According to industry associations, it costs about $10,000 to $12,000 per space to build a new multi-level garage. Even more staggering is the estimated $650-per-year cost of simply operating the structure - including lighting, cleaning, employees, elevators and gate equipment. Based on these estimates, there is little argument that the parking industry has a vested interest in utilizing techniques that provide not only efficient and cost-effective construction solutions, but also long-term value with respect to life-cycle cost. One solution, the VSLAB post-tensioning system used specifically to minimize life-cycle maintenance costs, was recently selected for one of the nation's most prestigious airports.
Located in the center of the metropolitan Washington, D.C., and Baltimore area and one of the fastest-growing airports in North America, Baltimore Washington International Airport (BWI) serves as a powerful economic engine for the region. With more than 52,000 people traveling through BWI each day in 2002, annual business revenues for central Maryland exceed $7 billion, with a significant portion of these revenues coming from rental car usage.
But, growth creates the need for greater infrastructure. By the late 1990s, this rapid increase in service led to growing concerns about parking availability at the airport. In 2000, the Maryland State Government and the Maryland Aviation Administration (MAA) announced a five-year improvement plan to upgrade the airport's functionality. The plan included provisions for a new state-of-the-art parking garage located less than two miles from the airport terminal that would consolidate the eight rental car companies into one central location.
The result of MAA's plan was the assembly of an expert team in Maryland to design the optimal solution for the new BWI Consolidated Parking Garage. Structural engineering firm Walker Parking Consultants and architectural design firm Michael Baker Architects were selected. According to Bill Reitner of Walker Parking, the first priority was selecting the most advanced design and construction materials to promote maximum durability and minimal maintenance for the structure's projected life cycle.
"After considering various options such as precast concrete and unbonded post-tensioned concrete," says Reiter, "we selected bonded post-tensioned, cast-in-place concrete to minimize joints and promote long-term durability."
The design consultants sought the expertise and proprietary technology of VSL, a firm specializing in the design, manufacture and installation of post-tensioning and special construction systems. And, while the project boasts a long list of accomplishments and accolades, including fast-track scheduling, preconstruction scale mockups, large-area concrete placements and architectural precast wall panels, the feature that sets it apart from other garages is VSL's VSLAB post-tensioning system.
POST-TENSIONING PRIMER
Post-tensioning is a technique used to counteract tensile stresses and deflections from externally applied loads. Unlike mild steel rebar, post-tensioning provides active reinforcement, accomplished by introducing a prescribed magnitude and distribution of internal loading using 7-wire, grade 270 ksi strand systems. Post-tensioned structures are known for their strength and durability. As compared to rebar-bearing concrete, benefits of post-tensioning include greater clear spans for the same member depth, enhanced crack control, larger floor-ceiling heights or reduced building height, less cladding, smaller foundations because of the reduced structural dead load, and reduced maintenance.
A solution implemented in the U.S. for more than 40 years, post-tensioning design for parking garages incorporates either an unbonded or bonded system. Unbonded tendons, comprised of single strands (monostrands) covered with a grease coating and enclosed in high-density plastic extruded sheathing, do not form a bond along their length in the concrete. The force in the stressed tendon is primarily transferred to the concrete by end anchors.
A bonded tendon, however, is comprised of multiple post-tensioning strands and, by design, forms a continuous bond along its length with the surrounding concrete slab, beam or girder. Bonding is achieved through cementitious grout that surrounds the strands. The grout acts with the duct that is encased in the concrete member to complete the bond path between the post-tensioning strands and the concrete member. Flat, corrugated HDPE ducts that house between two to five strands are used in thinner members such as slabs; whereas larger, round ducts (HDPE or galvanized metal) are used in beams and girders. Today, bonded post-tensioning systems are the industry standard for concrete bridge construction and are gaining popularity in the parking garage market.
