Following is a common sense analysis of the choices which plants have in regard to the issue of whether to anchor or design their blast/explosion resistant modules/complexes to slide.
At MBI we design & manufacture according to the requirements of the client. Some clients want the building to move in the event of an incident; some clients want the unit bolted-then the bolts will break in the event of an incident and the building will slide a few inches or less. Some clients want the building welded to the foundation for a secure hold. No answer is necessarily preferred, while all the solutions are correct—in accordance with the client’s specs and needs.
Anchored or Free to Slide Design?
Many blast-resistant modular buildings are designed to be permanent installations with their anchorage and foundations designed to resist the total anticipated blast loads. This design approach can result in quite large foundations.
However, in the case of modular blast-resistant steel buildings, some owners have taken the approach that the foundations and anchorages need only be designed for normal design loads (loads other than blast). In this case, the building’s anchorages are permitted to ‘break’ during a blast event (they act as anchorage ‘fuses’), but are designed to remain intact under other design loads. Alternatively, the building can be designed to be completely unanchored (free to slide) for both blast and other loading effects, subject to the local building official’s anchorage requirements for gravity, wind and earthquake loading. Whether a building should be anchored for blast, anchored for other loads (but not for blast) or completely unanchored (and free to slide), depends on the anticipated use of the building, whether or not potential down time is acceptable following a blast event, the amount of flexibility in the utility connections (power, water, wastewater, gas) and, most importantly, the owner’s tolerance to risk. Owners should make this decision on many factors, including risk, safety, cost, magnitude and probability of blast.
If the building is unanchored (free to slide) for blast loading, or only anchored for conventional loads with a structural anchorage fuse, the maximum sliding displacement, velocity, and acceleration of the building can be estimated using impulse-momentum first principles, simplified numerical integration methods or finite element analysis. The contents and personnel within the building should be assessed for these actions. In this case, the structural movement may result in impact/damage to attached utilities and building contents, as well as the possibility of injury to personnel, due to interaction with the structure, fixed equipment and internal moving objects (typically unrestrained and falling objects). In such interactions, the critical components of motion can be local accelerations, velocities and displacements that govern local forces and energies of impact, including the propensity to topple over and fall. Permanent fixtures and equipment should be designed to withstand the calculated local building motions as a result of blast loads. Anchorage and restraint techniques for nonstructural items have long been used for earthquake design (FEMA 412, FEMA 413, FEMA 414, SMACNA 1998). Attached utilities should also be designed to accommodate expected movements or fail in a safe manner.
As previously stated, the decision as to whether or not these displacements, velocities and accelerations are acceptable to an owner depends on the anticipated use of the building, whether or not potential down time is acceptable following an event, flexibility in the utility connections and, most importantly, the owner’s tolerance to risk. Since the building will act as an external pressure barrier, the design of internals need only consider the effects of movement. Definitive assessment criteria for interactions with personnel are not available, but criteria do exist (Baker 1983). Additional criteria for projectile impact (such as falling objects) are also available (TNO 1992). Note that architectural and nonstructural components may become debris hazards. TM 5-1300 provides some guidance on tolerance of mechanical and electrical equipment as well as personnel. For sensitive and critical equipment that must function during and after the event, verification by shock testing with the induced motions consistent with expected structural motions may be needed.
For modular buildings that are free to slide, the calculated permissible sliding displacement sometimes has been limited to 12 in. (300 mm), a generally accepted practice. The amount of displacement is very much an owner decision and is specific to the building being designed. In all cases, buildings that are not anchored for blast must have a high margin against overturning and the propensity to uplift should be calculated. In the case of significant uplift, application of pressure to the underside of the building should be considered, as this further adds to the overturning moment and magnitude of uplift.
Pat Lashley, PE, MBA –Vice President of Engineering