Protective Design Strategy of Blast-Resistant Structures

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The main strategy for blast-resistance structures design is to reduce blast demands, which inherently is achieved by reducing the deformations in structural and non-structural building components. This is accomplished through various techniques:

Providing sufficient protection by increasing protected standoff distances against external attacks. The most cost-effective solution for mitigating explosive effects to a building is to ensure the explosions occur as far away from the building as possible (increased standoff). Therefore, the site selection for new construction and site protection in existing structures is important in mitigation blast risk.

Many types of barriers are designed to resist the impact of a vehicle explosive. Among them are massive concrete barriers (Kontek 2008), concrete enclosed with steel plates (Crawford and Lan 2006), and soil filled corrugated metal (Crawford and Lan 2006). Each barrier is designed to absorb the large amounts of energy from an impact or blast with minimal effect on the facilities it is protecting.

The building shape and layout should be selected to minimize the effects of blast loading. Re-entrant corners and overhangs are likely to trap shockwaves and amplify blast effects. The reflected pressure on the surface of a circular building is less intense than on a flat building. When curved surfaces are used, convex shapes are preferred over concave shapes. Figures 1-2 present desirable and undesirable structural shapes and layouts of buildings when designing for blast loading.

Not many code resources are available when designing for blast loading. However, FEMA 427 does provide guidelines. The general design recommendations are not that different from what has been discussed here, in that blast resistant building designs should use simple geometries without sharp re-entrant corners and be placed on a project site as far from the lot perimeter as practical. The following discusses FEMA 427 recommendations for different types of framing systems:

In frame structures, column spacing should be limited. Large column spacing decreases the likelihood that the structure will be able to redistribute load in the event of column failure (Figure 3).

In frame structures, the exterior columns should be designed to resist the direct effects of the specified blast.

The frame structures system should be designed to resist the likely progressive collapse. In case of occurrence any localized failure must be considered.

It is not desirable to use transfer girders in design. Loss of a transfer girder or one of its supports can destabilize a significant area of the building. If transfer girders are required, it must be to add extra transfer systems.

In bearing-wall systems that rely primarily on interior cross-walls, interior longitudinal walls should be spaced to enhance stability and to control the lateral progression of damage. In bearing-wall systems that rely on exterior walls, perpendicular walls should be provided at a regular spacing to control the amount of wall that is likely to be affected.

The primary loading on the roof is the downward air-blast pressure. The preferred system is cast-in place reinforced concrete with beams in two directions. If this system is used, beams should have continuous top and bottom reinforcement with tension lap splices. Stirrups to develop the bending capacity of the beams closely spaced along the entire span are recommended. Finally, use two-way floor and roof systems.

Cast-in-place reinforced concrete is the structural system preferred for blast-resistant construction. This is the material and structural type used for military bunkers. The military has performed extensive research and testing of its performance. Concrete has significant mass, which improves response to explosions.

Generally, simple geometries and minimal ornamentation (which may become flying debris during an explosion) are recommended. If ornamentation is used, it is preferable to use lightweight materials such as timber or plastic, which are less likely than brick, stone, or metal to become lethal projectiles in the event of an explosion.

Ultra high performance concrete (UHPC) is known for its superior mechanical properties; compressive strength can reach up to 200 MPa (29,000 psi) and tensile strength up to 40 MPa (5800 psi). Also, the crack propagation can be well controlled due to inclusion of steel fibers in its cement matrix, leading to a higher ductility and energy absorbing capacity so as to make it an ideal material for structural members that are exposed to the constant threat of blast attacks. Previous experimental work conducted by Mao et al. and Wu et al., Barnett et al., Ibrahim Metwally, Schleyer et al., and Melançon confirmed the superior blast resistance of UHPC structures under high loading rate conditions such as explosion and impact compared to traditional normal and high-strength concrete.

Concrete-filled steel columns have high ductility and very good blast resistance [Peyman, et al., Ibrahim Metwally and Zhang, et al.). Concrete systems have significant inertia but are susceptible to shear failures. Steel systems have inherent ductility but are locally vulnerable open sections and connections. A combination of steel and concrete is ideal.

Blast-resistant design philosophy allows structural elements to undergo large inelastic (plastic) deformations under blast loading. A ductile structure that undergoes large deformations without failure can absorb much more energy than a brittle structure of the same strength. Tensile reinforcement between 0.5% and 2% of the cross-sectional area of the concrete element will usually ensure ductile behavior while providing the required strength.

Compression steel in flexural members serves two purposes. After a structural member is deflected by blast loads, it attempts to spring back or rebound. Dynamic rebound causes load reversal and, under certain conditions, can result in catastrophic failure.

Minor: Non-structural failure of building elements such as windows, doors, and cladding.
Moderate: Structural damage is confined to a localized area and is usually repairable. Structural failure is limited to secondary structural elements, such as beams, slabs, and non-loading bearing walls. However, if the building has been designed for loss of primary members, localized loss of columns may occur without initiating progressive collapse.

Major: Loss of primary structural components such as columns or transfer girders leads to loss of additional elements that are adjacent to or above/below the lost member. In this case, the building is usually not repairable.

About the Author

Ibrahim M. Metwally, PhD, PE, is a professor of concrete structures at the Concrete Structures Research Institute at the Housing and Building National Research Center, Giza, Egypt. He is licensed by the Wyoming Board of Professional Engineers in the U.S. and registered as a senior structural consultant at DRSO of the Ministry of Housing of Egypt.

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