May 9, 1997
Post-tensioned concrete for today's market
Post-tensioned concrete was used at the Redmond Town Center project.
Concrete is becoming an increasingly more efficient building material
By CRAIG D. OLSON, P.E. and LAURA N. SMITH, P.E.
When selecting a structural building system, it is important for the engineers and architects to understand the appropriate application of post-tensioned concrete and the effects that may result. If properly analyzed and assembled, concrete structures from high quality materials can provide a superior combination of durability, sound control and fire safety needed in today's building market. Considering the current market factors of cost options, material supply and lower floor-to-floor heights, and available developer financing, concrete is often selected as the more cost effective material over steel.
It is important to understand what post-tensioned concrete is and how it works. Concrete is very strong in compression and weak in tension, while steel is very strong in tension. Since the framing members in a building must resist loads through a combination of both compression and tension forces, concrete framing is reinforced with steel. Post-tensioned concrete seeks to mitigate concrete's natural weakness in tension by imposing a permanent compression load on the structural members. With this type of concrete system, high-strength steel tendons, in combination with reinforcing steel bars, are embedded and anchored in the concrete. When the concrete has acquired adequate strength, usually three or four days after placement, the tendons are tensioned, (stretched, like rubber bands) imposing a compression force on the concrete. These tendons remain stressed throughout the life of the structure, counterbalancing future tension loads. The presence of these tendons, in combination with conventional rebar reinforcing, enables the engineer to design shallower structural members with increased load capacities and less deflection.
Designers and engineers typically will choose post-tensioned concrete because of low material costs and availability, flexible column spacing and structural depth, and demanding construction schedules. This concrete system is also an efficient choice when issues regarding reduced sound and vibration, as well as future flexibility arise, as seen in residential, office and mixed-use facilities. However, the choice to use post-tensioned concrete is conventionally used when these building issues become a concern to the building design team.
With the high cost of imported structural steel, along with and the increased demand for structural steel in Asia, steel is at a premium in today's building market. The high cost of steel is also partially due to a decrease in the production of American steel. Because of the fluctuating market price of steel, concrete is becoming an increasingly more efficient building material. Northwesterners are fortunate that local concrete suppliers consistently produce a high-quality, strong concrete product at competitive prices. The industry continues to invest in research, testing and quality control in order to create a better product. With consistent testing and development, engineers are breaking new ground regarding concrete building systems. Such systems as post-tensioned ductile frames and flat-plate column slab joint systems, allow building designs to meet demanding building codes, while simultaneously providing a flexible, safe building. Post-tensioned concrete building systems are effectively addressing many of the design constraints presented by today's building designs.
The depth of the structural members depends on the column spacing and on the weight of the applied loads. Post-tensioned concrete is typically used for moderate to long spans with moderate floor loads, as seen in parking and residential buildings. In these situations, post-tensioned concrete framing may result in shallower depths than steel framing.
Construction schedule is often the driving force behind a project. Post-tensioned concrete buildings typically can be constructed quickly, due to the quick strengthening properties of the material. Form work in relatively simple successive floors can be built within a week of concrete placement. In conventional reinforced concrete, construction of successive floors must often be delayed until the concrete has gained enough strength to support its own weight, often 14 to 28 days. Steel buildings, well known for their fast speed of erection are currently penalized by extremely long delays in steel delivery. When all factors are considered, post-tensioned concrete can yield the quickest overall construction schedule.
For many buildings, such as residential, office and mixed-use, sound and vibration issues are common. In general, the ability of a structure to attenuate or deaden sound and floor vibrations is proportional to the mass of the structure. The massive state of concrete buildings allows for a more effective deadening of noise and vibrations. Therefore, concrete building systems generally are the more favorable system to address vibration and acoustical issues.
Another building issue to examine is the frequency of remodel and tenant improvements. When a building is remodeled, walls are moved and holes are cut in the floor slab to accommodate revisions to the wiring and plumbing. New adaptations are made to the building for changing lease arrangements or new architectural functions. For these reasons, post-tensioned concrete buildings are more difficult to remodel because the reinforcing steel is embedded in the concrete. This makes it difficult to locate the reinforcing and creates serious, costly problems if the post-tensioning tendons are cut. However, several things can be done to improve the future flexibility of a post-tensioned building. The location of the post-tensioning tendons can be marked on the under side of the slab, future stair block-outs can be added to the building during the original construction and sleeves for future electrical penetrations can be cast into the floor slab.
Post-tensioned concrete has been successfully used on a number of buildings. However, choosing a structural system can be a complicated process. Engineers must research all of the design issues presented by a building, calculate the loads and decide on the appropriate structural system for its intended use.
Post-tensioned parking garages can have economic lives of over 60 years. When designed for durability, actual facility life can be longer. A post-tensioned floor system in a garage is designed to control the entry of harmful moisture and contaminants. With the appropriate weatherproofing, post-tensioned concrete systems also protect the reinforcing steel. Steel frame parking garages typically require continual maintenance due to exposure to nature's elements.
