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Reinforced concrete construction in the Pacific Northwest

The Pacific Northwest region of the United States stands alone in sustainable design—Washington and Oregon enjoy the largest number of Leadership in Energy and Environmental Design- (LEED) certified buildings per capita by a margin of 2.5 to 1 over almost any other green state.

As the center for sustainable design, Pacific Northwest property owners and developers expect design teams to incorporate sustainable practices into their projects. High seismic considerations add to the challenge of meeting this expectation. Large demands on material needs—amplified by recent cost increases in transportation, labor, and materials—require creative thinking to eliminate inefficiencies and help keep projects within budget.

Several projects in Seattle have incorporated post-tensioned flat-plate floor slabs, high-strength concrete, and high-strength reinforcing steel in ways that not only meet seismic considerations, but also uphold this region’s growing green standard by reducing reinforcing tonnage, improving formwork efficiency, and incorporating strategic structural detailing.

Project name: Escala
Owner: Lexas Companies LLC, Seattle
Structural engineer: Cary Kopczynski & Co., Bellevue, Wash.
Architect of record: MulvannyG2, Bellevue, Wash.
Design architect: Thoryk Architecture, San Diego
Contractor: JE Dunn, Portland, Ore.

Escala is an 830,000-square-foot glass tower that is expected to be the largest residential building in Seattle upon its completion in 2009. While not the tallest condominium tower in town, the imposing 20,000-square-foot residential floors will break all previous records for total area. With 31 stories above-grade and eight subgrade parking levels, it already holds the record for the city’s deepest excavation in recent time with a bottom elevation of approximately 90 feet below street level.

Residential unit sizes will vary between 950 square feet to more than 3,000 square feet; topping out at 16,000 square feet at the penthouse level. Some balconies are as large as 1,000 square feet and will provide a unique extension of the living space by connecting residents with the outdoors. The structure consists of a cast-in-place concrete frame with 8-1/2-inch-thick, post-tensioned flat-plate floor slabs. Lateral loads are resisted by 30-inch-thick shear core walls and ductile moment frames.

Typical moment frame beams measure 30 inches wide by 24 inches deep with typical columns measuring 30 inches wide by 40 inches deep. Deflections at cantilevered balconies are controlled using 24-inch-wide by 24-inch-deep by 4-foot-long concrete outriggers supported off the columns for spans reaching out as much as 15 feet.

Forming system efficiency—Cost-saving options were considered as early as conceptual design. Structural engineer Cary Kopczynski & Co., (CKC) was hired directly by the developer, Lexas Companies LLC, to discuss several forming system options to reduce materials and labor, but more importantly, shorten the construction schedule. CKC worked with the developer and contractor prior to bringing an architect on board to ensure that the chosen system could work. A column-hung forming system was finally selected, as it was viewed as the best option to speed up construction. In order to take full advantage of the forming system’s efficiency, column faces were aligned so that the forming tables could slide in on either side. With this system, slight column offsets are allowed up to 1 foot, which offer a little more flexibility in column layout.

Collaboration between the engineer and the contractor is evidence that time-saving construction methods may be possible if the engineer has a clear understanding of the contractor’s schedule and construction techniques. It was estimated that the system saved the project two days per floor.

High-strength reinforcing steel—Constructability was further improved by the recent project-specific approval by the city of Seattle to allow the use of high-strength reinforcing steel for seismic confinement of high-strength concrete. The high-strength, 100 ksi steel offers a 40-percent reduction in column and shear wall confinement reinforcement as compared to 60 ksi steel; the highest grade of reinforcing allowed by the current building code.

A major frustration for both contractors and ironworkers, reinforcement congestion decreases quality and slows construction, with confinement reinforcement as one of the most time-consuming pieces to install. The change will take at least another two years to be written into the next building code cycle, but CKC saw the opportunity to take advantage of a pending code-change proposal written by the American Concrete Institute’s (ACI) seismic subcommittee approving its use.

