FIRE LOSS MANAGEMENT

FIRE LOSS MANAGEMENT

FIRE PROTECTION

Part 15: BUILDING CONSTRUCTION

The fire officer must be familiar with the basic principles of building construction both to understand the potential for collapse and to interact intelligently with the building community. Following is a brief summary of some major building construction considerations. (For further details, see Building Construction for the Fire Service, 2nd edition, Chapter 2, and the NFPA Fire Protection Handbook, Section 7.)

Ultimately all loads in a building must be delivered to the ground. Gravity is the eternal enemy of the building that is constantly attempting to bring the building down. The building is designed to resist the force of gravity, and it usually does, except in cases of fires and earthquakes.

BUILDING ELEMENTS

Fire attacks the system that resists the force of gravity. Some elements of the system are more vulnerable to fire than others. When a fire occurs, the building is as stable as the weakest element.

Principal structural materials are wood, masonry (stone, brick, and concrete block), steel, and reinforced concrete.

The principal parts of a structure are walls, columns, and beams. Walls and columns carry the loads of the building down to the ground. Beams carry the loads generated on each floor of the building to the columns or walls.

Columns carry vertical loads to the ground or foundation. Because columns take up space, suspension rods or cables in tension are sometimes used to “hang” certain loads in a building. The system must, however, provide for the tensile load to be carried over into a column or wall and delivered to the earth in compression.

Floors and roofs are supported on beams and girders. A girder is a beam that supports other beams. Since beams must act in both tension (usually at the bottom of the beam) and compression (usually at the top), they must contain excess material. In many cases the load can be carried on a lighter unit called a truss, which eliminates excess material. The elimination of excess material makes the truss vulnerable to fire and thus more likely to collapse than an equivalent solid beam.

Ail loads must be transmitted continuously to ground. This is accomplished by a multitude of connections in the structure. The importance of the connections varies. In some cases the failure of a connection may have only a local effect. In other cases the failure may be catastrophic—the building collapses.

The primary concern of fire-resistance provisions in building codes is that the building not collapse as a result of a fire. The secondary concern is that the structure limit the fire to a manageable size.

TYPES OF BUILDING CONSTRUCTION

There are five basic construction types. Various building codes subdivide these types further. The five types are fire resistive, noncombustible, heavy timber, ordinary, and wood frame. A fire report should use the terminology of the appropriate local code.

BUILDING CONSTRUCTION

Note that the commonly used word fireproof does not appear in the list of types, although it may appear in some codes. When designers first considered fire a problem they believed that constructing the building with noncombustible material was the answer. Such buildings were called “fireproof” and the misnomer has remained. Early “fireproof” buildings were found not to be so when put to the test of actual fires. During a fire in New York in 1835, merchants hurriedly moved their stocks into the “fireproof” Merchant’s Exchange. The merchandise burned and destroyed the building.

As technology improved, the term fire resistive emerged. In fire-resistive buildings major elements resemble those that have been subjected to standard fire endurance tests during which collapse and passage of fire were resisted for a certain period of time. There is no direct relationship between the time of the controlled test and real time during an uncontrolled hostile fire. Each of the elements of the building may meet fireresistance criteria, but it is most likely that the building as a whole was never analyzed for the total impact of a potential fire.

Fire resistance does not guarantee life safety—far from it. In the Iroquois Theatre in Chicago, which was advertised as “fireproof,” 602 people died; 145 died in 1912 in New York’s Triangle Shirtwaist fire, and the building is still in use. Hundreds of people have died in fire-resistive hotels, the most recent disaster being the Du Pont Plaza hotel in San Juan, Puerto Rico, in which 97 people died.

Fire resistance does not guarantee minimal fire loss; in fact, while achieving its designed fire resistance, the structure may be severely damaged. The Los Angeles First Interstate Bank building came through a massive fire with flying colors, yet the loss was well over S100 million. Repairing damage to a fire-resistive building may cost as much as or more than the structure’s initial cost.

Fire resistance does not guarantee control of toxic gas and smoke. For instance, a rated reinforced concrete floor could act as a very effective smoke barrier if the perimeter firestopping is adequate and all penetrations are effectively sealed (unfortunately, it is not too likely). An equally rated steel-bar-joist floor and ceiling assembly with an integral air handling system is likely to provide a path for smoke and gases to travel.

Fire-resistive assemblies are not necessarily noncombustible. Floors and walls of wood and gypsum board are rated “fire resistive” by Underwriters Laboratories Inc. In the past such structural elements were noted in the Fire Resistance Directory as “combustible.” This word has disappeared from most listings.

BUILDING CONSTRUCTION

Depending on how the fire resistance is achieved, different buildings of the same fire-resistance rating may exhibit different characteristics in similar fires. For instance, a fire-resistive floor of reinforced concrete absorbs considerable heat. A steel-barjoist floor and ceiling assembly of equal fire resistance will not absorb as much heat. This can affect the extension of a fire, as every Btu absorbed by the structure is one less available to keep the fire growing.

Noncombustible buildings, as the name implies, are supported by noncombustible structural elements. Protected noncombustible buildings have some limited fire resistance. Under some building codes, noncombustible buildings may have substantial combustible components—particularly cornices, mansards, and roofs— that in some designs are structurally necessary for stability.

