Case Study | 9.10.2015
Blast Analysis and Design of Light Gauge Steel Trusses in DoD Projects
Tennessee Air National Guard
US Marines, Quantico
Light Gauge Steel Trusses (LGST) are one of the most commonly used roof systems in current construction due to their light weight, long-span capabilities, design flexibility (many slopes and geometries), and fast erection times.
Starting with the 2012 version of UFC 4-010-01, “DoD Minimum Antiterrorism Standards for Buildings”, all roof systems must be shown to provide an adequate Level of Protection (LOP) against the applicable blast loads. Table 2-3 in the UFC 4-010-01 provides a list of “approved” roof systems that, when used in combination with the appropriate Conventional Construction Standoff Distance (CCSD), do not require blast-resistant design. Any other roof system not included in Table 2-3 must be analyzed or tested against the project applicable blast loads and shown to provide an adequate LOP. LGST are not included in Table 2-3 of the UFC 4-010-01. Therefore, to use a LGST roof system in a DoD project, the system must be analyzed or tested against the project applicable blast loads. This presents a design challenge for DoD projects due to the lack of simplified analysis methods and response criteria for blast. In addition, LGST are often a delegated-design (DD) item. This typically requires fabricators to design their systems to a specification after the design phase has been finalized and, in many cases, construction has started. As a result, if the blast-resistant design of the LGST is not promptly and properly coordinated, it could have significant impact on cost and schedule.
On recent efforts for the Tennessee Air National Guard (ANG) and for the U.S. Marines at Quantico, project construction was ongoing and Protection Engineering Consultants (PEC) was brought on board to perform blast-resistant design of several LGST for compliance with the UFC 4-010-01. A fast turnaround was needed to minimize impact on schedule and budget. Our engineers used their extensive experience with nonlinear dynamic analysis (NLD) and response of structural systems subjected to blast loadings to develop a simplified analysis method. NLD analyses were performed to determine failure modes and an acceptable response limit that satisfied the criteria for Low Level of Protection (LLOP) in the UFC 4-010-01. Our engineers were able to demonstrate that with minor connection improvements, the proposed LGST were adequate to resist the blast loads while providing the required level of protection. For more information on blast analysis of LGST for your next project, contact Marlon Bazan, Greg Rolater, or Aldo McKay at PEC.