Blast-Resistant Design
by Paul Deffenbaugh | 1 March 2022 12:00 am
Custom-designed metal buildings provide efficient solutions for chemical and oil and gas industries
Bouquot: What types of buildings have to be designed for blast loads?
Gomez: Buildings within petrochemical facilities often require blast-resistant design. The Occupational Safety and Health Administration (OSHA) refers to guidance documents from the American Petroleum Institute (API) and the AIChE’s Center for Chemical Process Safety (CCPS) for requirements on facility siting and mitigation of blast hazards in occupied structures.
Facilities are required to have facility siting studies performed to evaluate potential hazards to occupants within that facility. Confidential Facility. Confidential Location.
Credit: Photograph by IHI-Kiewit Joint Venture
Bouquot: Why are facility siting studies so important?
Gomez: To design a building that mitigates blast hazards, you have to know where those hazards are. The facility siting study is an integral part of blast design analysis because it details the blast hazards across the facility. This typically takes the form of blast contour maps which help companies identify the blast hazards on each of their occupied structures.
Bouquot: Can metal buildings be designed to be blast proof?
Gomez: No building is blast proof. A blast-resistant building is one that is designed for a specific blast loading condition and desired protection level. The design process requires the evaluation of several factors, including the building orientation and the building response criteria. Loads above that specified design load will result in additional damage and loads high enough over the design load can lead to structural collapse. No single building design will meet all potential blast loading conditions. However, our goal is to implement a design that severely reduces the likelihood of collapse, preserves lives and is cost effective. We do this by designing a building that will properly respond to the most likely blast scenarios.
Bouquot: Why are metal buildings particularly suited to help companies achieve their blast-resistance objectives?
Gomez: As a rule, metal buildings tend to be the optimum value for permanent buildings. Concrete buildings and blast-resistant modular buildings (BRMs) are significantly more costly while providing less flexibility for the end user. Every metal building is unique and custom to the needs of the owner and each building can incorporate virtually any dimension, so meeting a company’s special needs is not a high-priced add-on. A metal building’s structural system also provides maximum flexibility to create a variety of building sizes, shapes and plan layouts. If you want to have a crane or mezzanine in your building, this can readily be done. Additionally, metal buildings are easy to retrofit and modify. Refineries change all the time. These changes in usage can increase the blast loads within the facility. Metal buildings are relatively convenient to retrofit and adapt to higher loads.
In contrast, BRMs tend to be off-the-shelf solutions. BRMs are rated for a specific blast load—usually, appreciably higher than what is needed. BRMs are also significantly harder to modify and have much less flexibility in sizes, shapes, and layouts. You cannot get cranes or mezzanines into BRMs.
BRMs, concrete structures and metal buildings all have different blast loading ranges where they work well economically. Typically, BRMs are economical when the blast loads are in the range of 5-8 psi over pressure. Concrete structures are economical when the blast loads are more than 8 psi. Our typical blast-resistant metal buildings can withstand loads up to 5 psi. However, we have additional analytical tools at our disposal that can be implemented to resist even higher loads.
To mitigate the potential consequences of blast hazards, models produce realistic blast loading information for determining the extent of building damage.
Image courtesy of Thornton Tomasetti
Bouquot: Do you have some general tips for those trying to design for blast loads?
Gomez: A building should be designed to blast loads that are as realistic as possible. Blast loads are directional and decay with distance. The facility siting study shows the direction of the loading and how it decays. Realistically applying the loads on the walls and roof can potentially provide a substantial reduction in the cost of a structure. The walls that face the source of the blast will experience higher loads than the walls that are parallel to the direction of the blast. Because of this, I like to tell my clients to avoid designing the building as if a blast could come from any direction. This can be counter intuitive because engineers design buildings for wind or seismic loads coming from any direction. However, in the case of blast loads, this process can be costly, and the facility owners should already know in which direction a blast is likely to come.
Bouquot: What are some of the most common blast-related design considerations?
Gomez: Wind columns in endwalls will likely need to be hot-rolled wide flange or three-plate sections. Cold-formed wind columns typically only work for very low blast loads. Purlins can be used as compression struts but only if the blast load is low. Most blast resistant buildings will require HSS, pipe, wide flange, or three-plate members for the compression struts. These members should be at an elevation below the purlins to ensure that they only transfer axial loads and will not directly resist blast loads from the roof panels. Bracing is also a consideration. Rod bracing for the walls and roof can work for low to moderate blast loads. Angle bracing will be needed for moderate to high blast loads. Windows and doors require heavier support steel than would normally be required for environmental loads. These members are usually hot rolled.
Bouquot: What about the rigid frames? Are there any special considerations or issues with them?
Gomez: The amount of steel required for rigid frames is profoundly affected by how the blast load is modeled. Blast loads are transient in nature and degrade with distance from the source. If you assume that the blast load is instantaneously applied across the entire roof you will require more steel than if you applied the load on the half nearest to the blast and then applied it on the half farthest from the blast. The same logic is true if you were to cut the load up into fourths and apply the load incrementally with the timing and magnitude of the load application based on the wave speed. We have developed software that allows us to get considerably more granular. Our program allows us to adjust the load along every inch of the roof based on the wave speed. Thus, we can optimize the rigid frames to minimize the steel required while still resisting the blast loads.
Bouquot: What are your design tips for assisting firms in developing blast-resistant design criteria for metal building systems?
Gomez: Realistically, modeling the timing of the blast load and the response of the building can have drastic effects on the overall structure. Simplified finite element analysis with sweeping blast loads provides the most cost-efficient results for rigid frame designs. High-quality software makes this methodology easy to implement and optimize.
Additionally, modeling the timing of the building response can reduce the overall loads on the foundation system. Be sure to ask for more information on the load from the end user. Maximize the number of surfaces being designed for the side-on loads, if you can. You do not want to design every wall of the building for the maximum reflected load.
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