Western Wood Structures received an inquiry from the Facilities Project Manager of Alaska Airlines in January of 2006. He contacted us because the Bowstring Trusses in their Anchorage aircraft hangar were failing. These trusses had been evaluated and “repaired” less than three years earlier and as it was presented to us “the previous structural improvements were apparently insufficient to prevent additional failures.”
The first step in meeting the needs of this client was to make a site inspection to evaluate the condition of the trusses and to map out their configuration as the original drawings were not available. Based on this initial inspection, we recommended shoring of the entire roof system be installed immediately. We then completed a structural analysis which enabled us to make the required repair recommendations and our proposal to perform these repairs was accepted.
The original hangar was built in 1954 and the hangar was added on to several years later. As a result, there were two types of Bowstring Trusses in this building – Arch Teco Trusses that were constructed from solid sawn lumber in a multiple leaf system and Tim Trusses which were constructed from Glulam. The repairs were extensive and included the following:
• All of the lower chords in the Arch Teco type trusses were replaced. The lower chords of all trusses were post tensioned using ½’ high strength cable to reduce the tension stresses in the wood members.
• All webs that exceeded an L/d ratio of 50 were stiffened by adding new structural members with screws and epoxy.
• The top chords of all trusses were strengthened by adding structural members to the sides with lag screws and epoxy.
• All web to top & bottom chord connections were strengthened using structural steel shapes.
• T-Struts were added to the lower chords of all trusses because the post tensioning cables placed the lower chords in compression.
From experience we know that Bowstring Trusses of these types have been affected by a series of changes that have rendered many of them as “dangerous members” as defined by International Existing Building Code which is further explained here. The factors described in this article contributed to the problems in this building but issues which were specific to this project added further. For instance, over framing was added which increased the dead load significantly. In addition, a drifting condition was introduced to the structure when new bay doors were added to the ends of the building. Finally, the snow load in Anchorage had been increased by code from 40 psf to 70 psf since the original building had been built.
Now that the repairs are completed, the trusses are in better condition than they day they were built and Alaska Airlines can count on the use of their hangar for many years.
Western Wood Structures was sought out for this project because of our expertise in this type of work and because we carry both an engineer’s license in Alaska as well as a contractor’s license. And it’s always rewarding to receive a nice letter from a client following the completion of a project.