photo.jpg

## Collapse Threat in Construction The most serious collapse threat in buildings under construction occurs in structures that, when finished, would provide the greatest resistance to collapse: poured-in-place reinforced concrete structures. These buildings are built by erecting a wooden or fiberg...

## Collapse Threat in Construction The most serious collapse threat in buildings under construction occurs in structures that, when finished, would provide the greatest resistance to collapse: poured-in-place reinforced concrete structures. These buildings are built by erecting a wooden or fiberglass mold, also known as a form or formwork, in the shape that the concrete is to assume. Wet concrete is then poured into it. The unhardened concrete weighs about 150 lb per cu ft yet has almost no strength at this stage. The strength develops as the concrete dries or cures, molding itself tightly to the reinforcing steel. It takes about 48 hours, depending on temperature, for the concrete to harden enough to support its own weight. It is during this 48-hour period that the structure is most vulnerable to collapse due to fire. The floor below the most recently poured concrete is a veritable lumberyard of plywood and 4x4 posts. Because the forms are built as needed, there is often a good deal of scrap wood and sawdust lying around from the cutting.