Until MBI came along in the late 1990’s, steel reinforced concrete was considered the only option to protect against blasts. Today, steel-fabricated and factory manufactured blast-resistant buildings are the optimum solution for worker safety and equipment protection.
For over 50 years, the steel fabricated building concept has been used for offshore applications, which also call for high-end design challenges. Although a blast is typically not a large concern for offshore applications, hurricane-force winds and fire are of major concern. Offshore buildings can range from a small, temporary emergency shelter, to extremely large building complexes that can serve as living quarters for hundreds of workers. Although offshore applications for steel buildings require that they perform well under dynamic loading conditions, modern onshore steel-fabricated blast-resistant buildings have taken this concept to a new level.
Prior to 1999, blast-resistant buildings designed for HPCM facilities generally utilized concrete construction with a large amount of steel reinforcement. These buildings were primarily used as Control Rooms. Although this type of design and construction is still utilized today, it is widely recognized that steel construction offers advantages under dynamic loads, such as those resulting from blasts associated with a Vapor Cloud Explosion (VCE) — the type of accidental explosion that can be produced within HPCM facilities. Steel construction offers extremely high levels of ductility and energy absorbing capacity – something concrete does not do – and performs extremely well under dynamic loading conditions. Table 1 shows how concrete structures, trailers and light-gauge steel buildings perform when impacted by various blast pressures.
|Building Type||Peak Side-On Overpressure (psi)||Consequences|
|Wood-framed trailer||1 psi||Roof and walls collapse|
|Unreinforced masonry building(bearing walls)||1.5 psi||Complete collapse|
|Pre-engineered steel building||2.5 psi||Frame stands, but cladding and interior walls are destroyed|
|Steel-fabricated, blast resistant building||+25 psi||Per design basis (can be no damage or client’s stated limit)|
Unlike the typical pre-engineered steel buildings that has small to mid-sized structural members (with poor structural shapes for blast loading), today’s blast resistant steel buildings utilize medium to heavy structural members with ideal (highly energy-absorbing) structural response properties and heavy steel plate. These buildings can be designed for virtually any blast load and multiple Damage Response levels.
The lowest blast overpressures that are typically addressed in HPCM facilities are in the range of 1 to 2 pounds per square inch (psi). The term overpressure typically refers to a “Free-Field” overpressure, i.e., a pressure at a point without considering the obstruction that the building itself has on the pressure wave. Thus, the building interacts with the dynamics of this pressure to have an effect on all exposed building surfaces (front wall, roof, side walls, rear wall, and potentially the building’s underside if it is elevated off the ground). In contrast, the term pressure can also refer to the Reflected Pressure, which is roughly 2 ½ times the Free-Field overpressure and only impacts the building’s surfaces that face the blast source.
Even these relatively small overpressures are well beyond the design parameters of typical building designs — a 1-psi blast is considerably stronger than any wind load. In fact, some blast overpressures associated with a VCE explosion can impact building structures with Free Field overpressures in excess of 20 psi, or well over 50-psi Reflected Pressure, or the equivalent of over 2.8 million pounds of pressure exerted (virtually instantaneously) on a 10’x40’ wall facing the blast source.
Due to the enormous loads associated with blast overpressures, both the designs and the manufacturing techniques for steel fabricated buildings have gone well beyond traditional offshore building construction. New design analysis techniques have been applied including Finite Element Analysis (FEA) and Computational Fluid Dynamics (CFD) modeling. Structural shapes that have been used in the past are almost non-existent in today’s blast-rated steel buildings. Welding requirements have significantly increased the welder’s certifications and quality control procedures and mechanical integrity are more critical than ever before.
Minkwan Kim, PhD, Design Engineer