The approval process was not without its challenges. The city rejected CKC’s requests for a code alternate several times. Extensive documentation from leading experts, American Society for Testing and Materials approvals, conversations with building code officials, meetings with city planning personnel, and a personal letter from the CKC president certifying the suitability of the material for its intended use were required.

Approval was finally granted in July 2007. Escala is the very first project to incorporate 100 ksi confinement steel in North America. The use of 100 ksi confinement steel not only reduces rebar tonnage, but also improves construction speed, reduces labor, and in many cases reduces vertical reinforcing; further simplifying installation without compromising seismic performance. It was estimated that the use of 100 ksi saved the project 230 tons in seismic confinement steel alone.

Project name: The Cosmopolitan
Owner: Continental Properties, Bellevue, Wash.
Structural engineer: Cary Kopczynski & Co., Bellevue, Wash.
Architect: Mithun Partners, Seattle
Contractor: PCL Construction, Bellevue, Wash.

The Cosmopolitan is a 315,000-square-foot tower located in downtown Seattle that includes 25 residential levels over a 270-stall, eight-story parking structure, with residential units starting at the 10th floor.

Structure does double duty—Seismic loads are resisted by a code-unclassified, cast-in-place concrete shear wall core that was designed using displacement-based analytical methods. This provided more accurate lateral design loads and a better description of the behavior of the building during a seismic event. In addition, long-span post-tensioned slabs of variable thickness were introduced to serve two purposes: concentrate a large portion of the vertical gravity load on the shear core and allow for column-free floor space. The added gravity load "preloaded" the shear core and improved efficiency by reducing lateral overturning forces and tension steel demands by approximately 15 percent. The long-span post-tensioned slab was achieved by thickening the slab around the central core of the building from a typical 8-inch slab up to 18 inches deep, taking full advantage of reduced height requirements at the corridor around the central core. The design of the thickened slab, also called a drop head, created a support condition that allowed the slab to span much greater distances than with a conventional flat-plate system. The drop head allowed for an increase in slab-span capacity from 30 feet to close to 40 feet, while all building columns were pushed toward the perimeter. The exterior building columns and walls were then used as part of the fascia, which eliminated the need for exterior column and wall cladding, reduced building weight, and saved on fascia costs.

Drop head designs do not come without potential disadvantages, however. Post-tensioned quantities increase by about 0.25 pounds per square foot beyond what is required for a conventional flat-plate system. Depending upon the building type, the suitability of a drop head design needs to be thoroughly investigated to determine the added benefit of removing interior columns against increases in reinforcing. It is important that the engineer bring the possibility of this type of design to the table early so implementation is possible. Minor shifts made before drawings are well underway can mean the difference between living units with restrictive columns and those with floor plans that offer enhanced possibilities.

Project name: Mosler Lofts
Owner: The Schuster Group, Seattle
Structural engineer: Cary Kopczynski & Co., Bellevue, Wash.
Architect: Mithun Partners, Seattle
Contractor: JE Dunn, Kirkland, Wash.

Mosler lofts is a 234,000-square-foot, mid-rise residential loft building located in downtown Seattle that includes 12 residential levels over three subgrade parking decks and ground-level retail. Upon completion, Mosler is expected to be the first BuiltGreen and LEED Silver-certified residential condominium building in Seattle. Green features include high-efficiency elevators, use of certified wood products, hybrid flex car availability, CO2 monitoring, low-flow plumbing fixtures, natural day lighting, and dual flush toilets. A green roof is intended to insulate the building, filter rainwater, and provide outdoor space for residents.

The structure consists of a cast-in-place concrete frame with 7-1/2-inch-thick, post-tensioned flat-plate typical slabs. Seismic loads are resisted by 24-inch-thick shear walls located about the elevator core. Typical columns are 18 inches by 24 inches.

