Steel trusses are constructed similarily to wooden trusses in that they have a top chord, bottom chord which are connected by a web. The thickness and height, of the truss is governed, by the engineered use-span and load. The span is the free-unsuppported length. The manufacture of steel involves heat, fire. Alloys are added to give the ore more strength, but the process gives a major clue as why these members fail. The heat of a fire affecting these trusses either by conduction, convection, or direct flame impingement causes the temperature of the steel to rise to above its stable state. This stability exists up to around 900 degees farhenheit.
After this range is reached up to 1100 degrees the steel begins to react by slowly exceeding its ability to remain where it was placed. Above 1100 degrees F the steel begins to move- often as much as 1 inch per 10 feet of length. This unrestrained movement begins to change the load bearing configuration of the supports. The masonry pocket is compromised and changes the block/brick from compression into tension, which can never be allowed to happen as collapse will be imminent. When the steel sits on steel the total construction connection will be simultaneously affected and collapse will occur. Or the steel member loses its tensile strength and fails totally.
When the failure begins- the load shifting will begin to severely affect the ability for the truss to resist gravity's efforts to pull the steel back to earth. The extent of failure can be predicted by formula but we don't carry these formulas in our turnouts. So, how can we protect ourselves?
First, we must do a better job of size-up. I wish we could use the term reconnaissance because that is really what supposed to happen. Often times however, we transmit an incomplete assessment over the radio and then rush inside to quickly put the wet stuff on the red stuff. This paradigm is now playing too large a role in firefighter deaths. The construction of the past sixty years is less forgiving than that of its predecessors. Any light-weight members either wood or steel trusses will and do fail quicker than old style heavy timber or masonry/steel-H column construction. The fact that these assemblies occur in commercial structures should be a plus for us. There is less chance for the structure being occupied.
However, more firefighters are killed in commercial structures each year even though we fight more fires in residential construction. The initial reconnaissance should include whether the building is occupied at the time. If not then SLOW DOWN, use as few people as possible, have hose lines available, and stay away from the center of the structure. The best way to survey is from the perimeter walls. Using Thermal Imaging Equipment at this time will be very beneficial. All personnel inside the structure and also those outside should exercise caution during this period of assessment. Accurate and timely reports should be given to the Incident Commander giving location and conditions of the company or individual giving the report. When fire conditions are discovered use the full reach of streams. It is not safe fire operations to be inside the structure if you have hose streams, which can reach the seat of the fire while you are outside.
The information from after action reports involving firefighters being killed at these incidents indicate that little or nothing was showing at the start of the time line but suddenly without warning the roof or floor failed. This is very difficult to understand if we were laymen. The facts might be better understood if we factored in that during the response all firefighters were being charged with adrenaline.
Upon hearing the report that nothing or little was showing this adrenaline surge begins to wane and a sense of routine can take over thus suppressing the natural sense for caution. This behavior change must be trained, drilled, and instilled - no incident is ever routine. We must accept that we are not always required to rush into a situation. Sometimes it is better to go slow and observe everything then make decisions. Stay Safe.
