6.3.5 Structural Elements

Because direct explosion effects decay rapidly with distance, the local response of structural components is the dominant concern. General principles governing the design of critical components are discussed below.

Exterior Frame

There are two primary considerations for the exterior frame. The first is to design the exterior columns to resist the direct effects of the specified threats. The second is to ensure that the exterior frame has sufficient structural integrity to accept localized failure without initiating progressive collapse. The former is discussed in this section, the latter in the sub-section on structural integrity. Exterior cladding and glazing issues are discussed in section 6.4, Building Envelope.

Because columns do not have much surface area, air-blast loads on columns tend to be mitigated by “clear-time effects”. This refers to the pressure wave washing around these slender tall members, and consequently the entire duration of the pressure wave does not act upon them. On the other hand, the critical threat is directly across from them, so they are loaded with the peak reflected pressure, which is typically several times larger than the incident or overpressure wave that is propagating through the air.

For columns subjected to a vehicle weapon threat on an adjacent street, buckling and shear are the primary effects to be considered in analysis. If a very large weapon is detonated close to a column, shattering of the concrete due to multiple tensile reflections within the concrete section can destroy its integrity.

Buckling is a concern if lateral support is lost due to the failure of a supporting floor system. This is particularly important for buildings that are close to public streets. In this case, exterior columns should be capable of spanning two or more stories without buckling. Slender steel columns are at substantially greater risk than are concrete columns.

Confinement of concrete using columns with closely spaced closed ties or spiral reinforcing will improve shear capacity, improve the performance of lap splices in the event of loss of concrete cover, and greatly enhance column ductility. The potential benefit from providing closely spaced closed ties in exterior concrete columns is very high relative to the cost of the added reinforcement.

For steel columns, splices should be placed as far above grade level as practical. It is recommended that splices at exterior columns that are not specifically designed to resist air-blast loads employ complete-penetration welded flanges. Welding details, materials, and procedures should be selected to ensure toughness.

For a package weapon, column breach is a major consideration. Some suggestions for mitigating this concern are listed below.

• Do not use exposed columns that are fully or partially accessible from the building exterior. Arcade columns should be avoided.
• Use an architectural covering that is at least six inches from the structural member. This will make it considerably more difficult to place a weapon directly against the structure. Because explosive pressures decay so rapidly, every inch of distance will help to protect the column.

Load- bearing reinforced concrete wall construction can provide a considerable level of protection if adequate reinforcement is provided to achieve ductile behavior. This may be an appropriate solution for the parts of the building that are closest to the secured perimeter line (within twenty feet). Masonry is a much more brittle material that is capable of generating highly hazardous flying debris in the event of an explosion. Its use is generally discouraged for new construction.

Spandrel beams of limited depth generally do well when subjected to air blast. In general, edge beams are very strongly encouraged at the perimeter of concrete slab construction to afford frame action for redistribution of vertical loads and to enhance the shear connection of floors to columns.

Roof System

The primary loading on the roof is the downward air-blast pressure. The exterior bay roof system on the side(s) facing an exterior threat is the most critical. The air-blast pressure on the interior bays is less intense, so the roof there may require less hardening. Secondary loads include upward pressure due to the air blast penetrating through openings and upward suction during the negative loading phase. The upward pressure may have an increased duration due to multiple reflections of the internal air-blast wave. It is conservative to consider the downward and upward loads separately.

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.

Somewhat lower levels of protection are afforded by conventional steel beam construction with a steel deck and concrete fill slab. The performance of this system can be enhanced by use of normal-weight concrete fill instead of lightweight fill, increasing the gauge of welded wire fabric reinforcement, and making the connection between the slab and beams with shear connector studs. Since it is anticipated that the slab capacity will exceed that of the supporting beams, beam end connections should be capable of developing the ultimate flexural capacity of the beams to avoid brittle failure. Beam-to-column connections should be capable of resisting upward as well as downward forces.

