Oxidative stress

Appendix A presents a more detailed discussion of the structural effects of fire. • As floor framing and supported slabs above and in a fire area are heated, they expand. The people who designed the towers were not fools and knew all this. They designed the towers to survive much more serious fires than those that occurred on September 11. As a structure expands, it can develop additional, potentially large, stresses in some elements. If the resulting stress state exceeds the capacity of some members or their connections, this can initiate a series of failures (Figure 2-20). • As the temperature of floor slabs and support framing increases, these elements can lose rigidity and sag into catenary action. As catenary action progresses, horizontal framing elements and floor slabs become tensile elements, which can cause failure of end connections (Figure 2-21) and allow supported floors to collapse onto the floors below. The presence of large amounts of debris on some floors of WTC 1 would have made them even more susceptible to this behavior ...»