Since its construction in the mid-1920s, Seattle's Skinner Building has undergone numerous capital improvements to enhance the mixed-use building's operations and aesthetics. But three years ago, an earthquake shook the building to its foundation, and the building's owner and manager enough to put into motion the building's largest renovation project to date.
The 6.8-magnitude Nisqually earthquake — the most powerful seismic event to hit the area in more than half a century — shook the Puget Sound on February 28, 2001. Felt as far as 300 miles away, the quake caused an estimated $2 billion in property damage throughout the Northwest.
Although damage to the 203,872-sf Skinner Building was primarily cosmetic, the earthquake drove home the need for a $7.9 million seismic upgrade to enhance life safety for tenants and extend the building's lifespan. A team of consultants and contractors, including locally based architect Burgess Weaver Design Group, structural engineer Magnusson Klemencic, and the Seattle office of Turner Construction Co., was charged with developing a plan to seismically upgrade the building.
Located in the Emerald City's central business district on land owned by the University of Washington and managed by Unico Properties, the E-shaped Skinner Building is an eight-story mixed-use property, with office space on upper floors and commercial suites at ground and basement levels.
The building's most notable resident is the historic 5th Avenue Theatre, whose musical productions have attracted theater patrons for eight decades. The centrally located main auditorium spans nearly the full width of the building and reaches from the lower levels up through the sixth floor. With its main entrance and box office located just off 5th Avenue, the theater also maintains space for administration, rehearsal and dressing rooms, storage areas, and a prop and stage set workshop.
A mixed-use property constructed in the 1920s, Seattle’s Skinner Building has offices on the upper floors, retail on the ground floor, and a theater.
Design for the upgrade started in March 2002, with construction spanning from February 2003 through June 2004. Nearly all of the work was performed outside of business hours and around the theater's performance and rehearsal schedule.
Business by day, retrofit by night
Approximately 1,220 yards of cast-in-place reinforced concrete was pumped into the existing structure alongside steel X-bracing, steel struts, and steel framing. In some cases, concrete elements turned out so well that the decision was made to leave them exposed.
A mix of physical constraints, complex scheduling, and the highly technical construction necessary for many of the proposed elements added complication to the seismic retrofit. As the project moved ahead, it became clear that the best tools available for facing these challenges were the flexibility of the cast-in-place concrete and the team's ability to interact. "The team dynamic was great," says Burgess Weaver principal Henry Weaver, AIA. "Unico is one of our larger clients, and we've worked with Magnusson Klemencic and Turner on other projects, but not of this scale."
The first issue the team faced was how to solve the design issues of strengthening around the large open void of the theater, while overcoming the physical barriers of constructing between floors and across uses. "The main structural concept was to tie the rest of the building in with the concrete core — the theater — which is a four-walled concrete box," says Weaver.
With the void of the theater auditorium located so predominantly in the center of the building, all proposed improvements had to work around its perimeter, respecting the shell of the theater. The added structure, a 6–8-inch thick layer of concrete, essentially encapsulated the void, providing a more efficient path for both gravity and lateral loads.
As an older, existing building, the Building Team had to deal with interior build-out and built-up utilities that had been modified continuously through the years.
Scheduling was a bear. Not only did the building remain fully occupied for the term of the project, it was also necessary to juggle the various schedules of office, retail, and theater uses, while giving consideration to the many hotels in the area. Crews worked mostly at night, with work areas having to be screened and covered before work could begin. Everything had to clean and ready each business morning, which limited the number of productive hours of construction.
Phasing for the strengthening elements proved a problem as well. Considering that new concrete and steel additions would be permanent, whenever possible, the upgrade had to be coordinated with potential plans of the building owners and future expansions of existing tenants in order to minimize disruption.
The upgrade provided the structure necessary for the efficient transfer of significant seismic loads, while still maintaining the historic character of the building and imposing minimal impact on the existing interior build-out at both common areas and tenant spaces. This required an almost surgical placement of each new element. Locations were selected based on the least amount of disruption, the ability of elements to disappear back into the building, and the reality of constructability and material staging. When possible, retrofitting was conducted outside of occupied spaces.
Working together, the Building Team designed around known building constraints and made modifications as needed. Major design revisions were possible to the extent that, with construction already under way, significant proposed structural elements were shifted to accommodate unknown problem areas as they were identified.
The concrete choice
With the existing structure built primarily of concrete and with the need to have the stiffness of added elements be compatible with that of the existing structure, it became clear that cast-in-place concrete was the obvious choice for strengthening the building. This decision allowing for results that could not have been achieved any other way, says Burgess Weaver. Elements were able to grow or shrink in size and shape from area to area and floor to floor, simplifying construction by allowing quick reaction to uncovered discrepancies at individual locations.
In order to maximize the efficiency of exterior pump staging, formwork construction carried on in several different areas at any given time, allowing the focus to shift from one element to another to accommodate changing tenant schedules. The formwork itself caused minimal daily disruption and always provided full passage for tenants, with new shear walls located in isolated areas and the collector beams sitting at ceiling level.
The decision to go with hydrostatic placement rather than shotcrete kept disruption to tenants to a minimum. Crews pumped concrete by hose into a form, a virtually noise-free process that requires minimal staging area and does not result in the moisture release and messy rebound typical of shotcrete. Once forms for several elements were in place, concrete was pumped from street level to all scheduled pours through a process that grew in timeliness and efficiency as the project continued. The mix was designed to maximize the effectiveness of pumping up to 60 yards a night through 250 feet of hose, while still commanding a high cure rate and allowing shoring to be removed within three to four days of placement.
The Skinner Building's seismic upgrade effectively employed innovative construction techniques in a sensitive manner, maintaining the character of the building and helping ensure the viability of a treasured Seattle landmark.