By: James Chavanic, Candidate BAE/MAE Architectural Engineering (Structural) 2013, Pennsylvania State University
Pittsburgh International Airport Parking Garage
Precast Double-T Structural Defects
Pittsburgh, PA (1998)
Parking Garage, Precast Concrete, Double Tee, Carbon Fiber, FRP, Shear Failure, Performance Failure, Dapped, Prestress
The short-term parking facility at the Pittsburgh International Airport was built in 1992 by the Mosites Construction Co. of Pittsburgh, Pennsylvania. Owned by Allegheny County, the $26.6 million garage is constructed of precast/prestressed concrete members. In 1998, the garage was load tested after severe cracking was noticed in the first eight feet of each end in many of the double-tee beams. The tests showed that the beams were not meeting the specifications of the construction contracts and needed to be repaired per request of the owner. Of multiple options considered for the repair, a carbon fiber reinforced polymer (FRP) wrapping method was chosen based on expected life of the repair and limited disruption of parking garage use. Minimal disruption of garage use was a large deciding factor in the final decision since nearly one-third (~700) of the parking spaces would be unusable during the repair. (Belko, May 1998) This equated to millions in lost parking fee revenue during the repair. After four months of working around the clock, the repairs were completed just in time for the holiday travel season. At the time of completion, this project was the largest repair in the world using FRP technology. (Anonymous, March 1999)
|Figure 2: Aerial View of Pittsburgh International Airport (Image Credit: Bing Maps)|
Located in Allegheny County Pennsylvania, the short-term parking facility at Pittsburgh International Airport is a 90,000 m2 (~1 million ft2), 3-story, precast concrete structure. (Nanni, October 2001, p. 3) Based on total floor area, this is about one-third the area of the Empire State Building. Rental car services are provided on the first level while the second and third levels offer public parking, resulting in 2,100 parking spaces located within the structure. (Pittsburgh International Airport, October 2012) The second and third levels of the garage are constructed of precast/prestressed double tees bearing on precast/prestressed ledger beams. Concrete with a nominal compressive strength of 31 MPa (4500 psi) was used to cast the double tees. (Nanni, October 2001, p. 4) At each end, the double tees are dapped (notched) to allow the total structure depth to nearly equal the depth of the supporting ledger beams. Spanning 18.5 m (~60.7 ft.), the double tees were intended to have an asphalt topping installed to protect the concrete from the daily vehicle traffic. Since the third floor would be directly exposed to the elements, the asphalt topping was specified as being slightly thicker at this level. Naturally, the double tees were designed to support this added load. Figure 3 shows the geometry of the typical dapped end of a double tee beam.
|Figure 3: Typical Double Tee Dap Geometry [mm] (Image Credit: “Strengthening Dapped Ends of Double Tees with Externally Bonded FRP Reinforcement” used with permission of Antonio Nanni, 11-2-2012)|
Instead of being fitted with the asphalt topping on the second and third levels, the garage received an elastomeric vehicular-traffic coating supplied and installed by Martin Products East Inc. based in Grafton, Wisconsin. (Sawyer, December 1998) It was determined that this coating should be sufficient for protecting the concrete double tee beams, thus negating the need for the asphalt topping.
Five years after the garage was built, many deficiencies were discovered in the elastomeric coating. While inspecting the garage floors to determine where waterproofing repairs were needed in the coating, concerns about the structure’s ability to carry the design loads had arisen due to noticeable patterns of cracking occurring at the re-entrant corners of the dapped ends on the double tees. See Figure 4 and Figure 5. The cracks were observed on nearly every double tee in the garage. (Gold, Blaszak, Mettemeyer, Nanni, and Wuerthele, May 2000, p. 2) Questioning the structural integrity of the double tees, the owner requested that load testing be performed in the winter of 1997-1998 to assess the structure. Baker Engineering, an independent engineering firm with no prior ties to the project, was then contracted to perform load tests on four of the double tee beams. After performing the tests, flexural shear cracking was found to occur at approximately 1.5 m (4.9 ft) from the dapped ends. (Nanni, October 2001, p. 3) While the cracks resulting from the testing did not occur where the initial cracks were observed, they were severe enough that the double tees were determined to fail at approximately 75% of their design capacity. (Gold, Blaszak, Mettemeyer, Nanni, and Wuerthele, May 2000, p. 2) The differences in the locations of the cracks are illustrated in Figure 4 below.
