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With the imminent release of updates to the API’s Recommended Practices 752 and 753 (API RP 752/753), it is the appropriate time to examine two guiding principles that facility owners and operators should consider for occupied buildings.
The first, and most important, guiding principle is “The Golden Rule” — whenever possible, locate personnel in buildings away from hazardous areas. However, this is not always possible. Some processes require operators to be near the units they are responsible for, while others may not have the real estate to expand beyond their existing footprint. The second guiding principle is that occupied buildings should be designed to protect workers from hazards associated with the facility, which could include explosion, fire and toxic material releases.
Blast-resistant buildings have traditionally been quantified by the ability to resist blast loads for a prescribed level of structural damage. Focus has therefore been placed on the structural performance without fully correlating this to occupant vulnerability. Fire and toxic material hazards and their impact on building occupants is often overlooked, resulting in buildings that do not comply with the full intent of API RP 752/753. When selecting and/or designing a new building for your employees or critical equipment, you should choose a multi-hazard resistant building (MRB) that protects not only the structure but its occupants from multiple hazards simultaneously.
MRBs or BRMs?
A MRB is a building that offers a holistic approach to provide appropriate protection, peace of mind, and additional security for personnel and critical equipment located in environments with multiple industrial hazards. Historically, owners and operators have sought to find a building solution that protects against hazards beyond explosions without undertaking expensive and lengthy building projects. Since the BP Texas City explosion in 2005, the most utilized buildings for hazard protection have been metal blast resistant modules (BRMs). Due to their wide use in industry, BakerRisk initiated a testing program to determine the degree of protection these buildings provide against all industrial hazards, not just blast loads. This article summarizes notable results associated with BRM hazards from a series of explosion tests; concepts that are overlooked but paramount to understand risks associated with placing these buildings in high-risk environments.
Non-structural debris hazards
Modular buildings are viewed by owners as an attractive option to reduce the on-site construction time and disruptions to daily operations. BRMs are constructed similarly to shipping containers but have thicker corrugated wall panels and heavier steel frame beams and columns. While the construction is stronger than a shipping container, the structural members are still expected to deflect inwards towards the occupied space, sustaining a prescribed level of structural damage. This point is sometimes misunderstood, with owners and operators believing they are risk-free in a BRM, expecting little to no damage after a design-basis blast event, for example, 8-psi, 200-ms blast load for an 8-psi BRM. Such buildings are often incorrectly referred to as “blast-proof” because in order to absorb blast energy, the walls and roof of a BRM are designed to deflect. While a properly designed BRM will protect building occupants from structural failures during a blast, other secondary hazards are often overlooked that can result from the flexibility of the steel.
Non-structural items that may be necessary in a BRM such as cabinets, shelves, desks, electrical equipment, ducting and lighting can become sources of hazardous debris to building occupants, even at blast loads below the BRM blast rating. Single unit BRMs are typically between 10 and 14-feet wide and between 20 and 40-feet long. Due to their small size, non-structural items are typically anchored on or pressed up against exterior walls. Even for owners/operators with requirements to space items away from walls typically 12-inches, it is impractical for building occupants; therefore, items are once again butted against the walls and become potential non-structural debris hazards.
Common wall-mounted or near-wall architectural and electrical items are potential sources of interior debris in a blast event. Electrical boxes are anchored to the interior surface of exterior walls, and it is common for operator control panels to also be located at exterior walls to optimize the use of interior space. TV/computer monitors, cabinets, bookcases and other items with an elevated center of gravity are generally placed directly adjacent to an exterior wall surface. Overhead items such as drop ceiling components, lights, mechanical ductwork and vents, can pose as non-structural debris to building occupants throughout an entire building. Unlike wall debris, overhead items are prevalent throughout a building, regardless of the interior layout. Roof displacements associated with blast loads can quickly overwhelm the connections of these items, creating overhead debris hazards.
Figure 1 illustrates wall debris generated from a shock tube test of a BRM specimen, where non-structural items were anchored to an interior stud wall. The stud wall was spaced 2-inches from the BRM structural wall, but significant debris was created and thrown up to 10 feet from the wall at high velocities. Based on the structural performance of the BRM crimped wall, the damage would be considered low, demonstrating the existing disconnect between structural damage and vulnerability to building occupants.
The hidden risks of a blast-resistant building
Figure 1: Shock tube testing of a BRM wall with non-structural debris
Unfortunately, while API RP 752/753 does recognize the need to assess non-structural debris hazards, the current third edition provides no guidance on how to perform this assessment, or to correctly design the anchorage. Whether language is provided to address these hidden hazards in the next edition, it is essential for owners/operators to consider what vulnerability non-structural debris impact can have on occupied buildings.
Sliding during a blast event
BRMs that are not anchored to a foundation, or are only anchored for conventional wind loads, are susceptible to sliding during a blast event. This causes what is known as global accelerations to non-structural items, even those that are anchored off a wall surface. Global accelerations cause rapid deceleration to objects within the BRM, which can be compared to occupants in a car moving forward in their seat after braking suddenly. While the magnitude of the hazards from global accelerations is contingent on the BRM weight, dimensions, blast load and surface on which the BRM is located, BakerRisk’s testing program has proven that pressures less than a typical 8 psig design pressure exceed the Process Industry Practices STC01018 recommended design criteria of 0.2g for equipment. This means that for unanchored BRMs (or those anchored only for wind loads), non-structural debris hazards exist well below the design pressure and even for interior items that are adequately anchored. Additionally, building occupants seated in chairs can be moved at high velocity and standing occupants can lose their balance and fall due to these accelerations, impact interior walls or other hard items.
Occupant vulnerability in BRMs
The definition of occupant vulnerability, per API RP 752/753, is the “portion of occupants that could potentially experience a life-threatening injury or fatality if a potential event were to occur.” By definition, therefore, occupant vulnerability includes non-structural debris impacts as highlighted above. As summarized in the third edition of API RP 752, “owners/operators should understand the basis for the correlation between building damage and occupant vulnerability and assess its applicability.” Why does this matter?
Local wall and roof bending and global sliding displacement and accelerations of a BRMs can lead to significant non-structural internal debris hazards, as demonstrated in full-scale testing programs. These hazards become more severe as the BRM blast rating and response level increases. However, the stated response level does not directly communicate the hazard associated with deflection and non-structural debris, for example, a low response does not mean no risk, and a medium or high response will introduce more risk to building occupants). Such hazards are sometimes overlooked when siting a BRM. Rather than relying solely on a blast overpressure rating, a detailed structural analysis of a BRM may be required to predict wall accelerations to quantify these hazards and to properly site the BRM. To comply with one of API RP 752/753’s guiding principles, it is essential to evaluate the building’s protection for occupants not on the design rating, but rather on a vulnerability analysis that includes non-structural debris impacts.
Meet FORTRESS – The MRB design using modular construction
When you need a true multi-hazard resistant building but with the speed and convenience of modular construction, you need a FORTRESS. FORTRESS protective buildings are designed to be a MRB sited in areas with high blast, debris, fire and toxic hazards to ensure field operators can quickly respond and mitigate the event as a first line of response. When selecting and/or designing a new building for your employees or critical equipment, you should choose a building that protects not only the structure but its occupants from multiple hazards simultaneously. The important thing about FORTRESS MRBs is that, although they focus on building response, it is the vulnerability and safety to the people and/or equipment inside during an incident that takes priority.
To read the full technical paper, visit BakerRisk.
If you need a building that offers negligible occupant vulnerability regardless of the hazard, contact FORTRESS.
