In recent years, more nonmilitary buildings, particularly federal buildings and correctional buildings, are considering protection from terrorist type attacks as part of building security.
Equations have been derived to quantify blast intensity including one by Brode. This equation quantifies the overpressure in an unconfined setting. These pressures may be appropriate for designing the roof, sidewalls, and rear walls of the building. However for the side facing the blast, the front part of the blast wave is reflected off the building surface back into the wave effectively magnifying the pressure. Therefore, depending upon the configuration and the design parameters, structural elements facing a blast may require higher design pressures.
Blast loads also are unique in that the peak load lasts only a fraction of a second (generally measured in milliseconds) and the intensity of the load can be several orders of magnitude larger than conventional design loads due to wind or seismic events.
In addition, blast loads cause load reversals described as follows: First, overpressure (higher than atmospheric) resulting from the violent expansion of hot gases produces a layer of compressed air radiating from the source — the blast wave. Almost instantaneously, under pressure (lower than atmospheric) follows resulting in a rush of air back to the source to fill the void left by the suddenly cooled superheated air.
Because of the unpredictability of the blast intensity due to variations in the distance to the source and size of charge used, the most common blast design philosophies recognize that protection is not an absolute. The goal is to limit the extent of collapse, minimize loss of life, and facilitate evacuation and rescue. Casualties near the blast may be unavoidable, but preventing progressive collapse of the building reduces further fatalities. The design process should include a risk assessment to help determine what level of damage or potential injury is acceptable, considering public access to the building, aesthetics, and economics.
Bullet resistance can also have a high priority for many buildings, often more so than blast resistance. Unfortunately, much of the testing that has been conducted on the bullet resistance of concrete masonry assemblies is not available in the public domain. The most current published comprehensive study was conducted in Canada by the Canadian Masonry Research Institute and the Royal Canadian Mounted Police as reported in Resistance of Exterior Walls to High Velocity Projectiles.
Previous to the Canadian study, most published ballistic testing on concrete masonry walls was carried out during World War II to make sure that adequate protection was provided for transformers, switching stations, and other installations subject to sabotage. Recommended constructions for bullet resistance are 8 in. (203 mm) solid or grouted concrete masonry walls or 12-inch (305 mm) hollow units with sandfilled cores. Both walls provided equal protection under test conditions. In no case did bullets penetrate the opposite face shell of the masonry when tested with high-powered rifles, revolvers, and machine guns.
Building Standoff Distance
The distance between the point of detonation and the target, known as standoff distance, is typically considered the most important design parameter for blast resistance for the simple reason that an increase in standoff distance results in a marked decrease in load. For instance, 25 lbs (11.3 kg) of TNT at a standoff distance of 50 ft (15.2 m) produces a blast pressure of 365 psf (17.5 kPa) at Point A. Larger standoff distances also produce more uniform pressure distributions on the structure.
Unfortunately, a large, or even moderate, standoff distance is not always feasible due to site conditions, particularly in urban areas. In these cases,vehicle barriers are often used to keep vehicles off sidewalks and adjacent plaza areas. Materials such as concrete masonry can provide tough barriers that also enhance the streetscape. For example, concrete masonry units or segmental retaining wall units can be used to construct large planters, enhancing security while providing a small green space.
General Blast-Resistant Design Considerations
- Use symmetrical building plans when possible since they typically provide better performance than “L” or irregularly shaped buildings when subjected to blast or seismic loading.
- Use hardened walls and adjacent slabs in the entrance lobby, loading dock, and mailrooms to withstand a hand delivered package bomb, nominally a 25-50 lb (11-23 kg) explosive.
- Use a well-distributed lateral-load resisting mechanism in the horizontal floor plan, i.e., by using several shear walls around the plan of the building to improve overall seismic and blast resistance.
- Since the exterior facade is the occupant’s main protection from a blast, construct the exterior wall of a durable material, such as concrete masonry. If properly designed, the exterior wall can also assist in carrying the load of a damaged column.
- The amount of blast that enters a structure is directly proportional to the amount of openings in the structure. Limit door and window areas to protect the occupants. When this method is not aesthetically acceptable, use blast-resistant glazings, such as glass unit masonry, Mylar, or other window films to minimize injury caused by flying glass. Construct door and window frames integral with the structural components of the building.
- Avoid the use of reentrant corners and deep surface profiling. These can amplify blast pressures locally due to reflections of the shock wave, which combine with the initial blast to produce a greater pressure.
- Maximize standoff distance - erect barricades or other impediments to prevent car and truck bombs from getting close to the building.
- Design to prevent progressive collapse. Construct building in such a manner that if a supporting element (i.e wall or column) is taken out by a blast, the building will remain standing.
Specialized Concrete Masonry Units For Prison Wall Construction
Concrete masonry units are manufactured in many different shapes and sizes. Although conventional concrete masonry units are often used for prison construction, some specialized units may also be available which are particularly well-suited for prison construction.
Shapes intended to easily accommodate vertical and/or horizontal reinforcement include open-ended units and bond beam units. Open-ended units, such as the A- and H shaped units, allow the units to be threaded around vertical reinforcing bars. This eliminates the need to lift units over the top of the reinforcing bar, or to thread the reinforcement through the masonry cores after the wall is constructed. Horizontal reinforcement and bond beams in concrete masonry walls can be accommodated either by sawcutting out of a standard unit or by using bond beam units.Bond beam units are either manufactured with reduced webs or with “knock-out” webs, which are removed prior to placement in the wall. Horizontal bond beam reinforcement is easily accommodated in these units. Special Y-shaped and corner units were developed specifically for prison construction. The Y-shaped units (with one 90° angle and two 135° angles) were developed to allow one corner of a rectangular prison cell to be used as a triangular chase for plumbing, electrical and HVAC service. By truncating the cell corner in this way, all repairs and maintenance can be accomplished without tradesmen ever having to enter the cell, thus reducing additional security risks. The Y-shaped and corner units allow this construction, as well as construction of nonrectangular cells, without creating continuous vertical joints in the wall.