FIRE LOSS MANAGEMENT
DISASTER MANAGEMENT
SFPE
Part 13:
PHYSICAL BARRIERS TO FIRE EXTENSION
The two methods of halting the extension of fire are to put the fire out or to provide a physical barrier to the heat of the fire. The former is dynamic fire protection, which is either manual or automatic (these will be discussed in a future article); the latter is static fire protection. The distinction between the two is not clear-cut, however. For example, a static fire wall often requires the proper dynamic action of closing a fire door to be effective.
FIRE BARRIERS
The simplest way to control fire extension by barriers is to isolate the combustible material within a fireresistive room. This was accomplished with an old-fashioned “fireproof warehouse”-—a massive masonry building in which material was stored in cubicles, each packed as tightly as possible and then sealed behind a heavy door. The light switch was outside the door—there was no source of ignition. The concept was that any fire would consume all the oxygen in the room and smother itself before extension became a problem.
The system is not foolproof, however. 1 inspected one warehouse in which all the aisles were full of refrigerators in flimsy plywood and cardboard crates. A fire in this stock would have been a difficult problem mainly because of the lack of access. In another case, the system had been compromised by the installation of ventilating ducts. It was necessary to cut through the floor above and use distributors to control the fire.
Such a building is very inconvenient to work in and would be impossible to live in. For aesthetic reasons, modern architecture tends to favor more open spaces inside of buildings. Space requirements for assembly lines and massive research equipment, as well as the use of lightweight movable partitions for building flexibility, tend to decrease these static physical barriers to the spread of fire.
HORIZONTAL SPREAD
There are various guidelines as to the size of areas that should be subjected to a single fire. If these space requirements must be exceeded, the problem often can be overcome by installation of an automatic sprinkler system, but whatever the fire areas are, the separation between the fire areas must be complete.
Fire Walls
The hot gases generated by a fire resemble water behind a dam, the basic difference being that the gravity relationship is reversed. While the water behind the dam presents its greater pressure at the bottom of the dam, the heated gases from the fire present their greatest pressure at the topmost available point. If a dam has even a finger-sized hole in it and there is no Dutch boy around to put his finger in the dike, the dam will fail. The case is exactly the same for fire walls. A small hole in the fire wall, high up and subjected to the tremendous gas pressure generated by the fire, will permit a jet of fire to stream through the wall. If this jet of fire ignites material on the far side of the fire wall, the fire wall is useless.
The fire wall, therefore, must be complete in its structure. Where the fire wall joins the building enclosure wall the situation must be examined realistically to determine if fire can get around the wall. If the building walls are combustible or contain windows, the fire wall should be projected beyond the wall of the building or turn to the right or left, continuing for a sufficient distance to make it impossible for the fire to roll past the edge of the wall. If the roof structure is combustible, the fire wall should extend through and above the roof to make it impossible for fire to extend by rolling over the top of the wall.
Very often, architects whose experience has been limited to residential construction find it difficult to understand why fire walls should be extended through the roof. In residential construction the primary consideration is life safety (of the residents, not the firefighter), and, under pressure to reduce costs, some building codes foolishly do not require the fire wall to be extended through the roof. When a fire occurs, the occupants may well escape but the building may be destroyed as the fire slips over the fire wall and back down into the other sections of the building.
Fire Doors
Architects and building occupants find reasons—whether good or bad — to put holes in the barrier. The chief necessity is for a human passageway through die wall. The passageway must be protected by fire doors of a proper type.
At best, fire doors offer fire protection that is inferior to that of the solid masonry wall. The codes assume that there will be no flammable stock near the door and that the fire department will be on hand to extinguish any extension before it becomes unmanageable. If a passage through an industrial fire wall is no longer needed or used, the opening should be sealed with masonry.
NFPA 80, Standard for Fire Doors and Windows, classifies openings from “A” to “E.” The classification does not apply to the door, as is often supposed.
(Photos by author.)
Class A openings divide buildings or fire areas. Doors rated three hours are required.
Class B openings are in vertical enclosures (1 or 1½ hours).
Class C openings are in corridors and room partitions (¾ hour).
Class D and E openings are exterior openings.
Many three-hour fire doors are “tinclad wood doors” made of three layers of wood laid in opposite directions and covered with sheet metal. The unexposed side of the door has a four-inch circle of the metal cut away to vent the carbon monoxide generated from the wood. Such fire doors are installed in pairs, one on each side of the opening. The metal retains the structural integrity of the wood and prevents it from flaming. Subjected to intense fire, the wood will eventually break down, but it will resist the standard fire for the rated time. Other fire doors are “overhead rolling doors” that roll down when tripped by rated fire links. These fire doors operate when fusible links (metal links designed to fail when solder melts at about 165°F) release the door-operating mechanism and permit the door to close.
