Phase one of upscale $1.2 billion project to redefine urban living in Bellevue, Wash.

Bellevue, Wash., is transforming from what was once known as a quiet suburb of Seattle into a vibrant urban center of new mixed-use developments. Washington Square is a $1.2 billion "superblock" development that, once completed, will be home to a 10.5-acre European-inspired community in the heart of the city. One of the goals of the developer of Washington Square is to create a convenient, pedestrian-friendly community where residents feel as if they are a neighborhood within a neighborhood. When complete, this unique community will boast five condominium towers, walk-up town homes, a boutique hotel, office tower, fitness and amenities center, retail shops, and even a dog park.

As with any rapidly growing city with multiple projects, completion time is of the essence. Divided into four phases, the first phase of the Washington Square project includes 360 residential condominium homes in two 22-story towers, 26 two-story town homes, and 20,000 square feet of retail and restaurant space over four levels of below-grade parking. With phase one underway, DCI Engineers is tasked with providing inspired engineering solutions for this fast-moving project. The development is divided into five areas, including the floor framing system, building cores, underground parking garage, and two residential towers with multi-story peripheral structures that surround the towers for walk-up town homes and retail spaces. Each of these areas has its own challenges and required creative engineering solutions.

Floor framing

The construction for this project is all cast-in-place concrete utilizing two-way post-tensioned concrete slabs as the floor framing. In order to maintain a uniform ceiling height, stud rails are used to resist punching shear at columns in lieu of a column drop cap. The flat-slab framing system has multiple advantages: It minimizes the floor framing depth; maximizes the potential ceiling height within a story; allows the maximum number of floors to be built above ground level within the building height limit; and it results in less excavation for the underground construction. A post-tensioned slab also provides excellent vibration and sound insulation between floors, which is a very important consideration for any residential project.

The building cores

Lateral resistance to earthquake and wind loads is provided by full-height, cast-in-place concrete shear walls that encompass the vertical transportation core. Several studies were conducted to analyze the best configurations and locations for the walls. One consideration during the design of the core was that the developer wanted to avoid the "tunnel" effect that often occurs with traditional thick walls at the elevator entrances. To avoid this effect and make the area more aesthetically pleasing, DCI worked with the architects to relocate the core away from the elevators at the perimeter partitions that separate the unit spaces from the elevator lobby. It was possible to place a complete rectangular core-wall system at the outer lobby walls while expanding the lobby space inward because the team was able to leverage efficient, 7-inch shaft wall construction at the elevators. Advantages of the larger core included reduced wall reinforcement for both shear and boundary elements; elimination of an intermediate set of columns that would have been located where the non-structural exterior lobby walls had been; the ability to free-span the lobby slab within the core by virtue of two-way continuity; greatly improved sound isolation between the units and elevator lobby; and architecturally pleasing and accessible elevator door openings.

Hand-set forms were selected over jump forming because the tower heights, at 22 stories, were just shy of the cost benefit achieved by running core construction ahead of the floor plates. Jump-form systems can easily run $250,000 or more, but are intended to take core construction off the critical path.

Because earthquake forces controlled structural design—which is common for buildings on the West Coast under 30 stories—the concrete core, which extends the full tower height of approximately 220 feet, is the complete lateral bracing system. The high-strength core walls are 24 inches thick throughout. And, since the shear wall system is more than 160 feet tall and a back-up ductile moment-resisting frame was not employed, the International Building Code required that the core design conform to "super regularity" constraints. These prescriptive requirements promote alignment of a building’s center of gravity and center of rigidity by capping the magnitudes of torsional shears permitted on its walls to a specific percentage of their direct shears. To ensure this be true, modal analyses were performed on the tower using ETABS.

The garage

To accomplish the owner’s desire of completing the construction of the west tower three months ahead of the east tower, DCI was challenged to provide a design that would allow multiple sub-contractors to perform work simultaneously. The tight timeline coupled with the parking levels of almost 90,000 square feet each, required DCI to integrate specially designed construction joints into the pours in order to divide the large floor into four smaller, workable areas. As a result, the special joints helped to reduce cracks in the slabs and also allowed for different work, such as shoring, forming, reinforcing steel, concrete pouring, plumbing, and electrical to take place concurrently in different areas. One concrete pour joint was placed in the middle of the garage, which allowed the contractor to accelerate the construction of the west half of the garage and west tower.

Another challenge for the design of the parking garage was to accommodate the contractor’s desire to complete the flat garage floors before building the ramps between the floors. This typically would require double columns along the edge of the ramp and floor slabs. DCI came up with a unique construction detail and procedure that allowed the contractor to build a post-tensioned slab around an existing column to eliminate the potential double-column scenario.

The residential towers

For any project with residential towers over below-grade parking, the column locations and shapes are always a difficult issue for the designers. In a garage, the column locations are constrained by the parking stalls and drive aisles, while on a residential floor, the column locations are controlled by the unit layouts. Generally, it is highly desirable to place square columns by a corner, rectangular columns inside a wall, and round columns by the exterior windows or decks. The design team carefully studied the column locations in the garage and strategically placed the towers over the garage to meet three preset goals: Use a concrete core for vertical and lateral loads; minimize the number of columns while avoiding costly transfer beams; and utilize large cantilevered slabs for a unique and dramatic floor layout. Typically, the residential floor is 7 1/2-inch post-tensioned slab that spans up to 30 feet and cantilevers up to 10 feet. Thirty-inch by 36-inch garage columns were transformed to 12-inch by 48-inch columns to become part of the interior walls at the residential levels. Other garage columns were transitioned to smaller round or square columns, depending on their location in a room. Unbalanced loads due to column off-sets were designed to be resisted by the concrete slabs and the core walls.

The town homes

One of the unique design features for this project is the large areas of town homes surrounding the two residential towers. After comparing different framing options including steel, cold-formed steel, and concrete, the design team selected post-tensioned slabs for the town home floors and roofs. The columns at the town home levels are not in line with any garage columns, and are supported by the 12-inch post-tensioned podium slabs. These columns are 10 inches wide and are placed inside the partition walls. The lateral loads for the town home areas are resisted by the core walls and other strategically placed shear walls that eliminated the requirement for seismic joints.

Conclusion

Creating a completely unique urban living experience and redefining Bellevue as a city of its own was the shared inspiration for creating the Washington Square mixed-use development. With its own unique challenges, the dynamic, walkable "superblock" not only required a big vision, but most importantly, creative engineering solutions. For DCI, the biggest challenge was creating an innovative structural system that would improve the cost efficiency of the towers and creatively address the seismic needs, while maintaining the distinct vision of the developer. The Washington Square project has given DCI the opportunity to exploit traditional structural principles and provide the development team with innovative solutions that would keep costs low on an extremely tight schedule.

For DCI, the accomplishment of this project has been realized and appreciated through the simple notion that the ultimate goal in engineering is to appreciate and execute the architectural and development vision of the project through inspired engineering solutions.

J. Mark D’Amato, S.E., is a founding partner and president of DCI Engineers and can be reached at mdamato@dci-engineers.com. Tom Xia Ph.D., S.E., is a principal and technical director at DCI Engineers and can be reached at txia@dci-engineers.com.

Design & Construction Team

Project name: Washington Square
Owner: Wasatch Development Associates, LLC, Salt Lake City
Structural and civil engineer: DCI Engineers, Bellevue, Wash.
Architect: CollinsWoerman, Seattle
General contractor: Big-D Construction, Salt Lake City
Concrete sub-contractor: The Conco Companies, Kent, Wash.