Pier 54 is a turn-of-the-century pier in Seattle, Washington, along the Puget Sound waterfront, home to Ivar’s, a waterfront seafood restaurant. Concerns about the stability of the 100-year-old wooden pier, especially in the event of an earthquake, motivated Ivar’s to begin a series of significant seismic upgrades to the structure.
As a member of the Reid Middleton team, GeoEngineers performed a seismic hazard analysis as a first step toward developing geotechnical recommendations for the pier. GeoEngineers’ seismic specialists used a FLAC 2D model to analyze the site-specific responses of the soil used to anchor pier-supporting piles. Under earthquake conditions the soil posed a liquefaction risk, meaning it could lose stability, act as a liquid, and impose additional force the foundations would need to resist.
A traditional factor of safety modeling approach wasn’t possible for the pier, because it would have had to reconcile existing wooden piles with the new steel piles being used for reinforcement, two structures with very different stiffness and loading characteristics. Instead, the team used a performance-based engineering strategy for the model, which allowed them to consistently analyze how the two types of piles would interact. GeoEngineers then introduced various types of earthquakes into the model to simulate the soil/pile interaction in each case.
The team also understood that a new seawall, built just feet away from Ivar’s, would affect how the pier would behave in earthquake conditions. By taking a broader look and including both structures as a single system in their analysis, they were able to reliably design the seismic upgrade to meet the desired performance objectives under earthquake conditions.
Reid Middleton based their renovation designs on GeoEngineers’ modeling data and recommendations. The team replaced existing timber aprons with new steel pilings, precast concrete caps and deck panels. Ultimately GeoEngineers’ model demonstrated that the total number of necessary support piles could be safely reduced, decreasing overall construction cost.
By using advanced performance-based engineering modeling techniques, GeoEngineers showed the Pier 54 renovation could be safely completed with fewer piles than originally planned. Carefully examining the interaction between the two different types of piles and potentially liquefiable soils in a variety of specific earthquake scenarios led to geotechnical design recommendations that saved construction costs while maintaining a high standard of safety.