Post-Tensioning Parking Projects throughout the United States Showcase the Benefits and Acceptance of the Method|
By Theodore L. Neff, Executive Director of the Post-Tensioning Institute
Post-tensioning—an “active” reinforcement technique designed to counteract tensile stresses and deflections from external loads—is quickly gaining momentum in the concrete industry as a long-term, viable, cost-effective alternative to steel reinforcement. Nowhere is the shift in preference toward post-tensioning more apparent than in parking-garage construction. Millions of square feet of cast-in-place, post-tensioned parking structures are built in North America every year. An independent survey of parking garage construction in the United States for the year 2000 showed cast-in-place, post-tensioned systems were selected most often as a structural alternative to steel. The survey also revealed post-tensioned structures had a significant lead in market share in terms of both number of structures built and volume (number of spaces).
A number of the post-tensioned parking garages built today are constructed as free-standing structures, while others are constructed as part of hotels, condominiums, apartment and office buildings, and other facilities. The use of post-tensioning in parking structures offers several advantages, including initial and life-cycle cost savings, low maintenance costs, enhanced crack control, water-tight construction, smooth riding surfaces, lighting and security, fire resistance, aesthetics, reduced structural depth, longer spans, deflection and vibration control, seismic resistance, durability, and structural integrity. The project examples listed illustrate some of these advantages.
Commercial Confidence in Post-Tensioning
McKinney Place is a 13-and-a-half-story, 1,214-space parking structure that provides much-needed parking capacity in downtown Post-Tensioning Parking Projects throughout the United States Showcase the Benefits and Acceptance of the Method
Houston, Texas. The use of post-tensioning allowed for the long spans required of this parking structure design. The project site covered half a city block but was constrained by an existing building and three adjacent streets. Therefore, additional columns and a different structural system would not have been desirable. Initially, there were discussions about designing McKinney Place as a precast parking structure. However, once the owner was presented with the well-known durability advantages of cast-in-place, post-tensioned parking structures, he agreed that this design was the best way to proceed. The use of a cast-in-place post-tensioned structure provided the owner with a durable structure that was flexible enough to accommodate future tenant needs. The lateral load-resisting system uses frames in one direction and the ramps in the other, and the frames and ramps were designed accordingly.
Yet another example of post-tensioning success is San Mateo’s Main Street Parking Structure in San Mateo, Calif. The 159,000- square-foot garage, located downtown next to a commuter railroad right-of-way, included construction of a 380-space, five story (one below grade) parking structure, plus improvements to a pedestrian area between the garage and the Century Theatres Complex. The project also required the demolition of a 75,000- square-foot existing garage, construction of 3,500 square feet of ground-level retail space, installation of fire protection and security systems, and the removal and relocation of storm-drain sewer lines and other utilities located under Main Street between First and Third Avenues. The final construction cost of the garage came in at just over $12.2 million, which included adjacent street improvements (including the Main Street pedestrian area) and landscaping, demonstrating the cost effectiveness of post-tensioned garages.
By the late 1990s, a rapid increase in service at Baltimore Washington International Airport (BWI) led to growing concerns about parking availability at the airport. Subsequently, a five-year improvement plan was announced. The plan included provisions for a new state-of-the-art parking garage, which would consolidate the airport’s eight rental car companies into one central location. The new parking facility includes approximately 3.5 million square feet of parking space. More than 1 million square feet of the structure consists of elevated, post-tensioned, cast-in-place concrete.
Based on the recommendation of the construction team, the project’s owner, the Maryland Aviation Administration, chose a cast-in-place post-tensioned concrete system for the garage. This system was chosen based on the owner’s need for maximized durability and minimized maintenance over the structure’s projected life cycle. While standard commercial parking garages typically have 20- to 30-foot-wide by 54-foot-long bays, the column grid spacing for this project was increased to 60 by 60 feet to provide each car rental group with as much clear space at the bottom level as possible for staging and multi-directional traffic. Additionally, the floor-to-floor height was increased to 19 feet in order to provide customers with the feel of an open structure.
The facility holds 8,300 spaces and frees up more than 1,000 prime parking spaces in BWI’s terminal parking garage. More than 73,000 cubic yards of cast-in-place concrete were used to construct the foundations and superstructure during the two-year, fast-track construction cycle.
