Making A Structure Blast Resistant

More structures are requiring blast resistance than ever before. Blast resistance allows a structure to withstand an explosion while keeping occupants safe. Terrorist threats have been increasing the demand to design critical structures, landmarks, and even personal bunkers for blast resistance.

Estimating Risk of Blast Loads

While one could design a structure to maximize blast resistance with more and more reinforcement, it is far more economical to perform a risk assessment. A risk assessment determines the explosion design size for a structure. One of the most effective measures to mitigate risk to a structure is to limit vehicle access to a structure. By completely eliminating vehicle access to a structure you can reduce the blast load risk from 500 lbs of TNT for a compact car to 50 lbs of TNT for a container or parcel bomb which significantly reduces the design requirements and risk.

Estimated Blast Risk (TNT equivalent in lbs)

  • Small Package/Letter: 1 lb
  • Pipe Bomb: 5 lbs
  • Fedex Package: 10 lbs
  • Vest/Container Bombs: 20 lbs
  • Parcel Package: 50 lbs
  • Compact Car: 500 lbs
  • Full Size Car/Minivan: 1,000 lbs
  • Van/SUV/Pickup Truck: 4,000 lbs
  • Deliver Truck: 10,000 lbs
  • Truck with a Trailer: 30,000 lbs
  • Truck with Two Trailers: 60,000 lbs

A structures specific blast design criteria may be obtained from the federal agency client on a need to know basis.

Sources: IED Attack – Improvised Explosive Devices Homeland Security; Blast Loading On Structures

Finding The Optimal Balance

When designing a structure it is important to balance security, accessibility, initial and life-cycle costs, natural hazard mitigation, fire protection, energy efficiency, and aesthetics trade offs. Eliminating vehicle access may reduce blast risks, but it also can significantly reduce accessibility. Vehicles such as ambulances or fire engines may be prevented from gaining access to the structure during emergencies. The optimal balance for a given structure varies on a case by case basis and is dependent on many factors.

Different Blast Load Scenarios

When designing a structure for blast resistance, every possible scenario must be incorporated. Scenarios can include exterior building envelope blasts, interior blasts, and more.

Exterior Envelope Blast Threats

Background

The most common design scenario for buildings are exterior envelope blast threats. One example of exterior envelope blast threats is a vehicle bomb parked on the outside curb next to a structure. Blast threats on all sides of a building that are adjacent to a public street or adjacent property need to be considered.

When an explosive device is detonated shock waves expand outward in all directions. The peak intensity blast pressure decreases as the blast expands away from the source. As the stand-off distance increases the peak intensity blast pressure decays, but the envelope area that is affected by the blast increases. As the shock wave continues to decay it drops below atmospheric pressure and a negative pressure phase occurs.

FEMA - Explosive Blast - Figure 4-1
Source: FEMA 426 – Explosive Blast – Figure 4-1

When a surface blast occurs, as is the case with most exterior vehicle and package bombs, incident waves and ground reflected waves are generated simultaneously. The incident waves are the shock waves traveling directly through the air from the source. The the ground reflected waves is caused from the initial shock wave reflecting and being amplified by the ground surface. During a surface blast, both the incident waves and ground reflected waves are combined to form a single wave called the reflected wave. The reflected wave only delivers pressure to structures that are perpendicular to the path of the wave. Reflected pressure is always greater than the incident pressure.

The coefficient of reflection (Cr) is the ratio between the reflected pressure and the peak incident pressure. The reflected pressure is typically between 2 and 13 times the peak incident pressure and increases significantly closer to the center of burst.

Source: FEMA 426 – Explosive Blast – Figure 4-2

In order to determine the total energy that is delivered from a blast onto a building the impulse is calculated. The impulse is the integrated area under the pressure vs time curve.

Source: FEMA 426 – Explosive Blast – Figure 4-3

Structures that are parallel to the shock wave path such as the roof or adjacent walls only experience the incident pressure.

FEMA 427, Primer for Design of Commercial Buildings to Mitigate Terrorist Attacks (2003)

Source: FEMA 427, Primer for Design of Commercial Buildings to Mitigate Terrorist Attacks (2003)

Perimeter Protection

A buildings stand-off distance can significantly help to reduce blast loads. As the blast wave propagates over distance from the center of the burst the strength and speed decreases delivering reduced load to the structure. For this reason one of the most effective blast protection measures for a structure is to maximize an enforced and reasonable stand-off distance. Some measures to do this include screening vehicles that come within the stand-off distance, anti-ram bollards, and large planters on the curb.

Anti-ram protection systems need to be designed for the maximum vehicle size and maximum attainable speed. One effective way of reducing design requirements is to reduce the attainable speed near the structure. Two ways of limiting maximum attainable speed is the use of tight turns and required stop access points which limit acceleration distance. Vehicle size limits also can significantly reduce both the design requirements for anti-ram protection and the maximum blast risk.

For many structures, the trade off for reducing accessibility and increased costs is not worth the added security. When this is the case and the maximum vehicle size is not controlled, slightly increasing the stand-off distance may not be an effective way to increase protection. This is because maintaining and enforcing the stand-off distance that a substantially large vehicle bomb requires may not be reasonable. During these situations the design focus will need to be on structural damage mitigation.

