Editor's note: This article finishes summarizing a small portion of the 79-page Chapter 2, "Principles of Construction," of the 667-page third edition of Building Construction For The Fire Service, by Francis L. Brannigan. Part 1 was published in Firehouse® in February 1996, part 2 in July 1996.
(Facts about structures are printed in regular type. Firefighting implications are printed in italics. Page references are to Building Construction For The Fire Service, third edition.)
TRANSMISSION OF LOADS
"All loads must be delivered to the earth." This absolute maxim is often ignored by the proponents of one building material or another. The wood industry often shows a picture of a deep laminated wood beam only charred by fire. A twisted steel beam lies across it. The caption doesn't mention that many, many laminated wood beams are supported on unprotected steel columns.
It is important to understand how loads are transmitted from the point of application to the ground. Consider an ordinary brick and wood-joisted building with interior columns:
- A load is placed on a wood floor.
- The floor boards deliver the load to the joists.
- The joists deliver the load to a masonry wall at one end and to a girder (a beam which supports other beams) at the other.
- One end of the girder rests on a masonry wall. The other end rests on a column.
- The amount of the load transmitted to each support point depends upon the distance of the load point from each of the supports, the nearer points receiving the greater load.
- The structural engineer must calculate the distribution of the loads. The building does it automatically, according to the laws of physics. This example makes use of a simple masonry and wood-joisted building. Regardless of the size of the building or the construction materials, the principles are the same. Any weight on the roof of a giant high-rise building is transmitted to the ground through the structure of the building.
All loads must be transmitted continuously to the ground from the point at which they are applied. Any failure of continuity will lead to partial or total collapse.
Illustration by Christopher J. Brannigan
The building load supported on a too-short cast iron column built up by cast iron blocks was seen in Denver. Such a connection, like a child's building blocks, has no resistance to a lateral thrust.
There is a tendency to make light of partial collapse and consider it unimportant. A partial collapse, like any collapse, is very important to at least two groups those under it and those on top of it. The fatal collapse of the walkways in the Kansas City Hyatt Regency Hotel (page 60), which claimed more than a hundred lives, was "only a partial collapse."
Roofs are very important to the fire service. In some cases, the roof is necessary only to keep the rain out; in other cases, the roof is vital to the stability of the building. Roofs will be discussed in more detail as we discuss each type of building but a few vital points are given here.
In so-called tilt-slab concrete buildings, the roof is vital to the stability of the structure, and roof damage can cause wall collapse (page 373).
The recent, very necessary special attention to truss roofs should not cause firefighters to ignore the hazard of the sawn joist roof (page 181). Combustible metal deck roofs, when burning, are unsafe to be on, and firefighters cannot possibly or safely open the huge vent holes which would be required (pages 302-309; also see "Follow-Up Report" by Dave E. Williams on page 48 of this issue). Trusses are fully discussed in Chapter 12, pages 517-563. The subject will be summarized in a future issue. Single-ply (or membrane) roofing is a relatively recent problem.
To ventilate a ballasted roof, remove the ballast from an area several feet larger in all directions than the desired hole. Cut the membrane with a knife or scissors, not with an axe or power saw. Peel back the membrane and open the roof. If the membrane is glued to the roof, peeling is difficult.
A mechanically fastened membrane is loose between fasteners. In high winds, it can blow like a sail and throw firefighters or equipment off the roof. White membranes make roof edges difficult to see in blinding glare or ice and snow.
There are a number of different compounds used for the membrane. All should be expected to emit toxic gases, so self-contained breathing apparatus (SCBA) should be worn, even through a roof fire might seem to permit otherwise. Membranes are slippery when wet. Check for fire traveling under the membrane. Insulation may mask the possibility of a structural collapse. Installation is a hazardous operation due to the use of flammable compounds which emit heavy vapors. There may be a large quantity of hazardous flammable materials on the job site.
The simplest building might have walls of mounded earth and a roof of tree trunks. All other structures must have connections which transmit loads from one structural element to another. They are a vital part of a structure's Gravity Resistance System. The system is only as strong as the weakest link. Any failure lets gravity take over and collapse occurs.
