Lexas Companies had an ambitious concept for a 31-story condominium tower over eight levels of underground parking in downtown Seattle. Lexas and their architects wanted large open spaces unobstructed by walls and columns, expansive balconies, and private entries served by individual elevators. In response, Cary Kopczynski & Company developed a structural design for the Escala project incorporating both traditional and non-traditional features, including the first ever use of 100 ksi (kips/square inch) reinforcing for seismic confinement in North America.
Escala’s layout posed a challenge. Instead of being clustered in a central core, elevators were scattered in five locations to provide individual access to residential units. While creating a more personalized experience for residents, this elevator layout compromised shearwall opportunities. Ultimately, concrete shearwalls around the stair cores and adjacent to the elevators at each end of the building, combined with ductile moment resisting frames, were selected for the lateral force resisting system. Along with a gravity load carrying system of flat plate concrete slabs and columns, this seismic system of walls and frames integrated well with the architectural layout. The design of the open and spacious unit plans that were key to the Escala vision was developed with minimal interference from structural elements.
The structural system
A cast-in-place concrete frame was selected as most appropriate for the building. Key aspects of the structural system follow.
- Slabs: Flat plate, 8.5-inch thick at residential levels, 8-in. thick at parking levels. All slabs post-tensioned with 270 ksi unbonded monostrand.
- Columns: Variable size from 24 by 24-in. to 36 by 48-in.
- Lateral System
Dual system of shearwalls and ductile frames. Shearwalls 30-in. thick around stair cores and adjacent to elevators. Frame columns were 30 by 42-in. typical. Frame beams were 30 by 30-in. typical.
- Concrete: Columns varied from 14,000 psi (pounds/square in.) at base to 6,000 psi at top. Slabs varied from 10,000 psi at base to 5,000 psi at top. Shearwalls varied from 12,000 psi at base to 8,000 psi at top. Basement walls were 6,000 psi. Foundation mat was also 6,000 psi. Specified concrete test age varied from 28 days for lower strength mixes to 90 days for higher strength mixes.
- Rebar: ASTM A615 Grade 60 throughout except A615 Grade 75 in foundation mat and A1035 Grade 100 for seismic confinement.
Solving rebar congestion
Tall buildings constructed of reinforced concrete require high column concrete compressive strengths to prevent them from becoming unacceptably large. In regions of high seismicity, however, the use of grade 60 reinforcement for seismic confinement, the quantities of which go up linearly with increasing concrete strength, effectively limits concrete strength to approximately 10,000 psi due to the high rebar congestion created by the ties.
The compressive strength needed in Escala’s concrete – up to 14,000 psi in lower level ductile frame columns under combined gravity and seismic overturning forces – would have required unworkable quantities of 60 ksi ties. CKC decided that higher strength confining steel was the solution. International Building Council approval for rebar above 60 ksi yield for seismic confinement, however, was still several years away.
Escala’s layout posed a challenge. Instead of being clustered in a central core, elevators were scattered in five locations to provide individual access to residential units. While creating a more personalized experience for residents, this elevator layout compromised shearwall opportunities.
CKC approached the City of Seattle with the request for a code exception to use 100 ksi rebar. The city was concerned that the testing and research for 100 ksi seismic confinement was performed on columns smaller than those required for Escala. CKC argued that this difference was inconsequential. Opinions from other concrete experts familiar with the research, who agreed that confinement provided by the higher strength bar would be similar in larger columns as long as required steel areas were provided, supported CKC’s position. CKC was granted approval for 100 ksi reinforcement in Escala’s structural system.
Other design features
Beyond the seismic design, other key design features included the foundation, subterranean structure, podium levels, large residential levels poured monolithically, and column outriggers used to create long balcony cantilevers.
The foundation was a floating mat founded on native till. The soil bearing capacity was 15,000 psf (pounds/square foot), with a one-third increase for short term loading. Maximum applied loads under combined gravity and seismic forces were determined to create contact pressures well below these values; thus, a deep foundation was not required. Mat thickness varied from 9 feet under the building tapering to 7 ft. at the edges to reduce the shoring height. Grade 75 rebar per ASTM A615 was used throughout the mat, with the exception of vertical Grade 60 shear steel in some locations.
The subterranean structure required an excavation depth, which, at 90 ft., was the second deepest ever dug in the City of Seattle. Eight stories of subterranean parking were created, all of which were constructed with 8-in. post-tensioned flat plate slabs. To control slab shortening and the cracking that can result if shortening is restrained, several things were done. Prestressing levels were moderate at 160 to 180 psi, a concrete drying shrinkage limit of 0.030 percent at 28 days was specified, a disciplined curing plan was implemented, and the construction of basement walls, which were shotcreted, followed six weeks behind slab construction. Basement slabs were completed with almost no cracking or other signs of shortening restraint problems.
During the course of design, the contractor informed CKC of their desire to pour the residential slabs monolithically, without construction joints or closure strips. At 21,000 sq. ft. and 236 ft. in length, this was of concern. After review and discussion, however, stakeholders determined that this was acceptable, contingent on procedures similar to those used for subterranean levels being implemented. Prestressing designed by CKC was kept moderate at approximately 185 psi and supplemented with additional rebar where necessary, a concrete shrinkage limit of 0.030 percent at 28 days was specified, and a strict curing plan – including use of an evaporative retarder and two coats of curing compound applied at right angles to each other at all slab surfaces – was specified. No undue distress from slab shortening developed at any tower levels.
A key architectural design feature included long cantilever balconies of up to 13 ft. incorporated into the design of many of Escala’s units. To accomplish this, CKC designed outrigger beams cantilevering off of the adjacent columns, reducing the maximum balcony cantilever to approximately 10 ft. The outriggers were incorporated into the window wall system with minimal architectural impact.
Realizing the vision
The structural team at CKC worked closely with the owner, architect and contractor during the early design phases to create a structure that not only complemented the architectural intent, but also minimized construction cost and shortened the construction schedule. The resulting structural design accommodated the owner’s vision for a unique building with private elevators, spacious units, and large cantilevered balconies.
The use of 100 ksi seismic confinement reduced tie quantities in columns and shearwalls by as much as 40 percent when compared to Grade 60. Use of 100 ksi steel also saved additional bar, since the number of required supplementary seismic ties was reduced. This allowed a reduction in vertical bar numbers, further reducing tonnage and simplifying placement.
Escala has won four national awards for its structural engineering design. It was the first structure in North America to use 100 ksi steel for seismic reinforcing, which enabled the use of 14,000 psi concrete and created a significant reduction in column sizes. Escala paved the way for a new generation of tall buildings in seismic regions that can capitalize on the full range of opportunities high strength concrete and advanced material technology bring to our industry.
The structural team at CKC worked closely with the owner, architect and contractor during the early design phases to create a structure that not only complemented the architectural intent, but also minimized construction cost and shortened the construction schedule
Cary Kopczynski, P.E., S.E., FACI is CEO and senior principal of Cary Kopczynski & Company, based in Bellevue, Wash.