Several pivotal incidents within the last decade have made blast-resistant construction a universal priority. Concrete applications in the design of buildings for blast resistance are continually evolving, aided by Department of Defense initiatives. The Force Protection Equipment Demonstration program, for example, was developed in response to a directive from former Joint Chiefs of Staff Chairman, Army General John M. Shalikashvili, following the 1996 terrorist bombing of Khobar Towers in Saudi Arabia (Concrete Products, “ICF walls resist terrorist-grade blast exposure,” July 2003, pg. 12). The U.S. Army Corps of Engineers' Engineer Research and Development Center (ERDC) has also performed testing to determine the blast resistance of concrete block walls with various types of protective interior-wythe linings (Concrete Products, “Army Corps tests block walls' blast resistance,” October 2002, pg. 6).

More recently, the new $33 million Federal Building in Oklahoma City, replacing the Alfred P. Murrah Federal Building destroyed by a truck bomb in 1995, has been secured by means of perimeter waist-high architectural concrete bollards and high-strength, structural concrete walls featuring an exposed aggregate finish to offset a bunker-like appearance. A foundation has thus been established for concrete's greater role in blast-resistant building design. Concurrent with federal government work in blast-resistant construction, the Precast/Prestressed Concrete Institute's Architectural Precast Concrete Services Committee has weighed in with “Designer's Notebook: Blast Considerations,” from which the following has been adapted.

DESIGN CONCEPTS

In designing buildings for blast resistance, issues that demand attention include energy absorption, safety factors, limit states, load combinations, resistance functions, structural performance considerations, and most importantly, structural redundancy to prevent progressive collapse of the building. A design satisfying all required strength and performance criteria would be unsatisfactory without redundancy.

Structures of three or more stories are more likely to be subject to significant damage as a result of progressive collapse. The Engineer of Record needs to design the structure to contain any sustained local damage, while the structural system as a whole remains stable, avoiding damage to an extent disproportionate to the original impact. Such containment is achieved by means of structural elements that provide stability to the entire structural system by transferring loads from any locally damaged area to adjacent regions capable of resisting those loads without collapse. Transfer girders and the columns supporting them are particularly vulnerable to blast loading. Unless specially designed, this form of construction poses a significant impediment to load redistribution when the girder or supporting columns sustain damage.

To limit the extent of collapse of adjacent components: (1) highly redundant structural systems are designed; (2) the structure is analyzed to ensure it can withstand removal of one primary exterior vertical or horizontal load-carrying element (i.e., a column, beam or a portion of a load-bearing/shear wall system) without progressive collapse; (3) connections are detailed to provide continuity across joints equal to the full structural capacity of connected members (see Article 16.5-Structural Integrity in ACI 318); (4) floors are designed to withstand load reversals due to explosive effects; and, (5) exterior walls employ one-way wall elements spanning vertically to minimize blast loads on columns.

Strength and ductility (energy-dissipating capacity) are necessary to achieve high energy absorption, which is accomplished by means of appropriate structural materials and details. These details must accommodate relatively large deflections and rotation in order to provide redundancy in the load path. Elements with low ductility are undesirable for blast-resistant design.

Margins of safety against structural failure are achieved through the use of allowable deformation criteria. Structures subjected to blast load are typically allowed to undergo plastic (permanent) deformation to absorb the explosive energy, whereas response to conventional loads is normally required to remain in the elastic range. The more deformation a structure or member is able to accommodate, the greater the blast energy it can absorb. As member stresses exceed the yield limit, stress level is not appropriate for judging member response, as in the case of static elastic analysis. In dynamic design, the adequacy of the structure is judged on maximum deformations. Limits on displacements are selected on the basis of test data or other empirical evidence as well as blast probability and potential consequences. Because applied loads are not “factored up” to provide a factor of safety, a degree of conservatism is included to ensure adequate capacity.

As long as the calculated deformations do not exceed the allowable values, a margin of safety against failure exists. Since the actual weight of the explosive charge is unknown, the engineer cannot increase the design blast-pressure loading to achieve a margin of safety. Blast-resistant design requires that the loads from blasts be quantified by risk analysis and that structural performance requirements be established for buildings subjected to these derived loads. Methods to determine the blast loading and structural performance limits are established in TM 5-1300 for buildings exposed to explosions from TNT or other high-yield explosives in military applications and munitions plants. Typical threats for civilian structures vary from suitcase and backpack bombs (20 to 50 lb. TNT equivalent) to van or small truck bombs (3,000 to 5,000 lb. TNT equivalent). Generally, the smaller charge sizes are associated with vehicles that can be kept further from the building (60 to 100 ft.) by appropriately designed vehicle barriers.

Design codes contain special provisions for high seismic conditions, which may be used to address requirements countering progressive collapse that are associated with design for blast resistance. The desirable features of earthquake-resistant design (ductility, redundancy, and load redistribution) are equally desirable in blast design. The provision for seismic detailing, which maintains the capacity of the section despite development of plastic hinges, is also useful in resisting the effects of a blast. However, these provisions are not sufficient for blast design, as they are intended to protect against nonductile failure modes, such as buckling or premature crushing of brittle materials. The highly localized loading from a blast and the potential for different mechanisms/failure modes requires additional considerations. The engineer should design panels so that the full capacity of the section will be realized, and no premature failure will occur.

Building codes define the load factors and combinations of loads to be used for conventional loading conditions such as dead, live, wind and earthquake. However, no current building codes cover blast loading conditions. Blast loads are combined with only those loads that are expected to be present at the time of the explosion. Therefore, blast loads are not combined with earthquake or wind loads.