When examining buildings, pay particular attention to connections. Watch out for overhead loads, which must have multiple connections. Potential connection failures will be cited throughout this series but some are summarized here to get you started looking intelligently and warily at buildings. It's your life.
- In older heavy timber buildings, look at the connection of wooden beams to cast iron columns. They are probably very dangerous (pages 174-176). Cast iron columns in older buildings often have only gravity connections; they just depend on the weight of the structure to keep the support column in place. Any lateral thrust will displace the column. Nine Boston firefighters died when a gravity connection failed.
- In newer buildings, very often unprotected steel columns carry heavy laminated wood girders.
- In many older buildings, the connections may be adequate as long as the building is axially loaded. However, an eccentric or lateral load or shifting wall, floor or column alignment may cause collapse even after the building has been standing for many years. Undesigned changes in loading are dangerous and frequently cause collapse.
- Be especially wary of buildings in areas where there was, or is, no building code supervision, such as rural areas that suddenly become urbanized. A Los Angeles City firefighter died in a roof collapse where once there had been a vacant lot between two brick buildings. To provide a roof over the vacant area, mortar had been removed from between bricks. Pieces of shingle were hammered into the gaps. A ledger board nailed to the shingle pieces supported a wood joist roof. The makeshift assembly survived for a number of years until a fire destroyed it.
- Buildings of the same age probably have the same defects
- Sand lime mortar was used exclusively in masonry work until about 1880, and for many buildings after that date. Sand-lime mortar is water soluble. A firefighter operating with his unit in the basement of a building noticed that a hose stream had washed the mortar out from the bricks. He alerted the officer. The building was evacuated and shortly thereafter collapsed.
- Steel connections enter into the construction of almost every building. Steel is non-combustible but it has poor fire characteristics. The characteristics of steel ar described later in "Characteristics of Materials."
Unprotected steel columns supporting a concrete floor failed in a fire and plunged four Pennsylvania volunteers to a fiery death.
It was noted earlier that a load can be suspended on a thin tension rod as contrasted with the bulky column required to support it in compression. However, the load must be changed to a compressive load and delivered to the ground. This requires a series of connections.
Illustration by Christopher J. Brannigan
Many heavy timber buildings or timber interiors of masonry buildings were built under the concept of "self-releasing floors." Each of the girders can collapse without bringing down the others. A "dog iron" gives some slight degree of lateral stability but will not prevent a collapse. Other builders did not like this concept and chose heavily bolted connections. It is fair to characterize a building with self-releasing floors as "designed to collapse."
The vulnerable point is the connection most susceptible to fire. This may be floors away from the suspended structure. The steel tension rod connection of a failing wooden girder on a lower floor, to a steel beam supported on the walls, may be hidden in the cockloft (the space between the top floor ceiling and roof). The cockloft fire may cause the connection to fail, dropping the rod and its load. Thus, a cockloft fire might cause a lower floor collapse. Know your buildings!
SELF RELEASING FLOORS
Many codes require that wood joists in masonry walls be fire-cut. The end of the joist is cut off at an angle to permit the joist to fall out of the wall without damaging the wall.
The removal of wood lessens the inherent resistance of the joist to fire and can precipitate collapse. Wood joists often sag over time. Sometimes the joists are turned over to provide the desired "upward camber." In such cases, there can be very little wood bearing on the wall. This practice should be forbidden.
When the size of the building requires interior columns, buildings with timber interiors, are often built with self-releasing floors. Floor girders are set on brackets attached to columns. A wood cleat or steel dog iron (similar to a big staple) is used to provide minimal stability. Such a floor can be expected to release sooner than if it were tightly connected. Some designers rejected this idea and required tight connections.
Know your buildings! A building with fire-cut or self-releasing floors is designed to collapse. It is the duty of the incident commander and subordinate fire officers to see to it that firefighters are not under the structure when the designed collapse occurs.
OVERHANGING AND DROP-IN BEAMS
Structural design is often intended to be as economical as possible. The economy may be in material or in the work of erection. Consider a space three bays wide. There are two masonry walls and two lines of columns supporting girders. Sometimes it is more economical to let the two outermost beams overhang the girders by two or three feet. The gap is then closed by a beam dropped in and nailed to the overhanging ends of the outer beams. As a result, the drop-in beams are supported only by the nailing. They have no support underneath.