Buildings of heavy timber construction have masonry exterior walls and heavy timber interiors. The idea is that the heavy timber is slow to ignite and therefore the building is said to be “slow burning.” It would be better to call such buildings “delayed ignition.” The concept fails once the fire involves the building and the fire suppression forces cannot safely conduct an offensive operation. Then the massive amount of timber simply becomes a tremendous fire load. Many cities have suffered massive downtown fires in old, vacant, heavy timber buildings. The interior may be designed for selective collapse of floors in a fire to save the balance of the structure. Not all heavy timber buildings are truly mill construction. This type of construction combines heavy timber with a number of other features that are designed to limit the ability of the fire to overtake the structure.

Buildings of ordinary construction have masonry exterior walls and lightly constructed combustible interiors. The principal benefit of the masonry walls is the reduction of the conflagration potential. The interior is expected to collapse in a fire and may be required by code to be so designed; the so-called fire cuts on wood joists are typical examples of this.

Wood frame buildings are totally of wood structural construction. A noncombustible veneer such as brick does not change the nature of the building.

Code regulations that limit the type and size of construction are predicated on the type of building, the anticipated type of occupancy, and the anticipated level of potential fire risk. Estimates of the potential fire risk are based on very rough concepts of the fire load (or fuel load). For buildings of combustible construction, the basic fire load is the building itself; thus, such buildings may be limited by code in area and height. The fire department should have a good grasp of the fire load in specific structures in its jurisdiction.

Most studies on fire load are limited to combustible contents. Combustible structural elements, if present, are part of the fire load. Fire loads are often expressed in pounds (of ordinary combustibles) per square foot. When this is the case, all weights are converted to the equivalent of ordinary combustibles such as wood, which has a heat value of about 8,000 Btu/lb (metric 18,600 kilojoules/kilogram, or kj/kg).

For instance, plastics, which generally have a heat value of about 16,000 Btu/lb (37,200 kj/kg), are converted at the rate of 1 lb. of plastic to 2 lbs. ordinary combustibles. It is better to simply state the fire load per square foot in potential heat value terms— Btu’s or kilojoules. The potential heat values of many common fuels are given in tables of “calorific value.”

Fire loads vary considerably according to the occupancy, the specific location in the building, and other factors. An estimate of the fire load and the RHR (rate of heat release — how fast the potential heat will be given off) is essential for adequate prefire planning.

BUILDING CONSTRUCTION

FIRE-RESISTANCE TESTING AND RATING

Structural fire protection requirements in building codes are based on fire-resistance or fire-endurance ratings expressed in hours. The ratings are based on tests performed on the structural or compartmenting (separating) building components according to the NFPA 251 (ASTM El 19) standardized test procedure. The exposure is such that a temperature of 1,000°F is reach in 5 minutes; 1,700°F in 1 hour; 1,850°F in 2 hours; 2,000°F in 4 hours; and 2,300°F in 8 hours. The test is conducted in a special test furnace and continued until one of several criteria of failure is reached:

  • structural failure (inability to sustain the applied load),
  • integrity failure (development of a crack or opening through which flames or hot gases may pass during the fire test or a hose stream test), or
  • insulation failure average (average of heat transmission sufficient to raise the temperature on the unexposed surface by 250°F).

UL lists constructions that pass the test in the Fire Resistance Directory. It is important to understand that the manufacturer constructs the test specimen. The listing therefore includes everything found necessary to barely pass the test for the hourly rating desired. If stated details are omitted or modified in the field, the asssembly is not the equivalent of the assembly that passed the test.

Although the standard fire test curve represents only one set of fuel conditions, it serves as a useful means for the comparative rating of individual columns, beams, walls, partitions, and floor and ceiling assemblies. Again, it should be stressed that although the ratings are expressed in hours, there is no relationship between rating hours and hours of an actual fire assault on a building. In fact, the level of confidence is only that a four-hour, fire-resistive floor is somewhat better than a two-hour floor.

The burning of a fire load of 10 lbs. of ordinary combustibles per square foot (or 80,000 Btu/ft or 910,000 kj/ is the approximate equivalent of 1 hour of the standard fire test ASTM El 19.

If these figures are used cautiously and broadly rather than precisely, it is possible to determine that in a given situation the fire load is grossly excessive for the fire resistance of the building. Consider a building with floors rated “two-hour fire-resistive.” Such a building might reasonably be expected to successfully resist a fire involving a fire load of 160,000 Btu/ft (1,820,000 kj/m average. However, in the area under consideration the fire load is determined to be about 300,000 Btu/ft (3,412,500 kj/m) average. It can be reasonably concluded that the fire area is overloaded from the fire endurance point of view, even though die total structural loading is within permissible limits.

RATE OF HEAT RELEASE

Fire load represents the amount of fuel available to a fire. It is equally important to have some appreciation of how fast the fuel will burn. A pound of solid wood will burn much more slowly than a pound of the same wood shredded into excelsior. The term used to measure this difference is rate of heat release (RHR). Prefire plans should take into account structural or finish materials or contents diat might produce a hot, fast fire. A typical example is wood paneling installed in an unsprinklered high-rise office building.

Plans should include provisions for prompt use of heavy-caliber streams if necessary. For example, the Wilmington (DE) Fire Department is set up to use a lightweight deluge gun, which when fed by two 1 ¾-inch lines can deliver 300 gallons per minute. If the fire area is reachable from the ground, plans should contemplate an exterior attack to knock down the body of fire, with interior stairways closed and defended.

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