ACI 318 PLUS

The Strength Design Method of ACI 318 may be used to extend standard concrete strength and ductility requirements to the design of blast-resistant structures. The resistance of concrete elements due to high strain rates is computed using dynamic material strengths, which are 10 to 30 percent greater than static-load strengths. Strength-reduction or resistance factors are not applied to load cases involving blast. The plastic response used in blast design is similar in concept to the moment-redistribution provisions in ACI 318, Section 8.4 and the seismic criteria provided in ACI 318, Chapter 21. The seismic detailing provisions are applied to provide the necessary ductile response.

In addition to ACI 318 requirements, the following items should be considered for blast resistant design:

(a) The minimum reinforcing provisions of ACI 318 apply; however, the option to use one-third more reinforcing than computed should not be taken. The moment capacity of under-reinforced concrete members is controlled by the uncracked strength of the member. To prevent a premature ductile failure, reinforcing in excess of the cracking moment should be provided.

For panels, the minimum reinforcement ratio (percentage of reinforcing steel cross-sectional area to panel cross-sectional area) of vertical reinforcing steel should be equal to or greater than Building Code ACI 318 minimums required for Seismic Design Categories D, E, or F. If the risk potential for a blast is high, the minimum reinforcement ratio required for blast-resistant design (TM 5-855-1; Dahscweman 1998) should be employed. Generally, for concrete walls eight inches or greater in thickness, the recommended minimum reinforcement should be 0.25 percent each face. For concrete walls less than eight inches thick, 0.5 percent as a single row (on center line) of reinforcing should be the minimum specified.

(b) Code provisions for maximum allowable reinforcement are included to prevent crushing of concrete prior to yielding of steel. Code provisions also allow compression reinforcement to offset maximum tension reinforcing requirements. Because blast-resistant precast concrete panels typically have the same reinforcing on each face to resist rebound loads, maximum reinforcing provisions should not be a problem.

(c) The substitution of higher grades of reinforcement should not be allowed. Grade 60 reinforcing bars (No. 11 and smaller) have sufficient ductility for dynamic loading. Bars with high yield strength may not have the necessary ductility for flexural resistance and shop bending, thus straight bars should be used when possible. Welding of reinforcement is generally discouraged for blast design applications; however, when required it is for anchorage, ASTM A706 bars may be used.

(d) Development lengths should not be reduced for excessive reinforcement. Because plastic hinges will cause overdesigned reinforcing to yield, the full actual strength of reinforcement should be used in computing section capacities. The development of reinforcing should be computed accordingly.

(e) Criteria intended to reduce cracking at service load levels need not be applied to load combinations including blast. Cracking, as well as permanent deformations resulting from a plastic range response, are an expected result of such an unusual type of load.

(f) Some concrete elements are simultaneously subjected to out-of-plane bending loads in combinations with in-plane shear loads. For example, side walls must resist side overpressures acting into the plane of the side wall. Additionally, reactions from the roof diaphragm acting in the plane of the side shear wall must be resisted.

STANDOFF DISTANCE

In the event of blast, a commercial building's active and passive protection will greatly influence the extent of structural damage sustained and the rescue efforts of emergency personnel. For a given bomb size or charge weight, standoff distance is the key parameter determining the blast overpressures that load the building cladding and its structural elements. Accordingly, a primary tactic involves creating a standoff distance to ensure a minimum guaranteed span between the blast source and the target structure. The blast pressure is inversely proportional to the cube of the distance from the blast to the point in question: for example, if the standoff distance is doubled the peak blast pressure is decreased by a factor of eight (see Fig. 1).

Furthermore, for a similar charge weight, the greater standoff distance results in a longer loading duration than that from a shorter remove; and, the blast wave is more uniformly distributed across the building face. Currently, design standoff distances for blast protection vary from 33 to 148 feet, depending on the function of the building. This standoff distance, or setback zone, is achieved by placing at the site perimeter anti-ram bollards, large planters, low level walls, fountains and other barriers that cannot be compromised by vehicular ramming. In urban areas, the setback choices may be limited, whereas in suburban or rural regions, large setbacks around a building can be used.

Site conditions determine maximum attainable vehicle speeds and, therefore, largely dictate the vehicle kinetic energy applied to the impact that must be resisted by the standoff barriers. Both the bollard and its slab connection must be designed to resist impact loading at the maximum speed attainable. Conversely, if design restrictions limit the capacity of the bollard or its slab connection, then site restrictions will be required to limit the maximum speed attainable by the potential bomb-delivery vehicle.

While a setback zone is the most effective and efficient means to minimize the effect of a terrorist vehicle bomb attack, it can also deter rescue efforts since the barriers could restrict access by rescue and firefighting vehicles. In most urban settings, the setback distance from the street to the building façade is typically 10 to 25 feet, which does not pose any access problems for emergency vehicles. However, when designing prestigious buildings, including landmark office towers, hospitals and museums, the setback is often increased to 100 feet or more to create a more impressive public space. Provisions to allow emergency access should be included in the design of operational bollards or fences. If plaza or monumental stairs are used, secondary access must also be incorporated to allow entry.

Furthermore, public parking lots abutting the building must be secured or eliminated, and street parking should not be permitted adjacent to the building. Additional standoff distance can be gained by removing one lane of traffic and creating an extended sidewalk or plaza. The practical benefit of increasing the standoff, however, depends on the charge weight: if the charge weight is small, such a measure will reduce the forces to a more manageable level; if the threat is a large charge weight, the blast forces may overwhelm the structure despite the addition of nine or ten feet to the standoff distance. In that case, the larger set-back zone may not significantly improve survivability of the occupants or the structure. Yet, even where minimum standoff distances are achieved and site restrictions imposed, many aspects of building layout and other architectural design issues must be incorporated to optimize overall protection of building occupants.