Strategic structural detailing—The slab thickness at the parking levels was reduced by 1/2 inch to 7 inches in order to account for reduced gravity design loads, saving the project 225,000 pounds of concrete. Bolt-on decks were used in lieu of cantilevered concrete balconies to provide a thermal break, which significantly improved energy-efficiency calculations. Concrete shear walls were limited to the interior core of the building to provide more open space. Deep transfer beams were eliminated at the third floor by lengthening the columns between the first and third floors to capture the horizontal offset between them. The extended columns were then used as an architectural feature and demising wall between townhouse units on the ground floor. Roof drain locations were carefully coordinated to minimize concrete thickness without compromising roof-slab capacity. Concrete strength and reinforcing quantities were reduced as allowed by lateral and vertical load demands. Shear walls, columns, and slab soffits were left exposed to reduce finish materials and provide a more natural look. Foundation mat and spread footing reinforcing were reduced 20 percent by specifying 75 ksi reinforcing in lieu of standard 60 ksi reinforcing.

Green within our reach

While the solution to significant carbon dioxide emission reductions, energy efficiency, and energy independence are still years beyond our reach in terms of technology and government policy, certain measures to reduce the environmental impact of our buildings are currently available. From a structural engineering standpoint, a reduction in material demand, selection of methods geared toward shortening construction schedules, and reducing building volume without compromising architectural expression are all proven measures that contribute to happy clients and end users.

Close collaboration between all design team members is also key in creating green buildings. Both structural engineer and architect play a vital role in striking a balance between layout efficiency and aesthetics. As such, structural engineers will need to become much more involved in the early conceptualization of any proposed building to ensure that this balance is met.

Martin Maingot, P.E., has been a project manager with Cary Kopczynski & Company since 2005 and was recently promoted to associate of the Bellevue, Wash.-based structural engineering firm. He can be reached at martinm@ckcps.com or 425-455-2144.

Sidebar: Why go green?

Curbing the construction industry’s effects can help heal the environment

Building construction and operation have a profound effect on people’s lives and the health of the environment. They are a major source of pollutants that cause urban air and water quality problems, and also contribute to global climate change. According to a 2004 estimate by the United States Department of Energy (USDOE), carbon dioxide (CO2) emissions from buildings represent 38 percent of the total CO2 emissions nationwide. The USDOE expects this figure to increase by an average of 1.2 percent per year as more buildings are constructed and domestic population increases.

With respect to water quality, construction activities account for more water pollution incidents than any other industry, resulting from diesel, oil, paint, and solvents, as well as construction debris and dirt. Cleared land causes soil erosion, which leads to silt-bearing run-off and sediment pollution. Once absorbed by natural waterways, these substances poison water life and any animals that drink from them, including humans. Regarding energy consumption, 2004 USDOE estimates show that residential and commercial buildings consume 39 percent of the total energy needs of the country, with 71 percent of that energy in the form of electricity. Lastly, typical building construction projects in North America produce up to 2.5 pounds of solid waste per square foot of floor space.

These facts make it clear that there is a pressing need for environmentally sensitive buildings. Green buildings offer substantial reductions in materials, water, and energy consumption in addition to significantly limiting the development of the land they occupy. Green buildings also provide advantages that go far beyond the environment: reductions in operating costs, enhanced marketability, as well as increases in occupant comfort and productivity to help create a healthier, more sustainable community. In short, the essence of green building is to design, construct, and operate buildings to maximize their environmental and economic performance, both inside and out. While a significant part of this is achieved through efficient operation and maintenance of a building’s systems, it is the smart and creative initial layout and building design that ultimately take full advantage of building green. This is when structural engineers and architects can impact a project the most.
Many tangible examples of the green building advantage abound. For example, the Denver Dry Goods building was able to reduce operating costs by $75,000 per year by implementing energy-efficiency measures. Waste management costs were reduced by 56 percent and 48 tons of waste was recycled during the construction of a supermarket in Spokane, Wash. Hence, green building has economic, environmental, and social aspects that benefit all those involved, and to a greater extent, those who are not.