Architect: Mulvanny G2 Architecture
Engineer: Cary Kopczynski & Company
General Contractor: JE Dunn Construction Company
Reinforcing Bar Fabricator: Harris Rebar
Total Project Cost: $185 million
Total Project Size: 380,000 sq ft
Award: 2012 CRSI Award Winner – Residential Building Category
Photography: Cary Kopczynski & Company
STRUCTURAL FRAMING SYSTEM
Escala is a cast-in-place concrete structure featuring a lateral force resisting system of concrete shear walls, combined with ductile moment resisting space frames (DMRSF). This dual system integrated perfectly with the architectural layout, without compromising the open and spacious unit plans that were key to the Escala vision. Since the building does not have a central core, the combination of shear walls around stair cores at the building extremities, combined with four DMRSFs located at demising walls was a well received solution to a difficult problem. In addition, the layout of the dual system elements was coordinated with the contractor during schematic design to take full advantage of high production concrete forming systems. This facilitated a rapid structural frame construction schedule, allowing the work of other trades to follow behind more quickly.
UNIQUE DESIGN FEATURES
The unique architectural challenge of creating Escala inspired a similarly unique structural solution. Several of the key aspects of this unique structural system:
- The finely tuned seismic system required high strength concrete of 14,000 psi for columns and shear walls at lower levels of the building.
- Since required seismic confinement reinforcing (column ties and shear wall boundary ties) increases with concrete strength, it was determined that rebar congestion in the 14,000 psi columns and shear walls based on ASTM A615 Grade #60 steel would render them unbuildable. The structural engineer decided to pursue a radical solution to reduce rebar congestion and restore buildability – 100 ksi rebar for seismic confinement reinforcement.
- After nearly two years of research, analysis, discussions with industry leaders, and negotiations with the City, approval was granted. Escala became the first building in North America, and perhaps the world, to use ASTM A1035 Grade #100 reinforcing steel for seismic confinement.
- The use of 100 ksi rebar reduced seismic confinement quantities in columns and shear wall boundary elements by 40% when compared to Grade #60 rebar. In addition, 100 ksi steel reduced vertical bar quantities by six to seven percent, since the number of supplementary seismic ties (which must be hooked around vertical bars) was reduced, which further simplified placement.
- In summary, 100 ksi seismic rebar significantly reduced steel tonnage, improved constructability, facilitated easier assembly of columns and shear wall boundary elements, required less field labor, and lessened the demand on crane time and hoisting equipment by reducing weight. As high strength rebar becomes more widely used and its benefits more clearly understood, the long-term value to the industry will be enormous.
Escala was inspired by a world-class vision, which was to create large, dramatic open spaces unimpeded by walls and columns. Rather than clustering elevators in a traditional central core, the desire was to locate them closer to the building extremities and create private entries into each unit. The structural design fully accommodated the owner’s vision for a unique building with private elevators, spacious units, and large cantilever balconies. Parking is provided in eight subterranean levels, which required the second deepest excavation in Seattle’s history.
REASONS FOR CHOOSING REINFORCED CONCRETE
The structural design for Escala was developed with buildability and construction speed in mind. In addition, a high-strength low-shrink concrete mix was used to allow PT stressing in two days and eliminate the need for closure strips. This made it possible for the nearly 240 feet long, 22,000 square foot concrete slabs to be placed in one pour. The structure was carefully configured during design to allow the use of a column-hung slab forming system, eliminating the need for all slab reshoring and allowing other trades to follow closely behind the structure.
Escala is an excellent example of the economy, efficiency, and flexibility that cast-in-place concrete provides in high-rise construction. Implementation of the distinctive features mentioned above enabled Escala to take its place among Seattle’s most exclusive buildings.