Precast and pre-/post-tensioned systems are generally considered less desirable, unless members and connections are capable of resisting upward forces generated by rebound from the direct pressure and/or the suction from the negative pressure phase of the air blast.

Concrete flat slab/plate systems are also less desirable because of the potential of shear failure at the columns. When flat slab/plate systems are used, they should include features to enhance their punching shear resistance. Continuous bottom reinforcement should be provided 
through columns in two directions to retain the slab in the event that punching shear failure occurs. Edge beams should be provided at the building exterior.

Lightweight systems, such as untopped steel deck or wood frame construction, are considered to afford minimal resistance to air-blast. These systems are prone to failure due to their low capacity for downward and uplift pressures.

Floor System

The floor system design should consider three possible scenarios: airblast loading, redistributing load in the event of loss of a column or wall support below, and the ability to arrest debris falling from the floor or roof above.

For structures in which the interior is secured against bombs of moderate size by package inspection, the primary concern is the exterior bay framing. For buildings that are separated from a public street only by a sidewalk, the uplift pressures from a vehicle weapon may be significant enough to cause possible failure of the exterior bay floors for several levels above ground. Special concern exists in the case of vertical irregularities in the architectural system, either where the exterior wall is set back from the floor above or where the structure steps back to form terraces. The recommendations of Section 6.3.5.2 for roof systems apply to these areas.

Structural hardening of floor systems above unsecured areas of the building such as lobbies, loading docks, garages, mailrooms, and retail spaces should be considered. In general, critical or heavily occupied areas should not be placed underneath unsecured areas, since it is virtually impossible to prevent against localized breach in conventional construction for package weapons placed on the floor.

Precast panels are problematic because of their tendency to fail at the connections. Pre-/post-tensioned systems tend to fail in a brittle manner if stressed much beyond their elastic limit. These systems are also not able to accept upward loads without additional reinforcement. If pre-/post-tensioned systems are used, continuous mild steel needs to be added to the top and the bottom faces to provide the ductility needed to resist explosion loads.

Flat slab/plate systems are also less desirable because of limited two way action and the potential for shear failure at the columns. When flat slab/plate systems are employed, they should include features to enhance their punching shear resistance, and continuous bottom reinforcement should be provided across columns to resist progressive collapse. Edge beams should be provided at the building exterior.

Interior Columns

Interior columns in unsecured areas are subject to many of the same issues as exterior columns. If possible, columns should not be accessible within these areas. If they are accessible, then obscure their location or impose a standoff to the structural component through the use of cladding. Methods of hardening columns (already discussed under Section 6.3.5.1, Exterior Frame) include using closely spaced ties, spiral reinforcement, and architectural covering at least six inches from the structural elements. Composite steel and concrete sections or steel plating of concrete columns can provide higher levels of protection. Columns in unsecured areas should be designed to span two or three stories without buckling in the event that the floor below and possibly above the detonation area have failed, as previously discussed.

Interior Walls

Interior walls surrounding unsecured spaces are designed to contain the explosive effects within the unsecured areas. Ideally, unsecured areas are located adjacent to the building exterior so that the explosive pressure may be vented outward as well.

Fully grouted CMU (concrete masonry unit) block walls that are well reinforced vertically and horizontally and adequately supported laterally are a common solution. Anchorage at the top and bottom of walls should be capable of developing the full flexural capacity of the wall. Lateral support at the top of the walls may be achieved using steel angles anchored into the floor system above. Care should be taken to terminate bars at the top of the wall with hooks or heads and to ensure that the upper course of block is filled solid with grout. The base of the wall may be anchored by reinforcing bar dowels.

Interior walls can also be effective in resisting progressive collapse if they are designed properly with sufficient load-bearing capacity and are tied into the floor systems below and above. 

This design for hardened interior wall construction is also recommended for primary egress routes to protect against explosions, fire, and other hazards trapping occupants.