Oak Street Lofts, a four-story 32,000-square-foot mixed-use condominium project completed in the fall of 2005 used post-tensioned (PT) concrete slabs, reinforced concrete shear walls and columns, and mat foundations. The entire structural system of the Hood River, Ore., building is cast-in-place concrete with a unique trapezoidal shape designed by Holst Architecture, Portland, and Froelich Consulting Engineers, Tigard, Ore. Retail shops below and living quarters above made concrete the right choice for this structure.
The post-tensioned concrete system allowed maximum flexibility to develop the property. The fifteen condominiums in the upper three levels range from 800-square-foot studios to a 1700-square-foot penthouse. The building's 8-inch-thick structural slabs permit higher ceilings that made the retail and living spaces more appealing and marketable. Wood or steel floor framing requires at least a 24-inch floor depth. Another benefit to the flat slabs and ceilings was unobstructed ducting runs.
Homeowners can choose any type flooring system to cover the PT slabs without concern for load or deflection. They can paint directly on the underside of the bare concrete slab that forms their ceiling or leave the ceilings and concrete decks natural, highlighting concrete textures.
The design goal for the building was a clean architectural look for the exterior that served both retail and residential occupants. A veneer of brick softened the look of the structural concrete. The five tall, thin concrete columns accentuated the sharp lines of the concrete frame and highlighted the public plaza. The 8-inch concrete slabs cantilevered beyond the building envelope without obstructive and unsightly tension rods or the knee bracing that is often required for wood or steel framing. The plaza incorporated exposed concrete slabs, ramps, and planters that require minimal maintenance. The design included a balcony for each unit.
The downtown site for the project was between existing structures. One side of the building was directly against its neighbor. A four-story concrete masonry load-bearing wall was erected to provide a three-hour fire barrier at the property line. The column locations required some asymmetrical geometry to accommodate parking, retail, and living quarters. PT slabs allowed column grid offsets and geometrical irregularities that conventional framing did not. Shifting the PT tendon locations in the slab allowed a change in the distribution of the load with each column location. A steel or wood floor system would need additional beams and girders to handle the load from the offset column grids and the trapezoidal building shape.
The basement level has parking for the condominium residents. There is a concrete shear wall elevator core for access to the living and retail units above. The main floor includes a coffee shop and clothing boutique available to walk-in traffic.
Structural engineer and contributor, Timothy Terich, P.E., S.E., says, “Post-tensioned concrete provided the perfect match of strength, minimal depth, and performance. Froelich Engineers uses the PT concrete system for all mixed-use construction such as condominium and loft projects.”