THE FREESTANDING WALL

THE FREESTANDING WALL

FIRE REPORT

A Collapse Waiting to Happen

At 9:16 a. m. on March 23, 1990 in the Bay Ridge section of Brooklyn, New York, we responded to a valve (waterflow) alarm with a normally reduced response—one engine company, one ladder company, and a battalion chief. We expected to find a surge in pressure or a broken sprinkler pipe on arrival. We did find a broken sprinkler pipe, but the reason for the break was unexpected: A 100foot-long by 20-foot-high by 12-inchthick concrete block, freestanding wall, which was to be part of a secondstory addition to a warehouse, collapsed onto and through the roof of an adjoining one-story, 100-foot-by-100foot ordinary-constructed warehouse. Two people were killed and 16 injured, six seriously.

The officer of the first-arriving engine company made a quick and accurate size-up. He requested a full firstalarm response and notified the dispatcher of a wall collapse with numerous people trapped. This short preliminary report put the wheels in motion to bring the necessary fire department and interagency resources to the scene. Brooklyn dispatch called to the scene four engine companies, three ladder companies, three rescue companies (including Rescue Co. 3 with its specially equipped collapse apparatus), one rescue support ladder company, a field communications unit, three battalion chiefs, and one deputy chief. In addition, within seconds notifications were made to the police department, EMS, gas and electric utility companies, the buildings department, and the American Red Cross.

First engine companies were ordered to stretch precautionary hoselines to protect against ignition of leaking gas from broken lines. Units proceeded with the five-step plan for collapse situations: Site-survey, reconnaissance, and data gathering teams were established; surface victims were located and removed; voids were searched for known victims; and selected debris removal for possible victims. General debris removal was very limited (heavy construction equipment wasn’t necessary) because all possible victims were accounted for. All victims were located and removed in less than an hour. Although operations were conducted at great personal risk to firefighters and other emergency personnel due to the potential for secondary collapse, there were no real complications in the search and rescue procedures because of the way that the wall collapsed through the roof of the occupied warehouse.

An aerial view of the collapse site taken a few days after the incident. Note (1) the freestanding west wall opposite the wall that collapsed. Its toothed end, where it was to be tied into the front wall, is visible; this construction practice introduces a plane of weakness into a finished structure. The diagonal metal supports were added after the collapse; (2) the hole in the roof of the adjoining one-story warehouse created by the collapsing wall, illustrating the size of the collapse zone; (3) the scaffolding used to construct the concrete block wall, illustrating the approximate height of the wall at the time of collapse; (4) the 100-foot-long steel I-beam on which the wall was being constructed, visible at the base of the scaffolding.

(All photos by author.)

ANALYSIS

In general, a bearing wall acts as a column. It takes loads transfered to it from other structural members such as beams and girders and transmits these loads to a point of support. In this incident the point of support was a 100-foot-long steel I-beam on which the concrete block wall was being built.

Furthermore, a wall acts as a beam in that it takes lateral loads such as wind, a hose stream hitting it, or a ladder placed against it and transmits the stress perpendicular to the direction in which they are applied. When a roof or intermediate floors are placed on a bearing wall, they increase the ability of the wall to resist lateral loads or forces: They compress the wall and connect it to other portions of the structure. In addition,

when the end or cross walls are tied into the freestanding wall they give the wall more strength to resist lateral loads. These are the laws of physics.

BEARING AND FREESTANDING WALLS

Bearing Wall: An interior or exterior wall that supports a load in addition to its own weight. Part of the skeletal framework of a structure, it most often supports the floors and roof of a building. The collapse of a bearing wall is more serious than the collapse of a column, a flcxr or roof beam, a floor or a roof deck, or a nonbearing wall.

Freestanding Wall: A wall exposed to the elements on both sides and the top, such as a parapet wall, a property-enck>sing wall, an area wall, and a newly constructed exterior wall left standing without roof beams or floors. Of the three types of walls—freestanding, nonbearing, and bearing—the freestanding wall is the most unstable and likely to collapse at a fire because it has fewer supporting connections to the structure.

—From Collapse of Burning Buildings, by Vincent Dunn.

