Parking+Garage+Collapse+in+San+Antonio

San Antonio Parking Garage Collapse //Li Jiang, AE 537, PhD Candidate, Penn State University, Fall 2012 // toc
 * San Antonio, Texas - Feb. 14, 2011 **

Key Words:
parking garage, collapse, precast column, failure, grout, connections, instability

Introduction
At approximately 11:39 A.M. on February 14, 2011, the North Prow section of the UHS (University Health System) West Parking Garage in San Antonio, Texas collapsed during construction, with two construction workers injured (Figure 1). The six-storey parking garage structure is a precast/cast-in-place hybrid system with precast columns. On the north side of the garage, the North Prow structure is seperated from the main structure by an expansion joint thus limiting the collapse strictly to the partial garage construction.

According to the investigation by Walter P. Moore and Associates, the root cause of the collapse was the lack of grout in the precast column connections, both beneath the base plates and inside the splice sleeves. Other inadequacies were pointed out as discussed later in this document.



UHS West Parking Garage
Breaking ground in March 2010, the UHS West Parking Garage was a new construction project at the UHS Medical Center in San Antonio, Texas. The 10-story parking garage, which has the capacity of 3,000-plus-cars and was constructed by a joint venture of Zachry Construction, San Antonio; Vaughn Construction, Houston; and Layton of Phoenix, had been expected to be completed in the summer of 2011 ("Contractor Sounds", 2011). After approximately 11 months' construction, the North Prow section of the parking garage collapsed and the original planned completion date was consequently postponed.

According to the forensic investigation report by Walter P. Moore and Associates ("UHS West Garage", 2011), this parking garage structure is "a precast/cast-in-place hybrid system with precast columns supporting precast/prestressed beams and girders which are composite with cast-in-place/post-tensioned floor slabs." As the general contractor for the garage structure, Urban Concrete was responsible for "foundation installation, superstructure and precast design engineering, the precast fabrication, the precast installation and the poured in place concrete installation" ("UHS West Garage", 2011). The precast design and erection was in the contract with CEG Engineering and Precast Erectors respectively.

North Prow section of Parking Garage
The North Prow section of the parking garage structure is a 3X3 bay section (gridline O-R, 2-5, Figure 2) on the north side of the main parking garage structure. It has an plan area footprint of 16,000 sq.ft., which is approximately 13% of the plan area footprint of the entire parking garage. With an expansion joint, the North Prow section is separated from the main garage structure in terms of both gravity load and lateral load systems ("UHS West Garage", 2011). It has floors from level 4 to level 10. Twelve cast-in-place circular concrete piers of 15' height above ground were designed to connect and support the precast column members along gridline P-R (Figure 2). Every precast column has two members: lower member and upper member. They were designed to be spliced together at level 6 or level 7 ("UHS West Garage", 2011). Beams and girders were designed to be supported by column corbels (Figure 2).

Collapse
At approximately 11:39 A.M. on February 14, 2011 the North Prow section of the UHS West Parking Garage collapsed. According to the staff and witnesses statements, on the day of the collapse, nearly half of the North Prow structure had been erected but no floor slabs had been cast yet (Figure 3). More specifically, the precast column had been erected to the full height of level 10 between gridline O-R and 2-3; the lower column members had been erected along the gridline O-4 and O-5 (Figure 3). No beams or girders were connected to the finished structure between gridline O and P (“UHS West Garage”, 2011; Figure 3). Horizontal steel bracing was placed along the gridline O and P to provide the temporary support for the structure during the construction (“UHS West Garage”, 2011, Figure 3).

Just prior the collapse, workers were preparing the pier extensions to erect the precast column members along gridline 4 (“UHS West Garage”, 2011). One worker reported hearing a rumbling sound, and then approximately 120 employees working on the site were evacuated by construction supervisors ("Contractor Sounds", 2011). "Concrete columns fell atop vehicles, construction equipment and storage units leaving behind approximately 1,500 tons of fallen debris.." ("Contractor Sounds", 2011). Two workers, who were in the lifts at the time of collapse, were injured.

A surveillance camera nearby recorded the entire process of the accident. The video footage showed the collapse happened in two waves, with a segment of concrete columns and beams crumpling first. This video footage was later used by forensic exports as the evidence to investigate the cause of the collapse.

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Investigation & Cause of Failure
Walter P. Moore and Associates Inc., a Houston based structural and engineering firm,was requested by Zachry Vaughn Layton (ZVL) to perform an forensic investigation in order to determine the cause of the collapse. Four separate site visits were conducted: February 15-17, March 22-23, May 4, and July 6 (“UHS West Garage”, 2011). In addition to interviewing workers and witnesses who have been on the site at the time of the collapse, investigators also reviewed project documents and the security video of the collapse, tested samples collected from the rubble and built and demolished computer models of the structure at the time of the collapse. According to the final investigation report released by the UHS board of managers on August, 2011, two categories of problems were revealed: instability of the structure, and reduction of column joint moment capacity.

