Blast Design

Several industry documents provide guidance in the development of blast resistant requirements. Some of these documents guide the owner in assessing the explosion hazards for a facility (CCPS – “Guidelines to Vapor Cloud Explosion, Pressure Vessel Burst, BLEVE and Flash Fire Hazards”) and others provide guidance to design to the explosion hazards (ASCE – “Design of Blast Resistant Buildings in Petrochemical Facilities”).

However, when designing for blast resistance, unlike designing for environmental and gravity loads, following the guidance of any specific document is not mandated by a government authority. For the most part, facility owners utilize the guidance documents to develop a set of design requirements that are specific to a given facility and building. Therefore, each blast resistant building project results in a truly unique design to meet the requirements of a specific facility.

Our design team has knowledge of both the general industry guidance documents and the owner specific guidance documents. This knowledge is beneficial when navigating through building specifications and building requirements.

Design Process

The requirements and design methods for a Blast Resistant Building differ significantly from conventional building design. Utilizing conventional design methods to develop a blast resistant design can result in an inaccurate and/or significantly over designed structure. The following table is a comparison of the some of these differences:

Information Needed To Develop a Blast Resistant Design:

The information required to develop a blast resistant design is normally found in the building specification, which is based on information from a variety of sources. This list of sources includes:

Facility Hazard Assessment:

• Blast Load: Provided in terms of a pressure and duration (or impulse)
• Pressure (or Overpressure): Provides the magnitude of the peak pressure that would be applied to the flat roof surface of the building. Usually quoted in terms of free-field pressure or side-on pressure.
• Duration: Provides the length of time that the pressure wave will be applied to the building surface if the blast event were to occur.
• Impulse: Provides the energy imparted to the building structure by the blast loading. The impulse is a function of both the pressure and duration.

Owner’s Guidance or Risk Criteria:

• Building Response Level or Building Damage Level:Provides the designer with the level of damage the building can sustain in response to the design basis blast load. A critical building (example: Central Control Room) may require a design response level of “Low”, while a less critical building (example: Warehouse) may be able to sustain a response level of “Medium.”
• Risk Criteria:The owner’s risk criteria may be reported in terms of the probability of an employee or building occupant becoming a fatality during an explosion. The blast resistant pre-engineered metal building design completed by BakerRisk® and Rigid can account for this risk-based design approach.

Siting of the Building or the Facility Plot Plan:

• Building Orientation to the Blast Source: Building surfaces that face the explosion source will be subjected to a reflected overpressure which is greater than two times the free-field pressure. Understanding how the building is orientated towards the explosion source allows for this physical phenomenon to be appropriately accounted for in the design.

Static Design VS Dynamic Design

A static design procedure is appropriate for loads that are applied over a long period and do not vary significantly over time. However, blast loads do change dramatically over the course of the load application, which generally lasts only fractions of a second. Therefore, using a static design approach is not appropriate for a blast resistant building if an accurate and economical material design is desired.

The design basis blast load for a building at an industrial facility can have a magnitude of 2 psi (288 psf) or greater and duration of 100 msec (0.10 seconds) or less. Designing for these high magnitude/ very short duration loads using the same procedures (static design) that are utilized to design for wind or gravity loading results in significant over designs.

A dynamic design procedure provides an opportunity to account for the true interaction of the time varying blast load with the structure. This can include dynamic load increase factors, material dynamic increase factors, deflections in excess of the elastic limit, benefits of additional mass and variations of the blast loading at different building surfaces (reflected blast load).

Elastic VS Plastic Design

Blast resistant design generally allows the structural components to sustain deflections in excess of the elastic limit. This reduces material cost while still providing the level of protection that meets the owner’s requirements.

Unlike conventional building designs, a blast resistant building generally must only resist the design basis blast load once; the owner generally accepts some level of structural damage in response to the design basis blast event. Therefore, the performance of the building should not be judged by its appearance following an explosion but on the structure’s ability to protect the building occupants during the blast event.

Team Approach

In order to complete a blast resistant design a coordinated effort from all design and construction disciplines is needed. While the effective design and appearance of the exterior structural system is an important and critical portion of the design process, the overall blast resistant design must also be coordinated with the architectural and mechanical designs as well.

Our design team is experienced in understanding the requirements for blast resistant windows, blast resistant doors, mechanical openings, specialized anchorage systems, interior walls, building foundations and interior equipment. Not only are the implications of these blast loaded components incorporated into the structural design of the pre-engineered metal building, our design team works with the overall design and project construction team to ensure that the specialized requirements for these components are appropriately addressed