By Bryan Seamer, PE, SE
The seismic rehabilitation of York Hall at the University of California San Diego (UCSD) is an excellent example of the challenges faced by seismic engineers in earthquake-prone areas as aging building stock is modernized to prolong its functional life and seismic resilience. Designers must quantitively demonstrate compliance with modern seismic standards while qualitatively preserving critical pieces of social and community heritage.
Built in 1966 and composed of four seismically separate structures, the Donald Neptune and Joseph Thomas-designed York Hall is remarkable for its “structure as architecture” aesthetic. The 122,000 square-foot mid-century modern complex prominently frames the east side of historic Revelle College, its west wing perched atop a 300-foot-long column arcade and ringed with precast concrete fins that provide shading to exterior walkways at each level. It is one of the original buildings on the campus and has been referred to by administrators as “the heart of our campus.”
Everything that makes York Hall iconic also made it an unacceptable seismic risk for UCSD. The fluted columns, precast fins, and exterior CMU—components listed as “contributing features of historic significance”— all lacked the seismic ductility to meet the University of California’s (UC) seismic performance objective. When upgrading the building, preserving these historic elements was a top priority.
The UC Seismic Safety Policy establishes the seismic performance requirements for the retrofit of existing buildings across the University of California system. The standard establishes two performance levels that must be met based on the retrofit methodology described in ASCE Standard 41-17: Seismic Evaluation and Retrofit of Existing Buildings and the building’s Seismic Risk Category. Each of the seismically independent wings of York Hall is classified as Risk Category III based on the number of occupants. The first performance level is Life Safety for seismic demands based on a site-specific response spectrum with a 20% probability of exceedance over 50 years. The second performance level is Collapse Prevention for a site-specific response spectrum with a 5% probability of exceedance over 50 years. The UC Seismic Safety Policy also requires that hazards related to falling building components be abated as part of the seismic retrofit, a requirement that necessitated the assessment, restoration and repair of the hundreds of precast concrete fins that surround each building.
Also high on the university’s priority list was keeping the building operational throughout the renovation. As the main undergraduate biology and chemistry facility at UCSD, York Hall is one of the most heavily used buildings on campus. Classes and grant-funded research going on in the building couldn’t be disrupted or paused during the rehabilitation.
LPA Design Studios, the architect and engineer for the project, proposed an integrated, data-informed approach. Working as a unified team of architects and structural engineers, LPA led UCSD’s project management team through a process that explored multiple seismic retrofit strategies and balanced a broad set of priorities to minimize visible structural interventions, control costs, minimize operational and embodied carbon impacts, and maintain continuous operation of the building complex during construction. Seismic retrofit strategies were the starting point for each iterative design concept, so LPA’s structural engineers led the collaborative coordination sessions with the owner, architect, and contractor from the earliest phases of the project, ensuring that historical architecture, uninterrupted building systems, and constructability were respected with each subsequent evolution of the seismic retrofit strategy.
The result is an NCSEA-award-winning seismic rehabilitation that seamlessly updated the structural systems while preserving an important piece of regional history. In the process, the project achieved millions in cost savings and created a new model for seismic retrofits on the campus.
A Targeted Approach
York Hall was the first UCSD project to be retrofit under UC’s systemwide seismic risk mitigation program. Understanding the larger goals and providing multiple varied options were key to finding the right approach.
LPA worked closely with the UC Seismic Review Board and developed multiple retrofit options based on six campus priorities: seismic safety, historic stewardship, limiting potential disruption, phasing in coordination with campus user groups, maintaining budget and schedule, and reusing as much of the existing building as possible to reduce embodied carbon impact. Initial concepts ranged from “leaning” the colonnade-supported West Wing on the more structurally regular North and South Wings, to surgically targeting specific vulnerable elements in each of the buildings and many variations in between.
The informed design process helped UCSD mitigate risk at every step. By thoroughly understanding the university’s multi-faceted needs and providing clarity and transparency to the prioritization of those needs, designers were able to add confidence to the bidding process and ultimately save money. The team ultimately took a balanced approach that added capacity to some vulnerable components and fully replaced others to maximize seismic performance while minimizing construction cost and disruption. The work focused on three main areas: strengthening the fluted colonnade, restoring hundreds of concrete fins, and mitigating seismic shear forces in critical masonry walls. Crucially, the simplified retrofit process resulted in cost savings of about $3 million, which was later reallocated to other critical campus needs.
“Quadruple Columnectomy”–Strengthening the Fluted Colonnade
York West, a bar-shaped structure that lines Revelle Plaza, a major campus thoroughfare and gathering space, is as remarkable for the beauty of its structural system as it is for its significant seismic irregularities. Bringing it into compliance while preserving its delicate, gravity-defying architecture drew on computational firepower and the ancient craft of shipbuilding.
