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## Types of Buildings Collapses tend to occur in a similar fashion in similar types of buildings. Those with a protected steel or concrete supporting system resist total collapse rather well. These are called **framed structures**, since most of their weight is carried on the frame or skeleton....

## Types of Buildings Collapses tend to occur in a similar fashion in similar types of buildings. Those with a protected steel or concrete supporting system resist total collapse rather well. These are called **framed structures**, since most of their weight is carried on the frame or skeleton. Collapses that occur in framed structures are usually very localized—only the area between the two supporting members. Of course, even this small an area can kill you if you happen to be below the section that falls (fig. 22-9). ## Fig. 22-8. When steel is highly heated, it twists and sags, dropping its load. When cooled, it freezes in its new shape as it contracts. (Joseph Sperber) This leaves **Class 2 construction** (metal or noncombustible) as the least resistant to collapse when exposed to fire. This is mainly because large quantities of unprotected steel are used in its construction. If the steel were protected with some sort of fireproofing and/or encasement, it would no longer be a **Class 2 structure**, but a **Class 1 structure**. Steel's behavior under fire conditions creates several problems from the point of view of collapse. The first is that steel expands when it is heated. A 100-ft. long I-beam heated uniformly to 1,000˚F will expand 9½ in. lengthwise. This can push down columns or walls, as well as punch holes through walls through which fire can spread. When steel is further heated to about 1,500˚F, it will lose about half its strength, either dropping its load or twisting and sagging. Any beams that depend on it will fall. Moreover, when the steel is cooled back to its original temperature, it will contract back to its original length while retaining its distorted shape (fig. 22-8). If an I-beam were severely twisted or sagged, and then cooled by a hose stream, the steel would shrink, possibly enough so that the ends of the beams would no longer be resting on their original supports, and further collapse could occur.