This is not a new practice. I have seen it in hundred-year-old buildings. Take every opportunity to examine buildings being repaired or renovated to "undress the building" to see the hidden hazards.
Long wooden beams are not readily available for building needs. Shorter lengths are often spliced together with metal connectors to produce the desired length.
The resultant beam will carry its design load but the connectors may fall out when heated sufficiently, causing collapse. In some buildings, these connectors may have been made to look decorative. Take a second look! Some years ago, a jai alai arena in Daytona Beach, FL, was destroyed by fire. The roof was supported on laminated wood arches. The owners of the building had been convinced they had a sturdy heavy timber building. Pictures of the fire clearly showed that the arches had fallen apart at the connections.
These examples are but a few of the many connection failures which can occur, possibly catastrophically in a fire. Know your buildings. Trace the load from origin to foundation.
Ultimately, all loads are delivered to the ground through the foundation. The nature of the ground and the weight of the structure determines the foundation. Foundations can range from simple footings to grade beams or a foundation under the entire wall to foundations which literally float the building on poor soil. In some cases, wood or concrete piles are driven either to bedrock or until the accumulated friction stops the pile. Almost all foundations today are of concrete. In some locations, decay-treated wood is used for small houses.
Illustration by Christopher J. Brannigan
Most codes require a "fire cut" when wood joists are fitted into a masonry wall, to let the floor collapse without pulling down the walls. Notice how much wood is removed. This often leaves very little of the beam resting on the wall, thus making the beam liable to early collapse. Sometimes, when a building is being rehabilitated, sagging floor beams are reused by turning them over. This can leave very little wood resting on the wall. It is fair to characterize a building with self-releasing floors as "designed to collapse."
Any number of foundation problems can affect fire suppression in several ways. Masonry walls above foundations may develop severe cracks which make the wall vulnerable to collapse. Fire doors may not close properly. Openings may develop in fire walls or in floor-wall connections, permitting passage of fire. Dry-pipe sprinkler systems may not drain properly after "going wet." Wooden basement walls are of pressure treated wood which emits toxic fumes (in addition to the normal carbon monoxide) when it burns.
CHARACTERISTICS OF MATERIALS
Each material has its own fire characteristics. They are summarized here and will be developed more fully as the series progresses. At times, the objection is made that this is all negative. The good characteristics of any material can be obtained without cost from the trade organizations. We are interested in those characteristics which are hazardous to firefighters and not always found in industry literature.
Wood is combustible. Fire can spread rapidly over wooden surfaces, particularly when the wood is thin. Wood can be treated to reduce the surface spread by impregnating it with salts that retard ignition. It can be surface treated with flame retardants. These must be applied strictly in accordance with directions and, in the case of wood paneling, may be ineffective if the hidden back surface is untreated. In no way is the wood rendered non-combustible, although some codes use that term.
Wood treated to resist decay and insects emits toxic smoke. When the wood is carrying part of the load of the building, the loss of material can cause collapse.
Steel is non-combustible. It is very strong, thus members of small cross section can carry substantial loads. Mass of material provides inherent fire resistance; thus lightweight steel has very little fire resistance.
Despite its non-combustibility, steel has negative fire characteristics. Steel transmits heat readily by conduction. A metal box provides no protection to vital records.
At relatively low fire temperatures steel elongates significantly. (A rise of 1,000 degrees Fahrenheit will cause a 100-foot steel member to elongate nine inches.) If it can move the supporting wall or column, it will, probably causing collapse; if not, it will buckle. At slightly higher temperatures, depending on the load carried, steel will fail.
Steel which is required to be insulated from fire heat is protected by what is called "fireproofing." This is a poor word, since nothing is fireproof. The better term is fire resistive. Fire resistance is rated by the time in hours an assembly has withstood the Standard Fire Test ASTM E 119. THE HOURLY RATING HAS NO RELATIONSHIP TO REAL-TIME HOURS IN A FIRE (page 248).
In an unprotected steel building, the most important use of water, almost always, is to cool the steel constantly to prevent the structure from failing.