But at this construction site Murphy’s Law came into play. The wall under construction was almost at its maximum height. It was totally freestanding. The cross walls and roof beams were not built yet. The mortar joints of the concrete blocks had not yet cured to their maximum strength, but even if they had they wouldn’t have added much shear strength to the wall. There was no temporary diagonal bracing to give some lateral support to the wall. The wall under construction was at its most vulnerable point. And on the day of the collapse the wind was at 25 mph with higher gusts and it was blowing out of the west, perpendicular to the northsouth wall. So down it came.

It was a 90-degree-angle collapse. The wall fell straight out and the top of the wall hit the warehouse roof below at a distance equal to the height of the wall —in this case about 20 feet. The impact in turn collapsed a section of the warehouse rtxjf approximately equal to the area of the wall, 20 feet by 100 feet.

The warehouse roof hit by the wall was built with 100-foot-long, steel “I” girders, three feet deep and set 20 feet apart. They ran parallel to the front wall of the building and perpendicular to the collapsed wall. The collapsed wall was actually built on top of a steel 1-beam that was welded to the tops of these steel girders. Twenty-foot-long, 2-by10-inch wood beams spanned the distance between the steel girders and rested on the lower flanges. This gave a flat ceiling below and the girders protruded about two feet above the roof. The girders were able to withstand the impact load of the falling wall and did not fail. But the wood beams couldn’t resist the falling wall and failed throughout the area hit by it.

Because it was a 90-degree-angle collapse, the resulting debris was spread out over the relatively wide collapse zone and not concentrated in a smaller area as it might have had it

The 12-inch-deep steel I-beam on which the wall was being constructed. It was welded to the top of, and supported by, steel roof girders of the adjoining warehouse. Two-by-ten-inch wood roof beams rested on the lower flanges of these girders and failed on impact of the collapsing wall.An end view of same I-beam described above. Note the 6-inch-wide top flange and the 12-inch-wide channel iron welded to the flange to support the 12-inch concrete blocks.

been a curtainfall collapse. This made search and removal operations easier.

LESSONS LEARNED AND REINFORCED

  • Freestanding walls are the most dangerous.They cannot withstand lateral loads such as wind, ladders placed against them, or hose streams striking them.
  • Even if a freestanding wall is braced, it doesn’t ensure safety. The bracing may be inadequate to resist the loads; wood or steel bracing may be damaged in a fire; or the bracing-
  • to-ground connections may be weakened by rain or hose streams softening the ground.
  • A freestanding wall can fail at any time, and firefighters must be aware of the collapse zone at all times. Strategy and tactics must reflect this danger. In this case firefighters could have been called to the scene prior to the collapse to fight a fire or aid an injured worker and then have had the wall collapse on them. The collapse zone must be maintained; its dimensions should be at least equal the length and
Another view of the I-beam that supported the collapsed wall.
  • height of the wall. (See Collapse of Burning Buildings by Vincent Dunn, published by Fire Engineering Books).
  • The collapsed wall in this incident was built on a 100-foot-long, 12inch-deep, steel I-beam. Because the top flange of this I-beam was only 6 inches wide, a 100-foot-long, 12-inch piece of channel iron was welded to the top flange to support the 12-inchthick block wall. By outward appearance it would look as if it were a continuous, two-story-high masonry wall built upward from the foundation wall. This steel beam, heated by a fire, could easily twist or fail and drop the masonry wall built on top of it, causing a total failure of the second-story addition. The impact load of the second-story addition falling on the original one-story warehouse would cause it to collapse also.
  • A dangerous structural condition such as a wall supported by an unprotected steel beam must be noted
  • prominently on prefire plans and the firefighting strategy adjusted accordingly.
  • Inspections of buildings under construction and the regular inspection of completed buildings is a critical function of fire departments. It is during these inspections that structural hazards to firefighters should be picked up and entered onto prefire plans.
  • Setting up a command post and instituting the incident command system early at an incident helps everyone involved in attaining the best possible outcome.
  • The resources and expertise of support agencies outside the fire department are vital to successful collapse rescue operations.
  • There is an old saying in fire department circles, “They don’t call us because they did something right.” It certainly was true for this incident and most other incidents we respond to.

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