The chief difficulty encountered with fire doors is that for one reason or another they do not or will not close in the event of a fire. Fusible links are often missing, painted over, or replaced with rope. The general belief is that a piece of rope would burn through in time. The fire will be long past a fire wall opening before a piece of rope strong enough to hold back the door burns through. Fire door paths sometimes are blocked. Sometimes the bracket into which the fire door must fit is broken or bent so that the fire door cannot close properly. If the fire door must be pulled closed by weights, the weights often are not free to fall or are not rigged properly. As many as half the fire doors inspected in many occupancies are not able to work properly in a fire.
It is usually a truck company function to check that fire doors arcclosed, effectively separating the fire area from an adjacent exposure, and to close them if necessary.
Never advance through a fire door during a fire without first blocking it to prevent it from dosing behind you. At a department store fire, a rolling fire door closed behind an engine company. The retainer was bent so the door jammed tightly. It took six firefighters to get the door rolled open to free the trapped firefighters. While attending a fire chiefs meeting, I asked a local chief how he would approach a fire in the meeting room. He indicated a nearby corridor. I pointed out a barely noticeable fusible link in the ceiling that would release a concealed overhead rolling door. The bottom of the hidden door looked like just another piece of trim molding.
There is absolutely no excuse for management not to maintain fire doors properly. The management should be sure that any maintenance people and outside contractors understand the function of fire walls and that any openings in fire walls must be resealed even temporarily as work progresses. All personnel should be familiar with the proper way to test fire doors. Overhead rolling doors should be checked by building maintenance personnel. Once checked, they must be reset. A record of such inspections should also be maintained. A program should be implemented to assure that fire doors will close at all times. Otherwise the entire expense of subdividing the building and building the fire wall and the inconvenience of having a fire wall will have been totally wasted—all because of the inoperability of the “weakest link,”
Very often, deficiencies in firedoors are considered “minor” because, generally, it takes very littleeffort to correct them. Fire door deficiencies as seen are* invariably major deficiencies, and the effort required to repair them is no measure of the loss potential presented if they fail to operate. Any operating failure discovered should berepaired before the close of business on a priority basis.
Architects resist fire doors and often the “solution” permitted by building departments is to provide a line* of sprinklers across the opening. There is no evidence that this will stop a fire in an otherwise unsprinklercd building and certainly it will not stop smoke and gases. Deluge spray systems were provided in lieu of fire doors in the K mart warehouse in Falls Township, Pennsylvania. In 1981 a fire sent flaming aerosol cans rocketing through the deluge spray to extend the fire to other sections. The loss exceeded SI00,000,000.
The March 1966 issue of Firemen magazine provides an excellent illustration of the importance of properly functioning fire doors. It contains a wonderful set of color photos showing how fire doors played a part in preventing the extension of fire from an old combustible wing of a hospital to a brand new fire-resistant wing.
Other Openings
There are other openings in fire walls besides doorways. Ventilating ducts, conveyor belts, wire raceways, and pipes for various services all present problems w’here they pass through the fire wall.
Air conditioning ducts, particularly when added into a building after construction, present a real problem, as do ventilating systems of laboratories. Fire dampers are required to be inserted in the duct line at the point where the duct breaches the integrity of the fire barrier. Until recently, dampers operated only on heat-sensitive fusible links. Smoke detectors or devices connected to other alarm systems are now available and used to close dampers electronically.
Openings in fire walls even when protected according to existing standards remain weak points in the staticfire defense system. Units must be assigned to protect these weak points even if this reduces the effectiveness of the primary attack: If the fire passes the fire wall, there is a whole new fire problem.
For detailed information, see NFPA Fire Protection Handbook, 16th edition, Section 7, Chapter 9, particularly “Protection of Openings” on pages 7-120.
VERTICAL SPREAD
Fires spread vertically from floor to floor both inside and outside buildings.
Outside extension generally occurs when a large body of fire roars out of a window, sweeps up and over the spandrel (the space between the top of one window and the bottom of the next), and ignites the contents of the floor above—autoexposure. Buildings built before the recent emphasis on all-glass construction generally have spandrels of adequate size to prevent this. However, in more than one fire extension has occurred from floor to floor because of the installation of combustible acoustical tile after the building was constructed, providing such a tremendous body of fire at the ceiling level that the fire readily rolled up and over the spandrel and into the next floor. This is even more serious where the fire is less accessible, such as high up in multistory buildings and in rear courtyards. Two sad examples of this autoexposure problem are the fires in the Las Vegas Hilton and the First Interstate Bank in Los Angeles. Where outside extension is a possibility, window sprinklers, wire-glass windows, fire shutters, or structural barriers are available to contain the fire.
Interior vertical extension of the fire takes place via all vertical openings, pipe recesses, service pipe shafts, shaft openings, stairways, dumbwaiter shafts, vertical ducts, laundry chutes, and access stairs. The objective is to cut off every floor so that a fire on one floor will not extend to the next floor. A fire occurred in one floor of a fireresistive multistory tenant factory (loft building). The principal fuel was nitrocellulose. Everything combustible in the loft was totally destroyed. The cinder concrete floor was glowing red. The fire did not extend out of the occupancy of origin.
In good fire-resistant construction, a serious fire can be totally contained within the area of origin and may even self-extinguish.
