Publication: BIC Magazine
Blast-resistant modular (BRM) steelframed buildings are widely utilized in response to requirements of API RP-752/753 to provide a safe working environment in occupied buildings against the effects of explosion, fire and toxic gas ingress. BRM buildings range from single module structures to multimodule, multistory structures with floor areas over 10,000 square feet. This article examines several of the design issues for BRM buildings as presented in the American Society of Civil Engineers (ASCE) 2010 guideline, “Design of BlastResistant Buildings in Petrochemical Facilities,” to which I contributed. BRM buildings may be designed using dynamic structural analyses ranging from single degree of freedom to nonlinear transient dynamic finite element analysis (FEA). With continuous, fully welded connections, BRM buildings improve blast capacity by providing a high level of continuity. The designer should account for response modes, including tension membrane effects and plastic strain limitations, both of which can be more appropriately captured using a nonlinear FEA approach.
Structural components are designed to meet the desired acceptance criteria, usually expressed as high, medium or low response/ damage, as presented in ASCE 2010. Unlike light gage material, the walls in BRM buildings are usually three-sixteenths to fivesixteenths of an inch thick, often to a depth of 5 inches for onerous blast conditions. These wall panels exhibit greater levels of ductility and flexural capacity, coupled with a much lower propensity to buckle than light gage corrugated metal panels used in conventional metal-clad buildings. Vapor cloud explosions can result in relatively long duration events of up to 200 milliseconds, compared to shorter duration events associated with high explosives. Owners need to be aware of the overpressure and duration to which BRM buildings may be subjected. Many constructed in place buildings have heavy foundations designed to resist the total anticipated blast loads. However, for BRM buildings, owners often take the approach that foundations only need to be designed for normal design loads (other than blast) with the building’s anchorages
permitted to break during a blast event but designed to remain intact under other design loads. Whether a building is anchored for a blast, anchored for other loads or completely unanchored depends on anticipated use of the building, potential down time following a blast, the amount of flexibility in utility connections, building department requirements and owner tolerance to risk. For BRM buildings, the calculated permissible sliding displacement is sometimes limited to 12 inches, but this is a building-specific owner decision. In all cases, unanchored buildings must have a high margin against overturning with the propensity to uplift calculated. Overall building weight is very important to resist sliding and overturning effects. If possible, connections are designed to develop the full capacities of the connected members. Otherwise, the connection strength should be designed, at a minimum, with capacities in excess of the maximum transferable loads from the connected members. If a multimodule building is used, connections between adjacent modules, which can be welded or bolted, are critical. BRM buildings should be
evaluated for projectile resistance if projectile impact is a credible scenario. Interior modules of BRM complexes can be flexible and transportation/lifting analyses may be warranted in order to prevent damage to nonstructural components. Temporary bracing may be required and care must be taken when loading/unloading modules.
BRM buildings may be used as temporary structures and moved from site to site over their lives. Care should be taken to ensure accumulated damage due to rugged service and repeated transportation doesn’t result in a compromised blastresisting system. Therefore, routine inspections should be performed and reputable suppliers should be utilized. To contact the author, visit www. mmiengineering.com or call Paul Summers at (281) 810-5013 or email psummers@ mmiengineering.com. For more information on Hallwood Modular Buildings, visit www.hall woodmodular.com or contact Clayton Meaux Jr. at (337) 207-0593 or email@example.com.