control standards for bridge design (H-08-017).⁽⁸⁾
• Modifications to bridge inspector training specific to truss gusset plates (H-08-019).⁽⁹⁾

About one year after FHWA released TA 5140.29, the agency issued publication FHWA-IF-09-014, “Load Rating Guidance and Examples for Bolted and Riveted Gusset Plates in Truss Bridges.”⁽¹⁰⁾ As draft versions of that document were being reviewed, it was clear that more research was needed to rigorously validate several criteria suggested in the document. It was decided internally that FHWA would conduct a gusset plate research project. The project culminated in NCHRP WOD 197, which was used to develop new AASHTO LRFD specifications for the design and rating of gusset plate bridges.⁽²⁾ Figure 1 on the next page provides a visual timeline of significant events and responsible parties, beginning with the collapse of the I–35W Bridge and concluding with the official close of NTSB’s recommendation H-08-001.^[e]

Figure 1. Example I–35 deck truss connection showing gusset plate and truss member it connects.⁽¹⁾

1.3 Report Structure

The next chapter of this report, Evaluation Design, describes the evaluation methodology and key hypotheses, and provides a logic model for the Gusset Plate Project. Evaluation Design is followed by Evaluation Findings, which delves into the findings and results of this evaluation, followed by Recommendations, which offers recommendations for FHWA based on the results of this evaluation.

Figure 2. Timeline of gusset plate research activities.^[f]

^a The FHWA R&T Agenda is a website that provides a high-level overview and context of FHWA's R&T projects and shows the cross-cutting work of the Agency's offices. The site is organized around FHWA's strategic R&T objectives, which drive R&T programs in infrastructure, operations, safety, policy, planning and environment, Federal lands, exploratory advanced research, and innovative program delivery.

^b A variety of staff within FHWA were involved in the Gusset Plate Project. This was truly a collaborative effort with staff at FHWA Headquarters, the Resource Center, and Turner-Fairbank Highway Research Center (TFHRC) all contributing. This report does not distinguish the work performed by different offices within FHWA, but rather collectively refers to any and all work involving Headquarters, the Resource Center, and/or TFHRC, as "FHWA". See appendix A for more information on the different players involved in FHWA's infrastructure research.

^c Load rating refers to the periodic evaluation of a structure to determine the safe level of live load it can carry relative to that for which it was designed. Load rating is typically performed when there is a change of use (e.g., lanes were added to the bridge) or a change in condition (e.g., deterioration from corrosion).

^d Safety recommendations result from NTSB's investigative process and are essential to NTSB's mandate. In order to urgently address safety issues, NTSB often provides recommendations before the formal completion of its investigations. This was the case for I-35W investigation and the issuance of recommendation H-08-001. Recommendations draw upon the results of an investigation and may pertain to safety deficiencies beyond what is determined to be the probable cause of the accident. ⁽¹³⁾

^e The official close out process for an NTSB recommendation involves formal responses from the addressee, or the entity responsible for addressing the recommendation, documenting the action or actions taken to comply with the recommendation. NTSB's board is responsible for approving recommendation status changes, including formally closing out a recommendation. NTSB can close a recommendation for a number of reasons, depending on the action or inaction of the assignee. H-08-001 was classified as "Closed-Acceptable Alternate Action" by NTSB on November 14, 2013. NTSB defines this closed status as a response by the recipient that indicates an alternate course of action has been completed that meets the objective of the safety recommendation.^{(15, 16)}

^f Figure based on information from interviews with project staff.