The cooling does not cause failure of the steel if it is elongating, the cooling brings it back to its original dimension; if it is failing, the cooling freezes it in the failed shape. The best use of water is to cool the steel constantly, to prevent the structure from failing and thus reduce the hazard to firefighters and the loss. The burning contents are already a total loss.
STEEL IN NON-STEEL BUILDINGS
The fire characteristics of steel are important in most buildings, because steel is part of the structure of nearly all buildings.
Illustration by Christopher J. Brannigan
An important concern in today's buildings is clear space, without columns. One way is to carry loads up to the roof. The sketch shows a construction in a library. The mezzanine is built on a big beam, supported at one end on the wall while the other end is hung on a rod connected to a ceiling rafter. There is no way to estimate the stability of that connection in a fire. Temperatures up there would generally be much higher than on the floor, and the hazard would be invisible in the smoke. It is too late to detect these potential firefighter killers on the fireground. Know your buildings.
Lightweight wood trusses are held together with steel gusset plates or gang nails. Destruction of the fibers holding the gusset plate by pyrolytic decomposition (burning without flame) releases the plate and the truss fails.
Steel is often intermixed with other combustible construction elements. If the building is not required to be fire resistive, the steel is unprotected. Unprotected steel connections and supporting columns are found with most heavy trusses and laminated beams. If a steel girder supporting floor joists is restrained and cannot elongate, it will overturn and drop its load of wood joists.
Cold drawn steel cables lose their tensile strength at about 800 degrees F (lower than the temperature of a self-cleaning oven). Such cables are used to tension concrete, to hold back excavation bracing, to tie failing buildings together, and for elevators.
Concrete is non-combustible but not necessarily fire resistive. Steel is made fire resistive by applied "fireproofing." Concrete is made fire resistive, when required, by specified improvements in the mix. So-called reinforced concrete is actually a composite in which steel provides the tensile strength and concrete the compressive strength.
Many years ago, the Concrete Institute urged me to emphasize the following: Any failure of the bond between concrete and steel means that the structure is failing to some degree.>
Different types of concrete and concrete structures have different fire problems. Cement is a constituent of concrete. Do not use the word cement when you mean concrete. Fires in concrete buildings under construction may pose very serious hazards to firefighters, including total collapse of all floors (pages 342-359).
Plastics describes a wide range of materials. In general, they are easily ignited, produce greater amounts of heat than other materials and emit combustion products which may be more toxic than ordinary smoke. Some become pools of flaming liquid. Some erroneously consider rigid reinforced plastics (e.g., fiberglass) to be non-combustible. The glass is not combustible but the plastic will burn and leave a mat of fibers which, if disturbed, will be an itch hazard.
Some plastics are inhibited with chemicals to limit ignition but may melt or deform. If carrying a load, such as heavy tile on a foam plastic roof, the load can drop.
Fire load is the potential fuel for a fire. For combustible buildings it includes the building itself.
Fire load is measured traditionally in BTU (British thermal units). Fire load was first stated in pounds per square foot, since most fire load was wood and paper, which are taken as being 8,000 BTU per pound. The development of plastics and liquid fuels with BTU emissions of up to 23,000 BTU/pound made problems.
Since it is the heat, not the weight, that concerns us, it is better to speak of BTU per square foot. As a very rough rule of thumb, an estimated fire load of 80,000 BTU per square foot is the equivalent of the standard test fire at the one-hour fire resistance level.
RATE OF HEAT RELEASE
A more recently developed term is RHR (rate of heat release). This estimates how fast the material will burn. A pound of solid wood burns much more slowly than a pound of excelsior (wood wool).
Major sprinkler systems are hydraulically designed to deliver a desired flow of water to each square foot of an area. This is calculated to suppress fire in a certain fire load with a certain RHR. Huge losses have occurred in giant warehouses where the fire load has exceeded the design estimates. My statement of this situation is simply, "The BTU outnumbered the H2O."
Pre-planning should include consideration of the fire load and RHR. This might prevent the futile automatic stretching of small attack lines when big lines or heavy stream appliances should be used on the initial attack.
