6.4.1 Exterior Wall/Cladding Design

The exterior walls provide the first line of defense against the intrusion of the air-blast pressure and hazardous debris into the building. They are subject to direct reflected pressures from an explosive threat located directly across from the wall along the secured perimeter line. If the building is more than four stories high, it may be advantageous to consider the reduction in pressure with height due to the increased distance and angle of incidence. The objective of design at a minimum is to ensure that these members fail in a ductile mode such as flexure rather than a brittle mode such as shear. The walls also need to be able to resist the loads transmitted by the windows and doors. It is not uncommon, for instance, for bullet-resistant windows to have a higher ultimate capacity than the walls to which they are attached. Beyond ensuring a ductile failure mode, the exterior wall may be designed to resist the actual or reduced pressure levels of the defined threat. Note that special reinforcing and anchors should be provided around blastresistant window and door frames.

Poured-in-place, reinforced concrete will provide the highest level of protection, but solutions like pre-cast concrete, CMU block, and metal stud systems may also be used to achieve lower levels of protection.

For pre-cast panels, consider a minimum thickness of five inches exclusive of reveals, with two-way, closely spaced reinforcing bars to increase ductility and reduce the chance of flying concrete fragments. The objective is to reduce the loads transmitted into the connections, which need to be designed to resist the ultimate flexural resistance of the panels. Also, connections into the structure should provide as straight a line of load transmittal as practical.

For CMU block walls, use eight-inch block walls, fully grouted with vertically centered heavy reinforcing bars and horizontal reinforcement placed at each layer. Connections into the structure should be designed to resist the ultimate lateral capacity of the wall. For infill walls, avoid transferring loads into the columns if they are primary load-carrying elements. The connection details may be very difficult to construct. It will be difficult to have all the blocks fit over the bars near the top, and it will be difficult to provide the required lateral restraint at the top connection. A preferred system is to have a continuous exterior CMU wall that laterally bears against the floor system. For increased protection, consider using 12-inch blocks with two layers of vertical reinforcement.

For metal stud systems, use metal studs back-to-back and mechanically attached, to minimize lateral torsional effects. To catch exterior cladding fragments, attach a wire mesh or steel sheet to the exterior side of the metal stud system. The supports of the wall should be designed to resist the ultimate lateral out-of-plane bending capacity load of the system. 

Brick veneers and other nonstructural elements attached to the building exterior are to be avoided or have strengthened connections to limit flying debris and to improve emergency egress by ensuring that exits remain passable.