By Gregory Havel
Last month’s article discussed some of the forces and actions that can damage a building, cause its collapse, or destroy it. These forces and actions can cause a redistribution of building loads, causing loads that are normally axial to become eccentric or torsional. This redistribution of loads can overload structural members, cause connections to fail, and can cause structural collapse.
The building in the photos is a Type V (wood-frame) single-family residence built on the side of a hill; it has several additions using different types of framing methods. It was one-story on the A (front) side and two stories on the C (back) side. This structure was acquired for live fire training by a fire department in 2010. The all-day fire training event was conducted according to the requirements of National Fire Protection Association (NFPA) Standard 1403, Live Fire Training Evolutions (2007), with the coordination and assistance of fire service training instructors from the technical college district.
After the training evolutions (ventilation, live fire, and overhaul) were completed, the structure was cleared of all personnel, final fires were ignited, and exposures were protected while the structure was burned to the ground.
(1)
Photo 1 shows a view of the structure from the D side before structural collapse; rooms on both floors and the attic are fully involved by fire.
(2)
Photo 2 shows another view of the structure from the D side, after the roof collapsed. The roof on the A side collapsed into the rooms below. The fire weakened the connections between walls and part of the roof on the C side, and the roof slid over the exterior wall until it hit the ground, leaning against the C side wall. This placed an eccentric load on the balcony and exterior wall. This eccentric load initiated a chain reaction that took less than 10 minutes under fire conditions:
- The balcony collapsed.
- The exterior wall at the second floor was pushed inward.
- The exterior wall at the first floor was already partially burned away and collapsed.
- The second floor collapsed onto the first (ground level) floor.
At any incident that has compromised a structure, firefighters must remember the following:
- A building that still stands during or after the incident may not be stable.
- A building that still stands during or after the incident may collapse under the load and impact of interior operations or hose streams.
- A building that has partially collapsed may finish its collapse at any time.
- The length of time that a structural member can continue to resist gravity under undesigned eccentric or torsional loads cannot be predicted.
- Assume that a damaged or partially collapsed building will continue its collapse, right…about…NOW!
- When collapse occurs, we do not want to be either on top of it or under it.
We must include a continuous structural evaluation in our ongoing size-up during the incident. These points apply to any type of incident or event that can damage a structure, including but not limited to earthquakes, floods, high winds, fire, explosion, and impact by vehicle or aircraft.
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