FLOOR SLABS         

Treatments for conventional flat slab design are as follows:

1. More attention must be paid to the design and detailing of exterior bays and lower floors, which are the most susceptible to blast loads.

2. In exterior bays/lower floors, drop panels and column capitols are required to shorten the effective slab length and improve the punching shear resistance.

3. If vertical clearance is a problem, shear heads embedded in the slab will improve the shear resistance and improve the ability of the slab to transfer moments to the columns.

4. The slab-column interface should contain closed-hoop stirrup reinforcement properly anchored around flexural bars within a prescribed distance from the column face.

5. Bottom reinforcement must be provided continuous through the column. This reinforcement serves to prevent brittle failure at the connection and provides an alternate mechanism for developing shear transfer once the concrete has punched through.

6. The development of membrane action in the slab, once the concrete has failed at the column interface, provides a safety net for the post damaged structure. Continuously tied reinforcement, spanning both directions, must be detailed properly to ensure that the tensile forces can be developed at the lapped splices. Anchorage of the reinforcement at the edge of the slab is required to guarantee the development of the tensile forces.

COLUMNS

Treatment for conventionally designed columns to improve blast resisting mechanism:

1. The potential for direct lateral loading on the face of the columns, resulting from the blast pressure and impact of explosive debris, requires that the lower-floor columns be designed with adequate ductility and strength

2. The perimeter columns supporting the lower floors must also be designed to resist this extreme blast effect

3. Encasing these lower-floor columns in a steel jacket will provide confinement, increase shear capacity, and improve the columns’ ductility and strength. An alternative, which provides similar benefits, is to embed a steel column within the perimeter concrete columns or wall section.

4. The possibility of uplift must be considered, and, if deemed likely, the columns must be reinforced to withstand a transient tensile force.

5. For smaller charge weights, spiral reinforcement provides a measure of core confinement that greatly improves the capacity and the behavior of the reinforced concrete columns under extreme load.

TRANSFER GIRDERS

The building relies on transfer girders at the top of the atrium to distribute the loads of the columns above the atrium to the adjacent columns outside the atrium. The transfer girder spans the width of the atrium, which insures a column-free architectural space for the entrance to the building.

Transfer girders typically concentrate the load-bearing system into a smaller number of structural elements. This load-transfer system runs contrary to the concept of redundancy desired in a blast environment. The column connections, which support the transfer girders, are to provide sustained strength despite inelastic deformations. The following recommendations must be met for transfer girders:

1. The transfer girder and the column connections must be properly designed and detailed, using an adequate blast loading description.

2. A progressive-collapse analysis must be performed, particularly if the blast loading exceeds the capacity of the girder

GLAZING

Typical annealed plate glass is only capable of resisting, at most, 14 kPa of blast pressure and it behaves poorly on explosion. On failure annealed glass creates large sharp edged shards, resembling knives and daggers which cause injuries and casualties..

There exist better types of glazing that can resist some modest blast pressures. Thermally Tempered Glass (TTG) and Polycarbonate layups can be made in sheets up to about l-in. thick and can resist pressures up to about 200 to 275KPa. The greatest benefit of TTG is unlike annealed glass; TTG breaks into rock-salt sized pieces that will inflict less injury on the occupants. The failed Polycarbonate glass unfortunately remains in one piece,

EXTERNAL TREATMENTS

The two parameters that most directly influence the blast environment that the structure will be subjected to are the bomb’s charge weight and the standoff distance. Of these two, the only parameter that anyone has any control over is the standoff distance.

FACADE AND ATRIUM

The facade is comprised of the glazing and the exterior wall. Better glazing has already been discussed above and wall obviously should be hardened to resist the loading

Presence of an atrium along the face of the structure will require two protective measures. On the outside of the structure, the glass and glass framing must be strengthened to withstand the loads. On the inside, the balcony parapets, spandrel beams, and exposed slabs must be strengthened to withstand the loads that enter through the shattered glass.

OVERALL LATERAL BUILDING RESISTANCE, SHEAR WALLS

The ability of structures to resist a highly impulsive blast loading depends on the ductility of the load-resisting system. This means that the structure has to be able to deform in elastically under extreme overload, thereby dissipating large amounts of energy, prior to failure..In addition to providing ductile behavior for the structure, the following provisions would improve the blast protection capability of the building:

1. Use a well-distributed lateral-load resisting mechanism in the horizontal floor plan. This can be accomplished by using several shear walls around the plan of the building this will improve the overall seismic as well as the blast behavior of the building.

2. If adding more shear walls is not architecturally feasible, a combined lateral-load resisting mechanism can also be used. A central shear wall and a perimeter moment-resisting frame will provide for a balanced solution. The perimeter moment-resisting frame will require strengthening the spandrel beams and the connections to the outside columns. This will also result in better protection of the outside columns.

Several recommendations were presented for each of the identified features. The implementation of these recommendations will greatly improve the blast-resisting capability of the building under consideration.

LOWER FLOOR EXTERIOR

The architectural design of the building of interest currently calls for window glass around the first floor. Unless this area is constructed in reinforced concrete, the damage to the lower floor structural elements and their connections will be quite severe. Consequently, the injury to the lower floor inhabitants will be equally severe. In general, two sizes of charges can be discussed

1. To protect against a small charge weight, a nominal 300 mm (12 in.) thick wall with 0.3% steel doubly reinforced in both directions might be required.

2. For intermediate charge weight protection, a 460 mm (18 in.) thick wall with 0.5% steel might be needed.

Stand Off DISTANCE

The keep out distance, within which explosives-laden vehicles may not penetrate, must be maximized and guaranteed. As we all know, the greater the standoff distance, the more the blast forces will dissipate resulting in reduced pressures on the building. Several recommendations can be made to maintain and improve the standoff distance for the building under consideration:

1. Use anti-ram bollards or large planters, placed around the entire perimeter. These barriers must be designed to resist the maximum vehicular impact load that could be imposed. For maximum effectiveness, the barriers-bollards or planters-must be placed at the curb.

2. The public parking lot at the corner of the building must be secured to guarantee the prescribed keepout distance from the face of the structure. Preferably, the parking lot should be eliminated.

3. Street parking should not be permitted on the near side of the street, adjacent to the building

4. An additional measure to reduce the chances of an attack would be to prevent parking on the opposite side of the street. While this does not improve the keep out distance, it could eliminate the “parked” bomb, thereby limiting bombings to “Park and run”

INTERNAL EXPLOSION THREATS

The blast environment could be introduced into the interior of the structure in four vulnerable locations:

The entrance lobby, the basement mechanical rooms, the loading dock, and the primary mail rooms. Specific modifications to the features of these vulnerable spaces can prevent an internal explosion from causing extensive damage and injury inside the building.

1. Walls and slabs adjacent to the lobby, loading dock, and mail rooms must be hardened to protect against the hand delivered package bomb, nominally a 10-20 kg explosive. This hardening can be achieved by redesigning the slabs and erecting cast-in-place reinforced-concrete walls, with the thickness and reinforcement determined relative to the appropriate threats.

2. The basement must be similarly isolated from all adjacent occupied office space, including the floor above, from the threat of a small package bomb.