Know Your Enemy #39

A fire in the false work would cause the entire floor to collapse. That would cause the progressive collapse of all floors below, crushing every firefighter in the building.


There are three deadly winter hazards in my area of interest.

A post-tensioned concrete collapse will be total and kill all firefighters in the structure, and others close to it.

"We have a fire in some wood on a construction job. We are going to work with a 1 ? line." Could that be your last transmission?

What kind of attack would you make on a fire involving the wooden false work (form work shapes the concrete, false work supports the form work) including wooden I beams, which in a post-tensioned concrete job are supporting hard and dry concrete that is not attached to the building until the tendons are tensioned ("the cables are jacked") several days after the concrete is hard and dry. A fire in the false work would cause the entire floor to collapse. That would cause the progressive collapse of all floors below, crushing every firefighter in the building.

When I presented this hazard at the NFPA meeting in Philadelphia in 1971, Chester Babcock, the editor of NFPA's Fire Journal, refused to print it. "I can't believe they are building buildings which will totally collapse." Engineering News Record published it.

Read up on post-tensioned concrete in BCFS3, pages 350-57. Check every construction site for post-tensioned concrete. Big rolls of plastic covered steel cable that look like electrical cable is a good clue. Get cooperation from the building department to get a list of post-tensioned concrete construction.

In winter, the work area is heated to keep concrete from freezing and for personal comfort. LPG fueled heaters are usually used although I have seen scrap wood fires in barrels.

The heating can be an ignition hazard to the wooden false work and form work and the heating fuel used (see the picture on p 351 BCFS3). A fire should be declared defensive immediately on arrival. All operation should be outside the defensive zone.

At Biscayne Bay, Florida an outside fire in construction material extended to the false/form work of a post-tensioned building. The chief established a collapse zone but it was a real problem to enforce. This has been a problem elsewhere. An experienced fire officer was killed when the front wall of a building under defensive attack collapsed. The report did not say what the officer was attempting to accomplish.

A collapse zone should be set up at every defensive attack. Evacuation does not mean it is OK to stand at windows with hose lines. See the front piece in BCFS3 for a graphic example of the danger.

Whenever a collapse zone is established, which is uncontrovertibly the right and duty of the IC, a sector officer should be appointed to enforce the zone with police assistance if necessary. If there were some doubt at to this right in any state, I would like to hear of it.

You may have heard of friction between police and fire in New York City. It may to go back to a time many years ago, when the fire commissioner had the police escort the police commissioner outside the fire lines, due to his interference.

STACK EFFECT

On a recent very cold day in New York City, a fire occurred in a 4th floor apartment in a 22-story high rise apartment building. Fire alarm headquarters passed to the IC reports of smoke from persons in the following locations.

  • Apt. 12 "henry" reporting people trapped.
  • Apt. 14 "george" reporting people trapped.
  • Apt. 11 "edward".
  • Apt. 18 "frank".
  • Apt. 17 "adam".
  • Penthouse "D" on the balcony
  • Apt. 15 "frank".
  • Apt. 11 "frank" elderly female, difficulty breathing.
  • Apt. 18 "robert" female difficulty breathing.
  • Penthouse "A" & "B" reporting smoke.
  • Apt. 20 "charlie".
  • Apt. 10 "frank".

Note that the fire was on the fourth floor but the smoke distressed floors from the 10th floor to the penthouse. Why? The answer is the stack effect.

The stack effect is vertical air movement caused by the difference between inside and outside temperatures. The greater the difference the greater the effect. The more tightly closed the building is, the greater the effect.

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