PARKING GARAGE DESIGN
While monostrand post-tensioning systems are used in the majority of post-tensioned concrete building applications, bonded systems are becoming more popular with long-term owners for such projects as airports, hospitals, government agencies and universities. Approximately 3.5 million sq. ft. of bonded post-tensioning has been installed in the U.S. in building slabs since 1995, and this growth is attributed to the increasing interest in life-cycle economics.
Bonded systems offer a significant design advantage over unbonded systems, a factor that leads to life-cycle savings. This key design feature is related to the hardened grout as it locks the movement of the post-tensioning strands to that of the surrounding concrete. Hence, the force in a bonded strand is a function of the deformation of the surrounding concrete. This is the well-known concept of strain compatibility and internal equilibrium used in reinforced concrete design.
Another design advantage of bonded post-tensioning is the inherent capacity to provide resistance to progressive collapse. This may be especially important in the event of localized blast loading. Like mild steel reinforcement, a bonded post-tensioning tendon is capable of developing its force at a relatively near distance along its length. In the event that an anchorage fails or a strand is severed, the loss of tendon force is localized. The remainder of the tendon would retain its force at the development length away from the failure point, thus remaining functional. This functionality may be used in the design phase when planning for alternative load paths.
Bonded systems also offer several practical benefits, such as reduction of mild steel, particularly at the top of slabs. Minimizing steel content is especially important as most parking garage maintenance costs are due to repairs associated with spalled concrete and corroded rebar. Another benefit is complete encapsulation, since the strands are fully protected by cementitious grout, duct and surrounding concrete. The bonded systems also offer more flexibility in terms of structural modification for stairwell openings, utility access and future expansion.
BUCK STOPS HERE
Beyond longevity and flexibility, today's parking professional also must address both initial and long-term costs of materials and construction technologies. As such, WDP Associates conducted a life-cycle cost analysis between bonded and unbonded systems in 1999 for the construction of elevated parking decks and beams. Costs associated with initial construction and future repair expenses were used to compare the life cycles of the different systems for a 50-year expected life span. Several variables were considered, including estimated repair costs, concrete deterioration rate, time value of money and exposure to de-icing chemicals. The analysis indicated that substantial life-cycle cost savings are possible when a bonded post-tensioning system is selected in lieu of a traditional unbonded post-tensioning system.
FROM BONDED TO VSLAB
Recognizing the bonded post-tensioning system to be optimal for BWI, a design concept was selected for the 3.5 million sq. ft. of parking space - more than 1 million sq. ft. of which is elevated, post-tensioned, cast-in-place concrete. A one-elevated-level concept was selected to allow all rental car companies to operate from the same level. The structure is approximately 1,500 ft. long × 840 ft. wide and is founded on spread footings that are 3 in. to 6 in. thick × 16 ft. wide × 16 ft. long. The slab-on-ground is 6.5 in. thick (reinforced with welded wire fabric) with contraction joints at 20 ft. on center each way. The elevated deck consists of a post-tensioned concrete one-way beam and slab system. Slabs (5.5 in., thick) span 20 ft. between beams spanning 60 ft. to girders or columns. Concrete compressive strength (cylinders) at 28 days is 5,000 psi. Overall stability is provided by moment connections between the columns and beams and girders.
Building on the benefits of bonded post-tensioning, the VSLAB system provides total encapsulation of the strands using high-density plastic duct with watertight mechanical duct to anchorage couplers. Permanent end-caps (for both beam and slab tendons) are included to completely seal the anchorages. High-performance grout pumped through the tendons provides an additional layer of protection.
The VSLAB post-tensioning system worked particularly well with garage functionality requirements. While standard commercial parking garages typically have 20- to 30-ft. wide × 54-ft. long bays, the column grid spacing for this project was increased to 60 × 60 ft. to allow more flexibility for the rental companies. According to Walker Parking Project Manager Jason Gross, "The larger bay spacing was important because each rental group wanted as much clear space at the bottom level as possible to allow for staging and multi-directional traffic." Also, the floor-to-floor height was increased 19 ft. to provide customers with open-structure feel.