This concludes the abstracts from Chapter 2, "Construction Principles," of Building Construction For The Fire Service, third edition. Next we will take up the "Hazards of Wood Construction" with abstracts from Chapter 3. The 667-page third edition of Building Construction For The Fire Service, by Francis L. Brannigan, is published by NFPA, 800-344-3555; for autographed copies at a special reader's discount call 301-855-1982.
2 Firefighters Die In Chesapeake, VA, Roof Collapse
The National Fire Protection Association (NFPA) reports that two deaths resulted from the collapse of a 50-foot clear span wood truss roof on an auto parts store. The NFPA noted that the firefighters apparently had no knowledge of the type of roof. I will provide further information when the NFPA investigation is completed.
In the meantime, please READ and HEED Chapter 12, "Trusses," in Building Construction For The Fire Service, third edition. Particularly note the "Tactical Consideration" on page 532, "It cannot be stressed too often that heavy fire conditions can exist in concealed spaces, particularly overhead, and not be evident in the space below," and the four pages of "Tactical Considerations" beginning on page 557; senior officers might be well advised to READ and HEED the last paragraph of the "Tactical Considerations."
Earlier in this series, I pointed out that collapse is not the only hazard of buildings. Sudden rapid increase in fire is another. The Rockland County, NY, Fire Training Center has developed a great structure and program to demonstrate FLASHOVER. To get a description of the structure and further information, write to Fire Instructor Gerald Knapp, 9 Mackey Road, Garnersville, NY 10923.
Lessons Learned From The Milliken Fire
The January 1995 fire at the giant Milliken & Co. carpet mill in LaGrange, GA, was another costly example of the dangers of metal deck roof fires and another classic illustration of this type of roof deck fire as described in Francis L. Brannigan's book, Building Construction for the Fire Service, third edition. (See "On The Job Georgia," Firehouse, June 1996.)
Milliken is provided property coverage by Industrial Risk Insurers (IRI). The third-quarter 1995 issue of the company's publication, IRI Sentinel, discussed the "Tale of Two Fires," noting that the roof deck fires at a General Motors transmission plant in Livonia, MI, in August 1953 and the Milliken fire resulted from very similar reasons (see "On The Job Georgia").
Once the temperature of a fire impinging on a Class II metal roof deck reaches 800 degrees Fahrenheit for more than about five minutes, the tar melts and vapors from the roof assembly ignite. The fire will then "self-sustain itself" along the bottom of the roof deck, even in a sprinklered building. Buildings of this type are protected by typical "upright" sprinkler heads. These heads are designed to deflect the water downward. In a metal deck roof fire, however, little water hits the bottom of the roof deck, which would cool the metal deck from the bottom. In effect, the fire is running over the top of the sprinkler protection.
A metal deck roof incident will have the fire spreading and burning the flammable vapors generated when the asphalt, used to adhere the board to the metal deck, melts and vaporizes. Because the insulation board and various layers of felt and asphalt (built-up roofing) above the insulation block upward, and since the asphalt vapor is heavier than air, it will drop downward through the overlapped and unsealed seams in the corrugated or wide-rib decking. When it hits the hot environment inside of a fire building, it ignites beneath the roof. The heat of the vapor burning just below the metal decking causes more asphalt to melt, creating more vapor and away the vicious circle and self sustaining fire goes. This metal roof deck fire will continue to burn very hot and with huge volumes of dense black smoke.
Once started, a metal deck, self-sustaining roof deck fire can burn even in a completely empty building. This fire will burn until the fire reaches the ends of the building or until LOTS of water is applied to completely cool the metal deck to stop the self heating scenario associated with what is commonly called a Class II steel deck roof. It is almost like having asphalt in a frying pan.
Many firefighters fail to fully understand the seriousness of these types of roof deck fires and how quickly the self sustaining fire can begin. Of equal and sometime greater importance is how fast this type of fire will weaken the steel "bar joist" and steel beam supports, bringing the roof deck down on unsuspecting firefighters. Asphalt has a flash point of about 400 degrees F and a boiling point of 700 degrees F, per NFPA 325 and Factory Mutual Data Sheet 7-19N.
Dave E. Williams
Francis L. Brannigan, a Firehouse® contributing editor, was a fireground commander from 1942 to 1949. Since 1966, he has concentrated on the hazards of buildings to firefighters.