Architect: Weber Thompson, Seattle, WA
Engineer: Cary Kopczynski & Company, Bellevue, WA
General Contractor: Turner Construction Company
Concrete Contractor: Turner Construction Company
Total Project Cost: $60 million
Total Project Size: 300,000 sq ft
Floor System: Long span with core “drophead”
Framing System: Cast-in-place reinforced concrete frame with core walls
Award: 2016 CRSI Award Winner – Residential Building Category
Photography: Cary Kopczynski & Company, Bellevue, WA
Located in the heart of downtown Seattle near Pike Place Market, Viktoria is a 25-story upscale residential tower with 249 apartment units, six levels of parking above grade, and retail space. The owner’s desire was to create a contemporary, high-end urban environment with spacious layouts, minimal structural obstructions, and maximum usable square footage. The structural engineer, Cary Kopczynski & Company (CKC) responded with a long span structural system using a “drophead” around the core, without compromising other design features of the building.
With dazzling 360-degree panoramic views of the city and water, the entire 25th floor is devoted to social amenities, which include a rooftop terrace and indoor sky lounge. A dramatic butterfly roof cantilevers more than 20-feet from the enclosed portion of the sky lounge, and features an evergreen tree that pierces it through an oculus. The owner’s objectives were thereby met.
STRUCTURAL FRAMING SYSTEM
As with all structures, other construction options always exist, however, the residential use of the building dictated the use of cast-in-place (CIP) flat plate concrete and thus, other the options were not considered.
Viktoria is extremely unique structure. This 25-story upscale apartment tower has no internal columns, creating complete freedom in the interior design of every living unit, as well as the lobby, retail, and parking below.
UNIQUE STRUCTURAL AND/OR ARCHITECTURAL DESIGN FEATURES
Viktoria faced several design and construction challenges. One of the challenges was to find a structural solution to maximize the usable square footage in a tight building footprint. After numerous studies and analysis, CKC, the structural engineer, proposed a unique solution to eliminate all internal columns. By thickening the slab from 8-1/2” to 16” for a distance of six feet around the core perimeter and to create a unique “drophead”. All internal columns became unnecessary and slab spans of nearly 40-feet from the central core to the exterior glass line were made possible, providing completely open living units and parking layout without structural obstructions. In addition, the lack of internal columns produced numerous other benefits such as greater structural efficiency due to increased gravity load on the core, elimination of all transfer beams, and lower construction costs. This innovative approach was essential to the success of Viktoria’s efficient structural design.
Close communication between the general contractor and structural engineer was a key to the successful implementation of Viktoria’s unusual core design. The construction team went above and beyond to quickly adapt to its application. The structural engineer minimized the constructibility challenge by creating a buildable design and working synergistically with the contractor.
Due to close proximity to the adjacent buildings, the structural engineer minimized the loads at the building perimeter to avoid settlement effects. The structural design also minimized sideway motion during an earthquake to eliminate the possibility of contact with the surrounding buildings. State-of-the-art non-linear analysis tools were used to predict building drift and diaphragm deflection limits.
The architectural revisions associated with conversion from condominiums to apartments impacted the design team. The structural engineer achieved substantial cost savings by modifying the structural design in the process.
REASONS FOR CHOOSING REINFORCED CONCRETE
- Column-Free Layout. Viktoria used an innovative two-way slab system that eliminated all internal columns. By thickening the slab to 16-inches for a distance of six feet around the core and creating a “drophead”, spans of nearly forty feet from the core to the exterior glass were made possible. This provided completely open interior space in all units, which maximized interior design flexibility.
- Efficient Shear-Wall Core. With no interior columns, the core substantially supported the majority of the building’s dead load. This significantly reduced the net uplift forces at the core under overturning from wind and seismic loads. As a result, core wall reinforcing quantities dropped by over 12%. This rebar savings more than compensated for the drophead costs, creating a net savings to the project.
- Streamlined Structure. The drophead eliminated the need for transfer beams, which would have otherwise been required to relocate interior columns as they pass through the lower retail, lobby, and parking levels. Viktoria’s obstruction-free layout also streamlined the installation of non-structural interior walls, eliminating the need to build these walls around restrictive columns.
- Hidden Utilities. Water supply and most building utilities were placed in the slabs thus eliminating the need for dropped ceilings in the units, thereby maximizing floor-to-ceiling heights.
- High-Strength Reinforcing Steel. The structural design effectively combined ASTM Grade 60 and Grade 80 reinforcing steel (rebar) to minimize rebar tonnage and maximize constructibility. Grade 80 was used in the foundation mat, shear wall boundary elements, and column ties, Grade 60 was used elsewhere. The bottom line was an efficient structural frame that successfully combined high strength reinforcing steel (rebar) with a unique structural system, resulting in all project objectives being met or exceeded.