Emergency Roof Stabilization, Tustin Hangar 1
- Project Location:
Hangar 1 (Building 28), Former MCAS Tustin, Tustin, CA
- Project Description:
In early October 2013, Hangar 1 at the former Marine Corps Air Station (MCAS) Tustin experienced a partial roof collapse. Originally constructed in 1943, the hangar housed blimps that were used for Pacific Coast reconnaissance during World War II. At 300 ft wide at the base, 1060 ft long, and 178 ft tall at its peak, it is a massive structure.
Due to a shortage of available steel at the time of construction, Hangar 1 was constructed primarily of wood. The roof is supported by 51 reverse-catenary arched trusses spaced at 20 ft on center, with multi-ply dimensional lumber chords and webs, ranging in size from 3x8 to 4x16. Hangar 1, along with its sister, Hangar 2, was listed on the National Register of Historic Places in 1975, and was named a National Historic Civil Engineering Landmark by the American Society of Civil Engineers (ASCE) in 1994 with the distinction of being the “Largest Wood Frame Structures in the World.”
MCAS Tustin was closed as part of the Base Realignment and Closure (BRAC) process in 1999; however, Hangar 1 is still owned by the Navy pending completion of ongoing groundwater remediation activities.
During the partial roof collapse, the top four panels of the three northern-most roof trusses fell to the floor, creating an approximately 50 ft wide by 70 ft long hole in the roof and leaving the remaining portions of those three trusses cantilevered from their bases on each side. As a result of the partial collapse, the building was red-tagged and no one was permitted to enter.
Naval Facilities Engineering Command, Southwest (NAVFAC SW) awarded a task order under their Global Contingency Construction Contract to Kellogg, Brown, and Root, Services Inc, (KBR) and their design partner Jacobs Engineering (Jacobs)/Michael Baker International (Baker) Joint Venture (JV) to design and construct a system that would stabilize the remainder of the roof at the collapsed area. Given any delay increased the probability of further collapse of the remaining structure, time was a critical factor, in addition to safety in contract priorities; a 90-day period for stabilization was required by the Navy as further discussed below.
The Jacobs/Baker JV designed an innovative stabilization system utilizing steel cables that anchored to the four northern-most trusses and that were vertically supported by 190-ft tall modular steel towers placed on each side of the hangar. Four steel cables on each side of the hangar acted as guy wires, connecting to and supporting the top of each steel tower to resist horizontal movement. At its base, the steel tower was supported by a concrete raft foundation with the guy wires connected to helical piers installed in the soil. The designed stabilization system allowed all construction activities to occur from the exterior of the building.
In performing the design for the project, the wood structure of the hangar was evaluated in both the pre-collapse and post-stabilization configurations, ensuring that the loading imposed on the structure by the stabilization system did not substantially exceed what the building had experienced in the previous 70 years. Detailed three-dimensional structural models were developed for both configurations and were used to design the new components of the stabilization system, including the steel cables, steel towers, foundations, and all component connections, such as those of the cables to the existing wood trusses.
All of the construction activities that involved connection to the remaining roof structure, the demolition necessary to install those connections, as well as demolition of remaining dangling elements of the roof structure had to be carried out from work baskets suspended from one of two cranes on site, each approximately 300-feet tall. Under such challenging work conditions, safety of the workers was paramount, and KBR was diligent in taking the precautions necessary to ensure that the work was performed in as safe a manner as possible. Compounding matters further, the original roofing felt was classified as an Asbestos Containing Material (ACM), which necessitated further safety requirements to limit worker exposure, and to dispose of the demolished materials in a proper fashion.
Displacement monitoring of the building was a unique component of the project. NAVFAC SW, KBR, and the Jacobs/Baker JV agreed that a system capable of tracking the displacement behavior of the remaining wood structure and providing warning of any potential impending failure was an important safety measure. After the steel tower and cable stabilization system was in place, but before the steel cables were tensioned to their design loads, a surveying base station on grade and seven prisms located on the hangar roof above the northern-most seven roof trusses were installed on each side of the hangar. Prisms for displacement monitoring were also installed on the top of the modular steel towers. The surveying system took readings of the location of each prism every 15 minutes and all data was immediately posted to a website, where team members could carefully review data.
During initial cable tensioning, movement readings were carefully monitored to ensure that the stabilization system was performing as expected. Cable tensions were slowly increased, raising the building roof back towards its original pre-collapse location. Tilt meters were installed on each side of the roof opening on the second truss after stabilization was completed. These tilt meters provided continuous data acquisition and had alarm levels pre-set such that if a certain rotation was exceeded, team members would immediately receive a text message or e-mail notification of the movement. With all the data from these systems stored on the website, behavior of the building over time could also be monitored. In this manner, the design team could watch over the weeks following stabilization while the trusses with the missing crown portions were slowly pulled back up towards their pre-collapse location by the action of the tension in the steel cables. These displacement monitoring systems provided an additional safety measure once building access was later permitted, by giving occupants a real-time warning should significant movement occur.
