Case Study: I‑35W Mississippi River Bridge Collapse – Minneapolis (2007)
Project Overview
• Name: I‑35W Mississippi River Bridge (Bridge 9340)
• Location: Minneapolis, Minnesota
• Year: 2007 (collapse), opened 1967
• Project Size: Replacement bridge ~$234 million; commute impacts ~$165 million–$400 million
• Scope: Steel truss highway bridge undergoing resurfacing and maintenance during collapse
• Lead Agencies/Contractors: Minnesota Department of Transportation (MnDOT) /
Category of the Issue, Problem, or Challenge
• Structural Design
• Load Management
• Inspection & Maintenance
Summary of the Issue, Problem, or Challenge
On August 1, 2007, during peak rush hour and amid resurfacing operations, the I‑35W Bridge suddenly collapsed, dropping a central span (~300 m) into the Mississippi River, causing 13 fatalities and injuring 145 people. Resident and contractor vehicles and materials were also on the span at the time.
Root Cause Analysis
| Factor | Details |
| Undersized Gusset Plates | At nodes U4, U10, L11, the gusset plates were about half the thickness required, a design oversight never corrected. |
| Increased Dead Loads | The structure carried additional weight from successive resurfacing (concrete overlays, anti-icing systems) over the years. |
| Construction Load Concentration | At collapse time, stockpiled sand, gravel, equipment, and personnel added ~264 t on a limited bridge area (300 m²). |
| Design and Oversight Gaps | The design firm never revised those critical gusset plates; peer review was lacking, and maintenance inspectors failed to flag the problem. |
Impacts Due to the Issue, Problem, or Challenge
• 13 fatalities; 145 injured; 111 vehicles dropped with the span; many victims unrecovered.• Disruption to regional traffic networks; estimated direct and indirect economic losses in the hundreds of millions.
• Triggered national emphasis on bridge inspection, load rating, and safety protocols; MnDOT expedited a replacement crossing, opening in September 2008.
Corrective Actions Taken
- Full NTSB investigation and public reporting—recommended thicker gusset plate designs and better load management.Accelerated replacement project under MnDOT (Flatiron/Manson team) with modern redundancy and inspection provisions.
- Nationwide revision of design review procedures; emphasis on structural components critical to fracture control.
- New scrutiny for maintenance operations and restricted temporary loads during construction on existing bridges.
Lessons Learned
• Structural components that are underdesigned—even if stable for decades—can fail catastrophically when additional load is introduced.
• Overlay and construction weights must be treated with equal scrutiny to live loads and considered in daily inspections.
• Design oversight must include robust peer review and guard against assumptions based solely on initial safe service life.
• QA protocols must require real-time evaluation when existing structures are repurposed or loaded beyond baseline conditions.
Audit & Prevention: Project Control Questions to Ask on Future Projects to Help Control the Situation for Future Projects
Design & Review
- Are all structural connection elements (e.g., gusset plates, truss nodes) designed with sufficient thickness and redundancy?
- Have peer reviews or third-party checks validated all critical load paths?
Load Management
- Are dead-load increases (e.g., overlays, deicing systems) quantified and incorporated into load capacity analysis?
- Are temporary construction loads (materials, equipment) tracked and limited?
Inspection & Maintenance
- Do maintenance and inspection teams monitor for latent design weaknesses during renovations?
- Are temporary construction activities subject to independent QA approvals on existing spans?