Magazine

Bonded Post-Tensioning System Minimizes Maintenance for Garages

John Crigler and Don Kline

To build a multi-level garage costs about $7,000 per space, according to the International Parking Institute. Even more staggering is the estimated $650 per year cost of simply operating the structure. Although these estimates are eight years old, the parking industry obviously has a vested interest in utilizing innovative techniques that provide not only efficient and cost-effective construction solutions, but also value in terms of life-cycle costs.
The VSLAB post-tensioning system from VSL, designed specifically to minimize such costs, was recently selected as the answer for Baltimore/Washington International Airport (BWI).
Baltimore/Washington International is one of the fastest-growing airports in North America. But growth creates the need for greater infrastructure, including parking availability. In 2000, the state of Maryland and the Maryland Aviation Administration announced a five-year improvement plan to upgrade the airport's functionality. The plan included a state-of-the-art parking garage that would consolidate the airport's eight rental car companies into one central location.
Selected to design the optimal solution for the BWI Consolidated Parking Garage were the structural engineering firm Walker Parking Consultants and the architectural design firm Michael Baker Architects.
The first priority, says Bill Reitner of Walker Parking, was to select the most advanced design and construction materials to promote maximized durability and minimized maintenance for the structure's projected life-cycle. "We selected bonded post-tensioned cast-in-place concrete to minimize joints and promote long-term durability," he says.
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. The feature that sets this project apart from other garages was VSL's innovative VSLAB post-tensioning system.
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. Benefits, as compared to rebar reinforced concrete, include greater clear spans for the same member depth, enhanced crack control, larger floor-ceiling heights (or reduced building height) and reduced maintenance.
A solution in the United States for more than 40 years, post-tensioning design for parking garages can be either an unbonded or bonded system. Unbonded tendons, comprising 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.
A bonded tendon, however, comprises multiple post-tensioning strands and, by design, forms a continuous bond along its length with the 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 and five strands are used in thinner members such as slabs. 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.
Bonded systems offer a significant design advantage over unbonded systems, and this factor leads to life-cycle savings. Another advantage of bonded post-tensioning is the inherent capacity to provide resistance to progressive collapse. The bonded systems also offer more flexibility in terms of structural modification for stairwell openings, utility access and future expansion.
Beyond longevity and flexibility, today's parking professional also must address both initial and long-term costs of materials and construction technologies. Five years ago, WDP Associates2 conducted a life-cycle cost analysis between bonded and unbonded systems in the construction of elevated parking decks and beams. 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 square feet of parking space -- more than 1 million square feet 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 elevated deck consists of a post-tensioned concrete one-way beam-and-slab system. Slabs (5.5 inches thick) span 20 feet between beams that in turn span 60 feet to girders or columns.
Building on the benefits of bonded post-tensioning, the 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 post-tensioning system worked particularly well with garage functionality requirements. 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-feet by 60-feet to allow more flexibility for the rental companies. Also, the floor-to-floor height was increased 19 feet in order to provide customers with the feel of an open structure.
The new BWI Consolidated Parking Garage saw its first business in November and now serves as a one-stop rental car center, freeing up more than 1,000 prime parking spaces in the terminal parking garage. But beyond the cars, the parking spaces and the more than 73,000 cubic yards 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 Maryland Aviation Administration with minimal maintenance.

Article Abstract from March, 2004




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