|Figure 4: Typical Crack Locations in Double Tee Beams (Image Credit: “Strengthening Dapped Ends of Double Tees with Externally Bonded FRP Reinforcement” used with permission of Antonio Nanni, 11-2-2012)|
|Figure 5: Typical Repaired Crack at Dapped End of Double Tee (Image Credit: “Relevant Field Applications of FRP Composites in Concrete Structures” used with permission of Antonio Nanni, 11-2-2012)|
Before the double tees were load tested, the owner had planned to spend about $2.5 million repairing the elastomeric coating and the cracks that were observed in the concrete. Then, with hopes that the tests proved the beams adequate for the design loads, the original asphalt toppings could be applied to protect the structure. However, with the double tees exhibiting a failure when loaded to only 75% of their design capacity, the firm that conducted the testing recommended that the double tees be strengthened before placing any additional loads on the structure. (Gold, Blaszak, Mettemeyer, Nanni, and Wuerthele, May 2000, p. 2-3) This work would triple the cost of the repairs!
At this high of a cost, legal fingers were pointed in every direction regarding who was at fault for the performance failures. Allegheny County filed complaints and took legal action against both Mosites Construction Co. and Martin Products East Inc., citing that the coating failed because it was applied improperly and on a defective substrate. (Sawyer, December 1998) Mosites and Martin then filed counter-suits against R.W. Sidley Inc. of Thompson, Ohio, the precast design-build subcontractor for the garage. (Sawyer, December 1998) After analyzing the load tests, the county said the cracks were “indicative of shear failure” and were attributable to “improper and/or inadequate prestressing, improper and/or inadequate bonding between prestressed reinforcing strands and concrete, improper and/or inadequate prestress at the end zones of the beams, improper and/or inadequate concrete, and/or incorrect placement of the hanger and/or reinforcing bars relative to the beams’ dapped ends.” (Sawyer, December 1998)
Many construction related factors were taken into consideration when determining the repair system that would be used to strengthen the double tees. Of these, construction duration, garage use disruption, disturbance of double tee cross section, and repair appearance were the front-runners used in the decision making. Construction duration to complete the repair was important because the high travel volume of the holiday season was quickly approaching meaning the operations at the airport would want to be as smooth as possible for the travelers. Considering the millions in revenue that the 2,100 spaces bring in for the airport, minimizing the number of unavailable spaces was crucial to keeping the parking garage as profitable as possible. Since the double tees are prestressed with high-strength steel tendons spaced throughout the area of the cross section, drilling through the webs or reducing the cross-sectional area in any way was unacceptable. During the repair and once it was completed, it was desired that the work not alarm or cause concern from any of the people using the facility.
Possible solutions to reinforce the double tees were external post-tensioning, steel plate and steel angle bonding, and externally bonded FRP reinforcement. (Gold, Blaszak, Mettemeyer, Nanni, and Wuerthele, May 2000, p. 3) The following are some advantages and disadvantages of the possible solutions pertaining to their application in this particular repair.
Check out this article from STRUCTURE Magazine for more on external post-tensioning.
Steel Plate and Steel Angle Bonding
Check out this article for more information on adhesive bonding of steel to concrete.
Externally Bonded FRP Reinforcement
After careful evaluation of the aforementioned factors, externally bonded FRP reinforcement was chosen as the best option for the repair. All existing cracks larger than 0.50 mm (0.02 in) wide would need to be epoxy injected before applying the FRP reinforcement wrap. (Nanni, December 2001, p. 10) The unidirectional FRP reinforcement was to be oriented in a 0/90 degree orthogonal fashion so as to provide strength in both of the primary directions. Figure 6 below shows the layers and orientation of the FRP sheets at the dapped ends of the double tees.
|Figure 6: FRP Reinforcement Layout (Image Credit: “Strengthening Dapped Ends of Double Tees with Externally Bonded FRP Reinforcement” used with permission of Antonio Nanni, 11-2-2012)|
Since the use of carbon FRP reinforcement was still relatively new at the time of this repair, repair strategies needed to be verified through testing of the actual structure or a replica mock-up of it. To ensure that the selected repair strategy was sufficient for carrying the design loads, load testing was carried out on two of the repaired double tees, one at the second floor and one at the third floor.
