Building with concrete in Zone 4

Cement Producers Northwest

BELLEVUE -- Developers of multi-family as well as multi-story housing in seismic zone 4 can take lessons from recent local projects that incorporated durable materials design choices.

1995 witnessed completion of One Pacific Towers, another reinforced concrete residential structure thoroughly disguised by its site orientation and its attention-grabbing floor-to-ceiling windows. Affordable housing units are provided in the fifth floor where condominium levels begin in the combination retail and residential complex. PCL Construction Services built the 75-unit, $32 million concrete high rise project in barely a year.

The extensive use of concrete at One Pacific Towers was readily apparent before it was dressed in floor-to-ceiling windows.

Architectural treatment: Twist a building model 45 degrees for visual impact and, as a result, take advantage of a Puget Sound water view corridor. That was an approach chosen by the One Pacific Towers development/design group. The first and lasting impression seen by the public is the outer finishing touches and surface treatments. What they can't see is the solid reinforced concrete ductile frame that allowed the developer to build two extra floors which could not have been accomplished if steel framing were used.

Structural design: A mixed-use building challenges all engineers. The distances between upper columns where condominiums are situated couldn't be used down below where residents' parking and retail space need to go. A concrete framing system was designated for several reasons.

A concrete flat plate system is used for the residential floors where clear spans of 25 feet-27 feet are satisfactory, but the parking/retail levels require wider spans. So the concrete framing design changed at the fifth level, which is the transfer level for this project. The residential columns of upper stories are brought down to and transferred at the fifth floor level onto concrete transfer beams which provide adequate strength to handle 22 stories plus the fifth floor load itself. The transfer beams are grouted or ``bonded'' and post-tensioned. The post-tensioning process was performed incrementally, initially applied after a third of the building height was finished, then another one-third applied upon completion of each successive portion.

Contractor solutions: One Pacific Towers has a convoluted, curved floor plate configuration which lends itself to reinforced concrete flat plate construction. The project superintendent said, ``You can make any shape you want with concrete. That's hard to do in steel; it uses linear elements.'' McClone Construction provided much of the vertical and horizontal formwork as a subcontractor to PCL.

An important point he made about modern technology is economy. The high-strength concrete mixes available today contribute to formwork economy. Instead of changing dimensions of building elements, which increases forming costs, designers and contractors instead adjust compressive strength. The wide range of commercially available concrete mixtures allows changes in materials without changing dimensions.

Lone Star Northwest designed a variety of ready-mix concrete mixes that were prescribed for this downtown Seattle job to range from 4,000 psi for decks to 10,000 psi for the lower garage levels. There were incremental changes in compressive strength designations as building height progressed. Thus, floor to floor layouts and elements may be standardized. Standardization in production, such as tunnel-forming, contributes to affordable urban housing construction that is economical but also highly fire-resistant, quiet and durable.

One significant compliment the project manager paid to the design engineers was that, ``the structural detailing was fantastic, (the building) was very constructible.'' A building that is ``formable'' uses conventional sizes to minimize construction waste and maximize productivity.

Unlike low-rise concrete or masonry housing projects, construction of One Pacific Towers, at 27 stories tall, had to contend with the wind. When high winds reach around 30 mph, the tower crane can't ``fly'' supplies up to crews. Even with such deterrents, PCL completed another highly visible concrete-framed addition to urban vitality.

Project details

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