Gravity is a force that exists every day. It is present 24 hours a day seven days a week. Builders combat it from the time they begin to cut into the earth. The higher into the air that a building reaches the more gravity wants to drag it back down to the ground. Therefore, a builder must continually transfer loads down through the structural members back to the earth. Three of the traditional methods for load transfer are compression, tension, and shear.
Compression is direct stacking or loading. If you press your fist down into your palm it is the same. Concrete and wood trusses are excellent in compression, but will fail if shifted into tension. For example, if you have ever attempted to demolish a concrete slab or sidewalk you know this principle. In place the concrete absorbs the blows and eventually may crack. But lift the slab just a little bit and the work becomes quite easy. If you have ever seen wood trusses being installed you can also see this principle. Before they are in place and "racked in" with plywood they are extremely flimsy looking. Once installed however they appear quite solid.
Tension is when an object is hanging. When you perform chin lifts you are in tension. Items attached to ceilings are in tension. The undersides of trusses are in tension when loaded.
Shear is when an object is attached to a vertical plane. Pictures hanging on walls are in shear. Wooden decks attached to structures are in compression on the ends but in shear at the attachment to the building. A shed roof configuration is in shear. The pictures and the deck and the shed roof are dependent on the attachment system. If the connectors are undersized or held only by friction of nails the weight of the load can pull the picture, deck, or shed roof from its position causing a load shift and a failure. If you have ever responded to a deck collapse then you have seen how the deck pulls away and then down.
The builder is guided by the local building codes as they construct their structure. The building codes however are the minimum sized members or the maximum allowed spacing which keeps the building erect and supporting loads while still resisting gravity. Nowhere in these codes are fires and its effects considered in weight distribution or load shift if this configuration is compromised. The engineers also don't factor fire into their calculations.
During an incident when the effects of heat are affecting the components of a structure many dynamics are taking place. Concrete poured on Q-decking is being kept in compression and is quietly performing its task. Heat affects the steel decking causing it to heave or sag, which in turn causes the concrete to shift from compression into tension and therefore will fail. The extent to which it fails is solely dependent on time and temperature. Wooden trusses begin to fail as soon as the lightweight metal gussets begin to lose their tensile strength or ability to grip the wood. The truss is already set at the widest span allowable against gravity. The fire compromises this relationship and failures occur. The subsequent load shifting can cause sagging or abrupt total failure of the component or roof/floor assembly. This is also true with light- weight steel trusses. Anytime a firefighter is standing on an assembly in shear with fire affecting the connectors then that firefighter is standing in mid-air and their weight contributes greatly to how fast the collapse will take place.
Identify these conditions before the incident with good and complete pre-fire planning. Or consider them as you do your own size-up. How much fire is in the building and what effects is it having? If you do not know for sure whether a roof is trussed or not use caution. Perform vertical ventilation from the ends at the highest point. Stay close to bearing walls while inside. Trusses have a tendency to fail in v-shaped collapses. Remember wooden trusses always, ALWAYS, have a gusset exactly in the middle of the building on the top and bottom chord. This configuration fails most often. Concrete can and will fail with enough heat underneath. If it is too hot to kneel on the floor there is a good possibility there is fire under you. Unless you have definite rescues to be made it may be wise to operate outside of this area. Think as you go in. Gravity is waiting to grab you at the first opportunity. The trick is don't be above or below a failure. Stay Safe.