In order to pass the testing, the double tees had to successfully carry at least 85% of the design shear forces at the ends of the members where the shear forces are greatest. Figure 7 shows the device used to perform the testing on the double tees. The two members successfully passed the test and the repair work continued. In all, 16 beams had been tested, all of them meeting or exceeding the requirements.
|Figure 7: Load Testing Setup (Image Credit: “Strengthening Dapped Ends of Double Tees with Externally Bonded FRP Reinforcement” used with permission of Antonio Nanni, 11-2-2012)|
|Figure 8: Applied FRP Reinforcement Before Painting (Image Credit: “Relevant Field Applications of FRP Composites in Concrete Structures” used with permission of Antonio Nanni, 11-2-2012)|
As stated earlier, carbon FRP technology was still in its infancy at the time of this repair. Therefore, not much research had been done on the applications of the material making it difficult to choose for a repair when traditional methods that had years of proof were still readily available. However, the benefits of the carbon FRP reinforcement were simply too great to be overlooked. Carbon FRP was light weight, super strong, easily applied, and occupied minimal volume. Because of their light-weight and exceptional formability, FRP reinforcements can be quickly and easily bonded to even the most curved and irregular surfaces. The high strength-to-weight ratio of FRP composites makes them more structurally efficient than traditional strengthening materials. In addition, FRP composites are non-corrosive, nonmagnetic, non-conductive, and generally resistant to chemicals. (Belarbi, Bae, Ayoub, Kuchma, Mirmiran, and Okeil, 2011, p. 1)
Much has been done and researched in the use of FRP in general since the time of the repair at Pittsburgh International Airport. It has become much more widely accepted and is used as a repair medium in many different industries. An example of some of the recent research involving carbon FRP reinforcement can be found in this article.
A similar repair using carbon FRP was completed years after the repair at Pittsburgh International Airport. This repair took place on the two parking garages for Fountain View on the Plaza in Kansas City, MO. In addition to using CFRP laminates to remedy flexural deficiencies in the 6.5 inch thick, post-tensioned, flat plate concrete slab, this repair also utilized CFRP bars that are embedded into grooves cut in the slab. A more detailed report on the successful repair can be found here.
Deficiencies in the precast concrete double tees supplied for the project appear to be the clear cause in facilitating the need for this repair. Although choosing to not implement part of a design (such as the asphalt topping in this case) is typically never a good idea, the owner was probably thankful in this situation since the increased load may have just been enough to cause a more dramatic event or even worse, collapse. Although the double tees were not protected in the way they were intended to be, it appears that this was not a critical factor in the observed failures of the beams. For some reason, the double tees used in the Pittsburgh International Airport parking garage are not flared at their ends like most are. This “flaring” at the ends increases the cross-sectional area in the most critical shear zone of the beams. Had the beams been flared at the ends, enough shear capacity may have been present to prevent the beams from developing the severe shear cracking. There was some level of trust in the precast subcontractor to supply a product that was adequate for the intended use of the structure. Maybe the lesson to learn in this repair is to always check “the other guy”, no matter how specialized they are in their product or service.
Even though the short-term parking garage at the Pittsburgh International Airport did not collapse, the deficiencies present within it were a serious concern that called for immediate action. Significant visible cracking occurring in concrete at only 75% of its’ design capacity was simply unacceptable, especially in a parking garage where the structure is immediately visible to the public using the garage. While the beams may not have collapsed even when loaded to what they were designed for, the large cracks allowed plenty of room for corrosive salt water to reach the reinforcing steel during the winter months which surely would have caused even greater issues. Although expensive, the right call was made in repairing the garage.
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Belko, Mark. (May 19, 1998). “Repair Costs Mounting for Garage at Airport The Airport Parking Garage is Safe to Use, County Officials Stress”. Pittsburgh Post-Gazette, p. B-1.
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