JEC COMPOSITES MAGAZINE - Issue #33 - June 2007 - 40

FEATURE Walls with a downside “Construction” to the exact size they need to fit a shelter into a floor plan. A shelter option that provides greater size flexibility than cinder block or ICF is a system that consists of wood framing covered with a steel outer layer and a plywood inner layer. Like thermoset composite material, steel can be cut to the exact dimensions required by the floor plan of a house. But the cutting is usually done off-site by a steel fabricator. Then, the steel is attached to the framing system back at the building site. This could require screw holes to be drilled through the steel. Or workers could be supplied with powder-actuated nailer guns to drive nails through the steel, a process that can be both expensive and time-consuming. g…/… Composite-based storm shelters framing system covered with high-strength thermoset composite material on the outside (walls and ceiling) and plywood on the inside. The structure also includes a steel door with several deadbolts. Standard plans accommodate shelters as large as 2.5 x 3.7 x 2.5 m high. A structure this size is classified by the US Federal Emergency Management Agency (FEMA) as an “intermediate” sized safe room, but larger shelters can also be constructed. A key to the strength of the composite-based system is the wall sheathing material, consisting of a woven glass substrate treated with phenolic resin and pressed into a hard sheet measuring 1.2 x 2.4 m and weighing about 36 kg. Though they are extremely strong, the thermoset sheets are just 8 mm thick. The thin sheets help to minimize the overall thickness of the shelter walls. This is important because the thicker the walls, the less space there will be inside the shelter. The walls of the composite-based shelter – including a thermoset outer wall sheathing, timber board framing, a plywood inner wall sheathing, and drywall – are approximately 150 mm thick, making them only about 38 mm thicker than normal house walls. Thus, the thin composite panels offer minimal loss of wall space, allowing shelters to be constructed into rooms that serve other purposes. Within the storm shelter, walls are covered with drywall and painted so that the shelter looks like a typical room in the house. Under normal circumstances, the shelter can serve as a closet, pantry, bathroom, or utility room. But when a severe storm approaches, residents can seek refuge in the room, knowing that it will survive even if the rest of the house does not. JEC Composites Magazine / No33 June 2007 While composite-based systems offer minimal loss of wall space, this is not the case when the walls of an above-ground shelter are made of cinder block. Cinder block shelter walls are very strong but the cinder block alone is 20 cm thick, not including the drywall that must be added to the inner walls. Drywall can be glued directly onto cinder block, but the resulting inner surface is not strong enough to support shelving. So in many cases, the drywall is attached to timber board wood framing constructed next to the cinder block. This brings the total wall thickness to about 30 cm, which significantly reduces the usable floor space inside the shelter. Another wall option for above-ground shelters is insulated concrete foam (ICF), which is lighter than cinder block. Including drywall layers, ICF walls are about 23 cm thick – thinner than cinder block walls but still considerably thicker than the walls of a thermoset composite shelter. ICF construction is also more expensive than thermoset composite sheets. Besides providing high strength in a thin and relatively inexpensive package, thermoset wall systems give builders greater flexibility in sizing shelters. Builders can cut the material Aramid-based options By contrast, thermoset composite wall material is easily cut to the desired shape and nailed or screwed to the framing system. The entire job can be done at the building site by workers using standard construction equipment. Though they are extremely strong, the Norplex-Micarta StormBlocker composite-based thermoset sheets are just 8 mm thick. Like steel and thermoset composite material, aramid-based ballistic resistant materials can be attached to wood framing to form shelter walls. But aramid-based material is roughly three times more expensive than glass-based materials. As a result, shelters made with aramid-based materials cost up to 40% more than shelters with thermoset composite wall panels. Aramid-based material also has other downsides. Unlike customizable thermoset composite panels, aramid walls come in only a few standard sizes. In addition, it lacks the rigidity of its thermoset counterpart. Aramid-based material is flexible, so it stretches and recovers when force is applied to it. When hit by flying debris, thin aramid walls flex a good distance into a shelter before snapping back to their original shape, so these shelter walls must be significantly thicker than thermoset walls in order to absorb impact energy without extreme deformation that would endanger people inside the room. Additionally,

JEC COMPOSITES MAGAZINE - Issue #33 - June 2007

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #33 - June 2007