Case Study: Seattle SR 520 Floating Bridge Design Revisions Cause Construction Delays (2018)

Project Overview
• Name: SR 520 Floating Bridge Replacement
• Location: Seattle, Washington
• Year: 2018
• Project Size: $4.4 billion
• Scope: Replacement of floating highway bridge with enhanced seismic and environmental features
• Lead Agencies/Contractors: Washington State DOT /

Category of the Issue, Problem, or Challenge
• Design
• Structural & Environmental Engineering

Summary of the Issue, Problem, or Challenge
The complex floating bridge design required mid-construction design changes due to underestimated hydrodynamic loads and environmental impact concerns. The bridge’s floating pontoons and anchorage system needed redesign to accommodate stronger currents and updated environmental regulations, causing schedule and cost impacts.

Root Cause Analysis

• Initial hydrodynamic load calculations were overly optimistic.
• Environmental regulations changed during construction, requiring design adaptation.
• Late-stage design modifications lacked full integration with construction sequencing.
• Coordination challenges between environmental consultants and design engineers.

Impacts Due to the Issue, Problem, or Challenge
• 7-month delay due to design revisions and additional environmental assessments.
• Cost overruns approximating $15 million for redesign and mitigation measures.
• Increased stakeholder scrutiny and permit review timelines.

Corrective Actions Taken

1. Revised hydrodynamic load models using enhanced simulation tools.
2. Integrated environmental compliance into design and construction planning.
3. Strengthened communication channels between environmental and structural teams.
4. Established design change management protocols to minimize disruption.

Lessons Learned

• Complex environmental and structural design aspects must be closely coordinated.
• Early and continuous environmental impact assessment is crucial.
• Design changes during construction should be tightly controlled and communicated.
• Multidisciplinary collaboration reduces risks of late-stage redesigns.

Audit & Prevention: Project Control Questions to Ask on Future Projects to Help Control the Situation

• Are environmental regulations fully integrated into design at the start?
• Have hydrodynamic and other environmental loads been conservatively modeled?
• Is there a robust design change management system during construction?
• Are multidisciplinary teams collaborating effectively throughout the project?