Strict control of stressing and grouting operations was required due to the large quantity of strand involved (more than 3 million linear feet) and the aggressive placement schedule. Slab tendons were partially stressed (10 kips/strand) on the day after concrete placement to help control shrinkage cracking; they were completely stressed once concrete test cylinders, cured under job site conditions, reached a compressive strength of 3,200 psi. Partial stressing of beam and girder tendons was not required. These tendons were fully post-tensioned when the concrete compressive strength reached 3,200 psi.
STRESSED TO THE MAX
Stressing records and load-cell testing were used to ensure the initial force after friction loss in each tendon was within ±7 percent of the calculated value. According to VSL Project Engineer Lee Siridumrongphun, the team estimated short- and long-term losses based on ACI 318 and other technical sources. He added that realistic estimates are especially important at service loading areas because overestimation of losses can result in excessive camber and horizontal movement. Load cell monitoring helped calibrate and further refine the friction calculation coefficients. The friction (µ) and wobble (K) coefficients used were 0.15 and 0.0015, respectively, and a special two-strand hydraulic ram was used to stress the slab tendons. This allowed both strands to be stressed simultaneously and resulted in reduced friction losses by 15 to 20 percent.
Most of the top mild steel reinforcement in the slab was eliminated because of the use of bonded post-tensioning. With the exception of the presence of top rebar required in the slab at the first interior beam support to accommodate larger bending moments at the end-span, the slab is virtually free of the top mild steel typical of such projects. This elimination will alleviate the concrete spalling that frequently occurs when exposed to harsh weather conditions over time. Additional savings were realized by the elimination of corrosion inhibitor admixture.
Another interesting feature of this project was concrete placement sequencing. The original design layout called for 93 concrete placements at approximately 10,000 sq. ft. each. Building on expertise and careful preplanning, the team opted to increase the placement size to 33,000 sq. ft. Accordingly, each of the 36 placements averaged about 1,100 yd. of concrete, reports Paul Barry of Facchina Construction. Because of the large volume, most of the concreting was scheduled for early morning hours to ensure steady concrete delivery and to take advantage of cooler temperatures during the summertime. This change allowed associated bulkheads and joints to be eliminated, decreased the project schedule, and streamlined the post-tension stressing process.
The garage saw its first business in December 2003 and now serves as a one-stop rental car center, freeing up more than 1,000 prime spaces in the terminal parking garage. But beyond the cars, parking spaces and more than 73,000 yd. of cast-in-place concrete used to construct the foundations and superstructure during the two-year, fast-track construction cycle, the project boasts a long-term, cost-effective solution for the MAA with minimal maintenance.
Prepared by John Crigler, P.E., senior vice president, and Don Kline, P.E., branch manager, both of VSL. Part of Hanover, Md.-based Structural Group, VSL designs, manufactures and installs post-tensioning and special construction systems in new applications and retrofit. Other sources for this article include BWI's website (http://www.bwiairport.com/) as well as the articles "VSLAB Bonded Post-Tensioning Life Cycle Cost Analysis," Whitlock, Darrymple, Poston & Associates, Inc., June 1999; and, "Estimating Prestress Losses," Concrete International, June 1979.
BWI Consolidated Parking Garage
PROJECT PRINCIPALS
Owner | Maryland Aviation Administration |
Program manager | Parsons Transportation |
Architectural design | Michael Baker Jr., Inc. |
Structural engineer | Walker Parking Consultants |
Inspection services | Parsons Brinkerhoff Quade & Douglas, Inc. |
General contractor | Facchina Construction Company, Inc. |
Post-tensioning contractor | VSL |
Rebar placement contractor | Cortes Brothers Rebar, Inc. |
| Concrete supplier | D&G Brice Readymix |