- Project Justification:
This project was far different than the typical building design and construction projects that make up the majority of our industry. Due to its unique properties, the approaches and constraints of everyday design had to be temporarily put aside to make room for brainstorming and more creative solutions that would accommodate the unusual requirements and fast schedule of the project. During the proposal process, the design and construction teams had to collaborate extensively, identifying approaches to stabilization that would be structurally sound, could be constructed in a fast and safe manner, and that utilized materials that were readily available and did not require long lead times for acquisition and fabrication.
Time was of the essence on this distinctive project. The RFP was originally issued the week of Thanksgiving in November, 2013, with the contract awarded on Christmas Eve, December 24, 2013. Because the intent of the contract was to have the stabilization system in place as quickly yet safely as possible, design activities and early material source identification had to start immediately upon award, despite the holiday season. During the early design process, daily conference calls occurred between the Baker structural engineers and their partners and quality control reviewers at Jacobs. These calls established design standards, design approaches, modeling protocols, and laid out clear expectations as to the timing and quality of the design submittals. This collaboration between design firms led to a high quality final product. While Baker engineers performed the majority of calculations for the design, the Jacobs engineers provided invaluable contributions by acting as a sounding board for proposed design and construction approaches, by suggesting alternative design and modeling procedures that they felt would better reflect the real-world demands on and behavior of the structural systems, and by thoroughly reviewing each design submittal to ensure a quality product prior to delivery to NAVFAC SW. The efforts of Baker and Jacobs during this phase led to every design submittal being delivered on time, meeting the aggressive project schedule set at kickoff.
Due to the suspected instability of the remaining structure and the high possibility of further collapse, design and construction approaches that required access to the interior of the structure were immediately rejected during the proposal phase. Also rejected were any structural systems that would require long lead times for material acquisition, fabrication, and installation. This led to the utilization of modular steel towers, the type normally used to support tower cranes, as the primary vertical system on the exterior of each side of the hangar to support steel cables. These tower modules were a natural choice for the tight schedule because they could be purchased already fabricated with published available loading capacities, and because they could be easily and quickly erected upon delivery to the project site. During the proposal phase, KBR verified the availability of these components, and had a vendor identified for when the contract was awarded. The modular steel tower and steel cable system provided a solution that could be installed quickly and that did not have a high sensitivity to small changes in installed dimensions. Therefore, the system had the flexibility to accommodate unforeseen variances in the existing conditions of the hangar and site from what was originally assumed for design.
Once the use of steel cables was determined as the final system choice, KBR assigned a safety manager with over 20 years of experience on projects involving cables and rigging. He was available for consultation with the designers during the design phase and provided vital input as a reality check on the constructability of the proposed cable system and components, making suggestions as to preferred types of components and connections. This close coordination between KBR and the design team improved the safety and constructability of the design solution that was eventually built.
Efforts of the government stakeholders working closely with the design and construction team were also vital in delivering a superior solution in a short time frame. NAVFAC SW allowed submittal of an early design package covering only the modular steel tower system and its foundations, the review and approval of which permitted KBR to proceed with the purchase of the steel tower modules and preparation for the foundation construction prior to the government receiving the final design package. Government technical reviewers provided a fast yet comprehensive review of all design packages and communicated with designers, facilitating an efficient question-and-answer process that sped up the overall review periods. The review meetings during the design phase were attended by a range of government stakeholders, which allowed the design and construction team to take into account the various priorities that those stakeholders brought to the project. Overall, the review meetings exemplified team collaboration, as all parties were working towards a superior stabilization solution, constructed as quickly as possible without compromising quality or safety. NAVFAC SW also had personnel on site almost every day during construction to monitor progress and to work with the team to expedite approvals for challenges or roadblocks that emerged during that phase.
One additional area that required collaboration with and fast-tracking by the government was the installation of the foundation elements that supported the cable guy wires at grade. It was decided during the proposal phase to utilize helical piers for these foundations. Because the project schedule was so critical, the time necessary to perform a traditional geotechnical study was lacking. Historic geotechnical records from 1997 were used to develop a sub-surface understanding and preliminary geotechnical design. Based on that information, test anchors were designed and were drilled to demonstrate the foundation performance. Four anchors were installed at various depths and tested each for strength and displacement. The real-world test was combined with the engineering data from historical reports for a consolidated anchor design.
Each guy wire has two helical piers at its base, providing a high level of redundancy and safety. Additionally, the team had to be cognizant of the aforementioned groundwater plumes and the Navy’s ongoing remediation efforts. Through collaboration with KBR, the Navy, and their environmental consultant, piers were designed with a detailed understanding of the underground plumes. The result was significantly less risk to the environment and in the end no net change the existing cleanup strategy.