Post-tensioned concrete at work
Post-tensioned concrete was selected for the two Redmond Town Center parking structures currently under construction. These two-and three-story above-grade structures have a combined size of approximately 800,000 square feet to provide parking for 2,500 vehicles. In order to efficiently work with the parking planning module, the team selected a layout where the slabs span 20 feet and the beams span 60 feet. The engineering team recommended post-tensioned concrete floors because of the modular slab/beam spans combined with relatively light floor loads, the lack of large penetrations for mechanical and electrical systems and a low probability of future remodel and tenant improvements. The post-tensioned concrete minimized the expense of form work and resulted in meeting the aggressive construction schedule. Currently, two additional post-tensioned concrete parking structures are under design for the AT&T Wireless Campus at Redmond Town Center.
Architects today are creating high-rise "living spaces" which include many of the amenities and spaces found in detached homes. To achieve this, buildings require a floor slab span of 20 feet or more, typically achieved in the past by using thick reinforced concrete flat slabs. As an alternative, post-tensioning, with its long thin slabs, can offer architects and designers a new design flexibility by reducing the number of columns. Typically, a building with two-way slabs requires will require more weight of reinforcing, where as post-tensioning is a lighter system, requiring less reinforcing steel to achieve the same results. A lighter weight structure will reduce crane hoisting loads, form work sizes, column sizes and the size of the shear walls. Lighter structures combined with long, clear spans and increasingly waterproofed slabs, make post-tensioning a very desirable system in high-rise construction.
Mixed-use structures, facilities which provide multiple tenant functions at an economical cost, demand the examination of such issues as continuous remodel and sound/vibration adaptations, while respecting the building code. The current redevelopment of the King Street Station area has yielded plans for several new mixed-use facilities, such as the King Street Center. This nine-story, 450,000-square-foot building which accommodates office and retail space on the upper floors and parking space on the lower floors illustrates advantage of post-tensioned concrete systems. Located in the historic Pioneer Square area of downtown Seattle, the building must meet the historic district building codes, particularly the restrictions regarding building height. The need for long spans on the lower parking floors and the need to minimize the story-to-story height, led the design team to recommend a post-tensioned two-way floor slab system for the office, retail and parking floors. The post-tensioned system will also minimize the long-term flexibility needs of the building.
The NikeTown project in downtown Seattle is a mixed-use retail/restaurant building consisting of three levels of parking above three levels of retail space. The engineering team used a post-tensioned floor design to minimize the number of interior columns, optimizing both the retail spaces and the parking spaces. In addition, the architectural vision of projecting angular slab edges was achieved by cantilevering the post-tensioned slabs and by creatively detailing the slab edge conditions.
At times the best structural system for a building is a combination of steel and concrete construction. The Bellevue Galleria Project, a new high-profile mixed-use retail/theater/restaurant center on downtown Bellevue's pedestrian corridor, consists of post-tensioned concrete framing on the lower two parking floors and steel framing on the upper commercial floors. The long spans of the theater roof lead to the use of steel roof framing, while economic considerations and a desire to limit the excavation depth and the height of the lower levels lead to the selection of post-tensioned concrete below-grade.
Many office complexes have benefited from the advantages of post-tensioned concrete systems. The new AT&T Wireless Services Corporate Headquarters in Redmond is composed of seven office buildings ranging from three to five stories tall. There are three stand-alone parking structures. Post-tensioned concrete floor framing was used for all of the office buildings and for the parking structures to meet a rapid construction schedule and minimize structural costs. This concrete system also allowed for a high quality, flexible office space for a single tenant.
Future uses of post-tensioned concrete may include the use of post-tensioned concrete ductile frames, a system which KPFF investigated for a major new downtown hotel. These frames may improve the seismic response of mid-to high-rise buildings, an imperative building consideration to developers and A/E firms here on the West Coast. In addition, creative use of floor slab block-outs and thoughtful consideration of future remodeling needs of building tenants will make post-tensioning even more attractive.
Over the past 10 years, there has been an increase in the construction of post-tensioned concrete buildings in the Puget Sound area. By continuing to expand the uses of post-tensioned concrete, the building design and development community will continue to push the innovation and flexibility of these structural systems. Post-tensioned concrete will continue to be a viable and cost effective building alternative for many builders due to the healthy supply of concrete materials here in the area. The best structural material and system can only be selected when owners, developers, architects, contractors and engineers work together to select the best structural system based on the functional and economic aspects of each individual project.
Craig D. Olson, P.E., and Laura N. Smith, P.E., are both licensed structural engineers with KPFF Consulting Engineers in Seattle. KPFF Consulting Engineers provides structural and civil engineering services.
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