Facade Protection

When designing for blast protection your first line of defense is the buildings facade. Since the shock wave pressure decays over distance, the ground floor facade of a structure experiences the greatest pressure loads. When designing a building facade for blast loads engineers should consider adding dampening systems, designing systems to catch fragments, avoid the use of materials that break up easily, minimizing and reinforce openings, and adding structural redundancy.

All critical structural elements should also be located within the center of a structure to reduce risk through maximizing the amount of blast energy dissipation prior to contact. At risk structures should also be designed for progressive collapse to prevent structural collapse in the event that an attack damages structurally critical elements. Progressive collapse prevention design requires adding structural redundancies to allow for alternative load paths in case of a local failure.

In the case that a total collapse is not preventable, its important to allow time for occupants to escape the building. This requires using ductile materials that yield for an adequate amount of time prior to complete failure. By designing ductility into a structure it also prevents high load spikes by gradually transferring load from one load path to another reducing the risk of collapse. In order to prevent sudden collapse elements need to be designed to failure flexurally prior to shear to give adequate warning prior to collapse.

To also reduce risk architects can design buildings to eliminate occupied floors from being located on the most vulnerable lower levels of a building. Lobbies should also be designed to keep the welcome desk away from vulnerable exterior walls to increase the distance for a blast to decay.

Sources: Blast Safety Of The Building Envelope – Whole Building Design Guide;
Blast Loading On Structures´╗┐

Interior Blast Threats

Structures that commonly have interior access for vehicles include such as convention centers, stadiums, and parking garages are at increased risk for blast threats. Particularly when a public parking garage is located beneath a building it opens up the ability for vehicle bombs to directly target columns. One example of this happening was the 1993 World Trade Center Bombing where a 1,336 lb truck bomb was detonated in the world trade centers underground parking garage below the high rise structure. Fortunately, because the world trade center had designed redundancies, the damage from the attack was contained to the parking garage and did not cause a complete structural collapse.

When such threats are present it is critical to perform a progressive collapse analysis to prevent total structural collapse even when a primary structural element fails. For this reason structures that are exposed to blast threats need to have designed redundancy to minimize damage from blast loads.

Even when there is no vehicle access to a building, terrorists carrying container bombs can place explosive next to structural elements. Although these explosives are significantly smaller than vehicle bombs, they can still deliver a substantial amount of damage especially if structural redundancies aren’t in place.

When an interior blast occurs reflected blast waves increase the blast pressure in a room especially in small enclosed areas. In order to reduce the damage caused from interior blasts enclosures should allow for pressure to escape instead of reflecting off of surfaces. It’s also important to consider the smoke and fire risk of explosions. Enclosed areas need to allow for adequate ventilation and evacuation. Areas that are at increased risk for an interior blast should also remain inaccessible to the public and non-essential people.

Bridge & Dam Blast Threats

Bridges and dams are at significant risk for blasts. For the majority of bridges and dams, screening and limiting vehicles isn’t practical due to the need to maintain adequate traffic flow. Blast loads can occur from accidents with trucks carrying explosive cargo or from terrorist attacks using vehicle bombs. A bridge’s or dam’s structural resilience to blast loads depends on the construction material used and structural capacity.

Structural concrete is typically unable to handle significant strain without failing, however when concrete’s strain rate is great enough concrete’s strength increases dramatically helping to alleviate damage. Due to the massive peak pressure loads from blasts the concretes strain rate is significant enough to increase the materials dynamic compression and tension strength by as much as 4x and 8x respectively. The increase in strength is dependent upon the peak blast pressure making concrete an ideal material to handle blast loads.

Another critical design consideration for concrete bridges and dams is to prevent spalling from occurring. Spalling is the effect of pieces of material breaking off of a body. When concrete is exposed to high temperature loads, the water in the concrete can turn into steam creating internal pressures that induce spalling. When spalling occurs the reinforcement rebar which gives concrete tensile strength can become exposed and vulnerable. One method for preventing spalling and crack damage from high temperature loads is to use polypropylene fibers which give pathways for internal gas pressure to be released at high temperatures.

The rebar reinforcement is also susceptible to high temperature loads. At 1000┬░C rebar’s yield strength decreases to 1/3 of its strength at room temperature. Increasing the concrete cover can help better protect rebar by providing increased insulation. Increased cover also provides increased protection from corrosion caused by salting bridges

Modeling Blast Loads

At ASR we use LS-DYNA to simulate blast loads on structures and vehicles. LS-DYNA simulates high energy events and uses fluid-structure interaction analysis to model stresses. Using this we model the pressure from a blast wave on a structure and use the results to guide further design revisions. We are able to model transient loads to find the total energy dissipation capacity of structure using plastic design limits which allow for yielding but prevent collapse. For substantial loads where collapse is not preventable we are able to model time to collapse to ensure that there is enough time for a complete evacuation from a building.

Using LS-DYNA we also model impact loads which allows for structures to be analyzed to determine if they are bulletproof or if there is adequate protection from vehicle impact.

LS_DYNA also allows for vibration analysis to design structures for earthquake loads.

If you have a structure that is in need of advanced engineering analysis and design, our experienced engineers are ready to deliver the results you need. Contact us today for a free quote!

Sources:

Architectural and Structural Design for Blast Resistant Buildings

FEMA 426 – Explosive Blast

FHWA-HIF-17-032 Bridge Security Design Manual – June 2017

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