A final area of significant challenge on the project was determining the in-situ configuration and condition of the hangar structure post-collapse, so the stabilization system could be correctly designed to interact with and support the remaining roof. As access to the interior of the building was prohibited until the stabilization system was in place, the designers had to rely on photos, plans, and previous evaluations of the hangar to draw conclusions as to existing conditions. Shortly after the roof collapse, NAVFAC SW engineers had briefly entered the building and taken photos of the remaining structure at the north end of the hangar, including the intact structural elements and those that were suspended from the edges of the new opening. These photos were provided to and closely studied by the design and construction team to glean as much information as possible. As for drawings, the City of Tustin has in its possession a large library of drawings for the hangar. Not only are numerous copies of the original design drawings from 1942 housed in this library, but as-built drawings from 70 years of repairs and renovations to the hangar are also represented. Finally, over the years both the Navy and the County of Orange had commissioned numerous reports on the condition of the hangar and its potential reuse after the BRAC process. These reports were made available to the design and construction team and provided relevant information as to the extent and timing of previous alterations made to the original hangar construction. It was a significant effort to sort through the voluminous available information and collect the findings that were relevant to the design and construction of the stabilization system. Despite the best efforts of the design and construction team in this area, some of the as-built conditions that were revealed once construction began did differ from assumptions made during design. When that occurred, the designers and KBR quickly and successfully modified the design to fit the existing conditions.
- Special Circumstances:
With such a distinctive project and such a short performance period, the only way to bring it to successful completion was for all stakeholders to work as a team and to commit to expediting all efforts as much as possible. As previously mentioned, during the design phase daily communication occurred among design team members and between the design team and KBR. Jacobs/Baker and KBR were collaborating every day on design and construction decisions and were constantly making adjustments and incorporating new information in response to recommendations from one another. During the construction phase, the designers still played an integral role, being on site during the most critical construction steps and fielding daily calls that included progress reports and answering questions from the field. The design and construction process was collaborative from beginning to end.
NAVFAC SW was also an integral player in the successful design and construction of the stabilization project. While they were the client of KBR and Jacobs/Baker, they were willing to work in a collaborative fashion to provide information to the project team and to help overcome some of the normal obstacles of permitting and scheduling that are inherent in a project with so many stakeholders. NAVFAC SW worked hard to expedite reviews of design submittals and was willing to talk through design and construction challenges to come to a solution that was mutually agreeable to both them and the project team. The structural engineers of NAVFAC SW were particularly engaged on the project and several conference calls were held that included the structural engineers from both Baker and NAVFAC SW to discuss concerns and solutions to technical issues in the design approach. This project is an outstanding example of the success that can be achieved when the government partners with construction and design firms in the private sector and leverages the strengths of people in both the public and private spheres.
The efforts of the design and construction team were recognized by the government in the interim “Contractor Performance Assessment Report” (CPAR) that was issued prior to closure of the contract. The design portion of the project received an “Exceptional” rating, which is rarely awarded. The interim CPAR specifically mentions the teamwork that was essential to making the project successful, stating “Contractor did not have any conflicts between anyone of the project team and maintained the same team throughout the project duration. Partnering between the contractor, their two subcontracted A/E design firms, construction subcontractors and the Government was always positive.”
- Project Attachments:
Please see paragraphs above and addition pictures below.
- Award Citation::
Rapid responsiveness, flexible and innovative solutions, and the utmost focus on personnel safety are hallmarks we strive for on every project. The MCAS Tustin Hangar Stabilization exemplifies these strengths of the public and private sectors working together toward a common goal of stabilizing this historic structure.
- Suggested Award Summary:
While every project is unique; it’s not every day you get an opportunity to work on the world’s largest wood structure. KBR and the Jacobs/Baker JV joined to support NAVFAC SW’s need to stabilize the 1000 foot long, 300 foot wide, and 180 foot tall WW-II blimp hangar.
In October of 2013, the north end of the topmost roof section partially collapsed creating a 50 foot by 70 foot hole in the roof. Because the remaining structure was deemed unsafe for entry, the team needed a stabilizing solution built entirely from the outside. Additionally, NAVFAC desired to enter the facility as soon as possible. The design solution would require using readily available materials and construction techniques with minimal on-site labor.
The KBR/Jacobs-Baker team held a brain-storming session where numerous ideas were discussed. Ultimately the solution was found by using remnants of the economic down-turn. Tower cranes were suggested as a potentially quick method to erect a shoring tower. The supporting towers are strong, large, flexible, rapidly assembled, and readily available. We proposed use of cable rigging and helical piers to stabilize the arches and cranes towers because they could be installed just as quickly as the towers. Virtually all of the necessary components were “off the shelf” items and could be on site in a few weeks.
KBR then turned to Mr. Crane for an outstanding rigging crew to construct the towers and rigging. The rigging crew had experience doing ACM demolition, selective demolition while suspended from a crane, and extensive rigging experience. KBR and Mr. Crane worked closely together to choreograph the stabilization while suspended from a 300 foot tall crane. Each step was meticulously monitored using the latest automated surveying techniques for building stability and personnel safety. Ultimately the project was completed with zero safety incidents and outstanding construction quality.