The city of Phoenix also has a car rental facility success story. The Consolidated Rental Car Center at Sky Harbor International Airport is designed to service a 40,000-car rental fleet – earning the designation as Arizona’s largest building. The structure’s position under the flight paths of one of the nation’s busiest airports imposed height restrictions on the project. At the same time, designers needed to make room for 5,600 covered parking spaces and nine freestanding rental car service centers.
From the air, the consolidated rental car facility resembles a person with outstretched arms holding a large ball. Designers created massive 61- by 61-foot bays supported by columns and beams. A 5-inch post-tensioned slab is used between the beams. Because of the project’s complicated design, large transfer beams were necessary to distribute the load. One large, 85-foot-long beam supports a large area with heavy bus loading. The 7-footwide, 6-foot-deep beam is heavily reinforced with 115 tendons. This $256-million project used 130,000 cubic yards of concrete, 10,000 tons of rebar and 760 miles of post-tensioned tendon.
Out of Sight and Mixed-Use Success
Sometimes post-tensioning’s success is simply below the radar screen, such as an underground-entry pavilion in Monterey, Calif. This unique project involved a multi-level, cast-in-place, post-tensioned concrete parking structure placed completely underground and concealed by the reinstallation of the original landscaping. Innovative post-tensioning design solutions maintained efficient use of the underground space and economy in the design.
The building is 202 by 275 feet in plan, with three below grade parking levels and a plaza at grade for a total of 220,800 square feet. Typical parking decks have 6-inch one-way concrete slabs that span 20 feet between 14- by 36-inch-long beams. The plaza framing follows this bay spacing, but with an 8-inch deck on 24- by 48-inch beams. Perimeter concrete walls provide soil and lateral support. Post-tensioning benefits such as shallow floor-to-floor heights and long column-free spans were critical on the parking levels of this project, making post-tensioning a logical choice. Reduced floor-to-floor heights in an underground building provide direct savings in excavation, temporary shoring and retaining wall costs.
Because post-tensioned construction is well-suited to variability in form, this project features custom beam sizes to maximize the load-carrying capacity of the members within the restricted height. Warped parking decks provide adequate drainage. Customized plaza level reinforcement supports the precise load patterns of fire-truck-accessible roads, several 45,000-pound magnolia trees and extensive planting. Post-tensioning also helped achieve architectural effects. A slender, post-tensioned, cantilevered frame marks the entry, and a post-tensioned walkway canopy will be added to the above-ground landscape. The thin canopy profile is achieved with a double cantilevered, post-tensioned form designed specifically for this site. The elegance of this canopy feature reflects the creative potential of post-tensioned design.
Down the road is yet another example of how post-tensioning proved to be the optimal solution. The Pike at Rainbow Harbor Parking Structure in Long Beach, Calif. links the vibrant nightlife of Pine Avenue and activity of the Convention Center with the serene beauty of the Aquarium of the Pacific and the colorful boats in Rainbow Harbor. The seven level parking structure provides 2,211 spaces. This 475-foot-long cast-in-place, post-tensioned concrete structure is designed in a “streamlined moderne” style, with metal panel accents similar to those used on the Aquarium of the Pacific Parking Structure located just across the street. At the east end of the structure, six geared elevators form a 140-foottall tower and foot-wide starburst, echoing the shape of the starburst light.
The Future of Post-Tensioning
Innovative projects such as these demonstrate the possibilities for post-tensioning in parking-garage structures are only beginning to be realized. As acceptance for post-tensioning continues to grow within this segment of the market, it is important to stay abreast of new trends, developments and guidelines for the technique. The Post- Tensioning Institute’s Design, Construction and Maintenance of Cast-in-Place Post-Tensioned Concrete Parking Structures is an excellent source of further information on the subject and provides a comprehensive reference for the design, construction and maintenance practices that will ensure
long term durability and minimize life-cycle costs of cast-in-place post-tensioned concrete parking structures. For more information, visit
Editor’s note: In this issue of Design Cost Data, Dale and Associates Architects, P. A. have submitted a post tensioned parking structure.
The Arts District Parking Garage, located in Meridian, Mississippi is featured on page 36.
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