The major reason that was found to cause the collapse was the lack of grout in all column connections which contributed to the overall instability of the structure. As found by the forensic team, the shop drawings produced by CEG wrote clearly about the requirements of the grout installation: "the space beneath the column base plate is to be dry-packed with grout immediately after erection of the column;" "no more than two levels may be erected before column are fully grouted and that connections are to be completed as erection progresses" (“UHS West Garage”, 2011). Except the column-to-pier connections along gridline O which only require grout installed beneath the base plate, all column-to-pier and column-to-column connections in the North Prow require installation of grout both beneath the base plate and inside longitudinal bar splice sleeves in order to complete the connections (“UHS West Garage”, 2011). Installation of grout beneath the base plate (Figure 4) is critical to transfer compressive forces between structural members; installation of grout inside of the longitudinal bar splice sleeves is essential to transfer tensile forces across the splice plane. As both requirements of grout are satisfied, the loads will be fully transferred and the separate column pieces will act as an integrate and solid column, supporting the structure as planned. Although the requirements of grout had been identified specifically in the documents, the forensic team found no evidence to show the column connection had grout installed either beneath the base plate or inside of the splice sleeves in the North Prow section. As a result, the lower and the upper column members remained separate instead of functioning effectively as one solid structural member. Without the awareness of the situation, the temporary bracing plan for the North Prow was based on the assumption that all column connections would be fully grouted as the construction progressed. Thus, the temporary support by the steel bracing didn't help much in the collapse.



Additionally, other irregular conditions causing reduction of column joint moment capacity were found on site after the collapse (“UHS West Garage”, 2011):
 * Enlarged base plate holes: 4 of the 10 column-to-pier splices connected at the time of collapse contained at least one enlarged base plate hole;
 * stripped anchor bolt threads: column-to-pier connections were observed in the post failure condition to show anchor bolts with stripped threads;
 * Pier extension confinement ties: at least 2 of the pier extensions appeared to lack the #4 confinement ties that are to surround the anchor bolts and longitudinal bars spliced to the precast column above. The shop drawings specified that all pier extensions in the North Prow were to contain these confining ties;
 * Shim pack height: The height of some shim packs, used to support the self weight of the lower column members on top of the pier extensions, may have exceeded the elevation of the lower anchor bolt nuts. This condition would have allowed excessive rotation of the lower column member.
 * Modified C-050 Column: The lower column member installed at gridline Q-3 (column member C-050) was delivered to the site missing one of the three anchor bolts to connect it with the upper column member. The missing anchor bolt was to be post-installed using epoxy and temporary shoring was to be used to ensure that the anchor bolt did not experience load for at least 24 hours. However, daily construction reports did not indicate use of temporary shoring and load may have been applied shortly after installation of C-050.

Therefore, the lack of grout prevented the integration of column members through column-to-column and column-to-pier joints, which enabled the large rotations and displacements of structural members and consequently reduced the stability of the entire structure. Regardless of the reduction of column joint moment capacity causing by some irregular conditions, there would not be a fatal defect of the entire structure as long as the grout had been installed correctly.

Lessons-Learned
The collapse of UHS parking garage at San Antonio was not the first case of precast concrete construction failure due to lack of attention to grout installation. One of the major causes of t he Nov. 15, 2002 Fishers Place parking garage collapse was the lack of grout in the column-to-pier connection ("A Responsibility" 2003, Figure 5). However, the grout requirements doesn't seem to raise the attention of the industry. Looking through OSHA regulations, there's not a specific regulation about grout installation in the precast concrete construction ("Requirements", 1985). After 8 years, the same grout issue happened on the parking garage at San Antonio and caused the collapse of the North Prow section of the garage.

After the 6-month investigation, it was found that all design requirements had been clearly written and organized. As a result, the inspection and management of construction work becomes a critical part in the case. With experienced people (e.g. project engineer, superintendent, and forman) on site, thorough planning of the work that is planned for each day is very important to ensure the construction procedures and corresponding trades are correctly arranged; effective inspection of the work that is expected to be completed at a day is critical to ensure the construction work is correctly performed and will function as planned, and is a must for successive construction procedures and the entire project performance.

According to the forensic investigation report, the general contractor on the garage project was Urban Concrete, and its subcontractor Precast Erectors was responsible for the column work. Urban's engineer, CEG, designed the precast portion of the project. Since the collapse, steps have been taken to ensure all work was completed according to plan. Zachry Vaughn Layton representatives were observing all grouting, foundation and pier cap installation work as it's done and providing daily written reports ("Error cited", 2011). Project managers from Broaddus+Muñoz were confirming compliance before the next day's work could begin.

Other similar parking garage collapse cases can be found as Tropacana Casino Parking Garage and Pittsburgh International Airport Parking Garage.

Conclusion
On February 14, 2011, the North Prow section of the UHS (University Health System) West Parking Garage in San Antonio, Texas collapsed during construction, with two construction workers injured. According to the forensic investigation by Walter P. Moore, t he primary cause of the collapse of the North Prow section of the UHS West Garage was the lack of grout in the column connections, both beneath the base plates and inside the splice sleeves (“UHS West Garage”, 2011). Other issues were revealed in this investigation such as the enlarged base plate holes and the lack of confining ties in the pier extensions, however they would not be harmful to the entire structure if the grout had been performed correctly. Regulations for grout installation in precast construction becomes very important for the entire industry. Yet it had not been addressed by OSHA to date in their erection guidelines. Effective and thorough planning and management of construction work is a valuable lesson to be learned from this failure.