Navigating Existing Building Codes for Seismic Upgrade of Marine Structures Supporting Buildings

Abstract: Marine structures and building structures are designed with different codes and methodologies. When marine structures support buildings, the engineering team must interpret multiple design codes, and propose an approach to the Authority Having Jurisdiction (AHJ). When a project involves the alteration and seismic upgrade of an existing marine structure, navigating the codes is further complicated. For example, the International Existing Building Code (IEBC) defines thresholds that trigger upgrade of existing elements based on the change in seismic lateral loading on that element. This ‘grandfathering’ clause was written for buildings designed using deterministic force-based analysis methodologies and is based on demands on individual elements. Whereas marine structures in high seismic regions are designed per ASCE 61 using performance-based design and displacement-based methodologies. Other avenues for nonlinear analysis of existing building structures exist in ASCE 41, but its guidance is not directly applicable to marine structures. There is currently no similar, nationally recognized standard defining when marine structure modifications trigger upgrade of the existing elements. In lieu of published guidance, this leaves the engineering team and owner responsible for proposing a methodology suitable to the AHJ.

The Port of San Francisco (Port) has recently undertaken a large seismic evaluation and retrofit program to upgrade the reliability of the seawall and marine structures along the downtown waterfront. These historic piers are comprised of a marginal wharf (approximately 40 ft wide) and a transverse pier (extending about 800 ft into the bay). Both marine structures support the historic building. One key strategy to these retrofits includes cutting a seismic joint between the marginal wharf and the pier, through both the marine structure and building. This seismic isolation significantly improves structural behavior of the wharf, where demands are highest, and therefore reduces overall seismic risk. However, this joint also introduces a significant modification to the pier and building. The Port and engineering team therefore considered whether the pier structure is also required to meet new design standards. The Port has successfully implemented seismic joints at similar structures along the waterfront; however, those projects inherently included a larger retrofit scope due to changes in building use and occupancy. For this voluntary seismic retrofit of the wharf, the team desired an innovative and defensible way to improve the overall seismic performance without bringing all the marine structures up to current code.

This paper discusses the impact of the seismic joint on the existing pier, code interpretations considered by the team, advantages and disadvantages to each code approach, and the preferred methodology that was ultimately selected by the Port. The team developed a consistent approach for implementation across the broader San Francisco waterfront. Thus, this approach is applicable to other engineers, facility owners, and AHJs when considering the extent of retrofit required due to voluntary seismic upgrades or other alterations to marine structures supporting buildings.

How does this project / topic contribute to the advancement of the industry and profession?: This topic addresses a void in current design standards. Marine structures are often modified during their lifespan, but whether existing elements need to be upgraded to new design standards is not clearly defined, particularly when a building is also involved. This paper discusses various approaches for interpreting codes in this gray area and provides a preferred methodology that can be used by others in the industry.

Does the project / topic implement new and innovative techniques, materials, technologies, and delivery methods?: Yes, it provides an analysis and design approach that is not defined in national design standards.

What was the most challenging aspect of your project / topic and how did you handle it to ensure success?: The most challenging aspect is trying to navigate a consistent path through various design codes and communicate this to AHJs that only have building experience. We handle this with detailed conversations between the engineering team, the Port, and through soliciting input from industry peers.

Who is the target audience for this paper?: Structural engineers/designers, port authorities