The 300-foot long, two-story concrete lift slab and CMU shear wall structure is supported in its entirety on an open colonnade that creates a shaded breezeway (Fig. 2). Thirty-five fluted, cast-in-place concrete columns support the heavy upper structure and, along with the cast-in-place second floor slab, create a bi-directional pinned-base space frame as the building’s primary seismic force resisting system. The upper floor and roof lift slabs bear on perimeter CMU piers that, along with several interior concrete shear walls, provide seismic resistance at the upper levels. Because the upper-level shear walls don’t extend to the ground or align with the fluted concrete columns below, the seismic overturning forces and moments on the colonnade are substantial. This unique building configuration resulted in multiple irregularities including soft story, torsion, and in-plane discontinuities in the vertical LFRS.
LPA structural engineers, with oversight from seismic peer reviewer KPFF Consulting Engineers, used non-linear time history analysis and linear dynamic analysis to model seismic performance and determine that four of the fluted concrete columns lacked the capacity to resist the deformation demand caused by the building’s vertical irregularity. A combination of high-axial demands resulting from overturning forces from shear walls in the stories above and bending caused by seismic displacement of the rigid structure above necessitated strengthening. A traditional retrofit approach with FRP or steel column jackets was considered, but due to the historic nature of the vulnerable columns, a more dramatic solution was needed. The four columns would need to be replaced entirely—a “quadruple columnectomy,” as the team dubbed it.
The geometry of the columns made this challenging. The hexagonal columns narrow slightly from base to mid-height before dramatically flaring outward to create a series of cloister-like domes in the exposed concrete soffit.
To match the unique form of the original columns, fiberglass molds were created in situ prior to demolition. To provide structural integrity to the molds, PCL Construction, the design-assist contractor team, borrowed classic maritime engineering techniques learned from the local shipbuilding industry, adding parallel ribs to each fiberglass mold. After shoring the upper levels of the building, the columns were demolished, new rebar cages and structural steel wall-to-column connection hardware installed, and concrete poured into the forms, perfectly preserving the original form of the colonnade (Figs. 3 and 4).
The design of the seismic load path from upper floor shear wall boundaries to the heads of the rebuilt flared columns was equally challenging. A two-piece structural steel connection assembly was custom designed at each column, with the specific arrangement of through-bolts, embedded rebar and steel plate weldments designed to transfer seismic forces without disrupting the existing slab reinforcing (Figs. 4 and 5).
Fins to the Left, Fins to the Right—Restoring Hundreds of Concrete Fins
Eight-hundred and five vertical, story-tall precast concrete fins ring the upper floors of three of York Hall’s buildings, providing sun shading for exterior walkways. These critical components of the buildings’ history have been slowly deteriorating through exposure to marine air and direct sunlight for over 50 years. Like with the column arcade, designers used a bespoke approach that relied on technology.
After a comprehensive condition assessment of each fin, designers determined that nearly 75% of them showed signs of deterioration that could lead to failure of the steel connections that tie into the concrete floor plates at top and bottom. Each fin weighs nearly 500 lbs., so ensuring that they could not fall was a life-safety imperative.
The lightly reinforced fins had several types of damage, including spalling of the precast concrete body, corrosion of the steel components at top and bottom connections and cracking at the location where the perimeter guardrail attaches. LPA worked closely with specialty contractors to create a “kit of parts” to efficiently address each required repair and created specific procedures for each unique concrete deterioration and steel corrosion condition (Fig. 6).
Shotcrete in Disguise—Mitigating Seismic Shear Forces
The final major piece of the seismic retrofit puzzle was the addition of new shotcrete and masonry shear walls at critical locations throughout each building. This proved challenging because the uniquely colored and textured masonry blocks were identified as a contributing element to the building’s historical significance. Strengthening these walls without working inside the building—a campus priority—or changing the exterior aesthetic required special attention by the design team.
The solution was to add a combination of shotcrete and multi-wythe CMU directly to the exterior face at key shear wall lines, with new CMU blocks specially fabricated to mimic the color and texture of the historic masonry (Fig. 7). Where seismic demands necessitated new shotcrete walls, the concrete was clad in historic CMU veneer to preserve the original mid-century architectural aesthetic. This approach allowed the building’s interior to remain functional during construction, without sacrificing seismic performance or compromising the historic character of the Revelle College Historic District.
Takeaways
As building owners in California and other seismically active zones work to modernize their buildings and prolong their functional life, York Hall shows how it can be done efficiently and affordably without losing the architectural heritage that these buildings represent (Fig. 8).
The first big takeaway is integration. By working together as a unified architecture, engineering and ownership team, LPA and UCSD were able to rapidly develop and evaluate multiple design concepts very early to ensure that the project’s goals were optimized.
Secondly, communication within the design team and between the design team, owner, and builder was critical. Each design solution was multi-faceted and relied upon the focused expertise of many individuals across a large project team. In many cases, the willingness and openness of the structural engineering team to lead early iterative ideation was the only way to meet the university’s needs for the project. ■