FEATURES
BUILDING CONSTRUCTION
Photo by Frank Brannigan
Photo by Frank Brannigan
The current practice of designing buildings with lighter and lighter materials and substituting geometry for masses has brought about significant strides in the architectural field—it has also brought about significant problems in the firefighting field.
When our ancestors arrived here, huge forests supplied inexpensive building beams and columns of every desired dimension. Structures were massively built of wood and masonry, without regard to their weight.
Times have changed.
Charlie Suedeshoes, the real estate developer, is meeting with his architect: “I want 5,000 tons of one-story stores, 15,000 tons of three-story office buildings, and 30,000 tons of two-story one-family houses.” An imaginary conversation, of course, but not as far from the mark as you might think.
BEWARE THE TRUSS
As Frank Brannigan says,
The weight of materials and the weight of the structure required to support heavy materials are now an important cost consideration. Suppliers of materials lighter than those previously used proudly brag about the weight savings. The Empire State Building weighs 23 pounds per cubic foot. Modern high-rises weigh as little as 8 pounds per cubic foot. Some are so light that they sway enough in the wind to make top floor occupants “seasick.” In a number of cases, weight has been added back into the building in the form of “tuned mass dampers,” huge computer controlled weights that move back and forth to counter the effect of the wind.
Many of the techniques for lightening buildings and providing large open areas without columns have been available for centuries. The hammerbeam truss, for instance, used in churches and other monumental gothic structures to eliminate columns, was invented for Westminster Hall in the 14th century, and is still standing in London today. In recent years, increased material costs have forced builders to use these more economical construction techniques almost exclusively.
“Very interesting,” you say, “but what’s that to us?” Nothing— except that the whole interior fire suppression environment is changing and many fire departments are totally unaware of the potentially disastrous consequences.
TRUSS PRINCIPLES
Consider a building span of 20 feet. With beam-type construction, the building would have two 10-foot beams extending from the walls on opposite sides to a column support in the center. Assume that each beam can carry 1,000 pounds.
If the column is removed, the beam will then be 20 feet long. A beam this length, of the same material and cross section area, can carry only 500 pounds (double the length equals half the capacity).
Suppose we cut off the column, leaving a stub at the junction of the beams. If we tie a tension member from the stub to the beam ends at the wall, we have formed a triangle (a truss) and, in effect, restored the load-carrying capacity of the beam, yet removed the obstruction of the column from the floor below.
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500 LBS.
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TYPES OF TRUSSES
Trusses make use of the triangle principle. The triangle is the only rigid geometric form. Given three lines with all ends touching, only one shape can be formed—the triangle. Thus, the triangle is inherently stable.
The top and bottom members of the truss are called chords. The compressive connecting members are called struts. The tensile connecting members are called ties. Connections are called panel points. As a group, the struts, ties, and panel points are called the web.
Trusses can be built of wood, wood and steel combined, or steel. There are not many concrete trusses, though huge concrete trusses are in the Tampa, FL, Airport. Cast iron was used for compression members in early metal trusses and may be found in 19th century structures.
FIRE RESISTANCE
Massive wood and masonry structures can be inherently more fire resistant than lighter structures of the same materials. I use “fire resistance” in its basic sense, meaning the inherent capacity of the structure to resist collapse when fire attacks it.
By some method or other, the builder put together a system to resist the law of gravity. Fire attacks that system. If all the components were equal, the building would fail all at once, like the “one horse shay.” However, one component is usually much more vulnerable to fire than the others and fails first. This component is most often a connection, such as the unbelievable common wooden lintels in otherwise massive masonry walls, or the steel connections in massive wooden trusses or arches.
The fire vulnerability of connections is conveniently overlooked by those interested in pushing a particular building material. Unfortunately the strength of connections is often missed by some in the fire service who do not realize that the building is the sum of its parts, and, like a chain, is only as strong as its weakest link.
The inherent fire resistance of one structure may be absent in another, similar structure. Therefore, experience, whether personal or institutional, may not be transferable from one structure to another. To put it simply, what you know may not be so.
Let’s distinguish between two uses of the term fire resistance. Rated fire resistance is a quality ascribed to a wall, floor, or column assembly that has been tested in a standard manner to determine the length of time it remains structurally stable (or resists the passage of fire) when attacked by a standard fire.
Inherent fire resistance is the structural members’ resistance to collapse by fire because of the nature of their material or assembly.
A heavy wood beam takes longer to fail from a fire than does a 2X4. A light steel beam will absorb heat faster and thus fail sooner than will a heavier beam. A masonry wall, itself quite fire resistant, may fail because of the lateral thrust of an elongating steel beam. The inherent fire resistance of a structure has never been formally rated or required by law.
TYPICAL TRUSS TYPES
By experience and tradition, we in the fire service have developed some criteria for determining the inherent fire resistance of a particular type of construction—one standard being how long we can stay in the interior of a structure on fire. Unfortunately the criteria are, to put it politely, imprecise. To put it bluntly, many times we risk life and limb without really knowing the degree of risk and without any risk/benefit calculation. The generally accepted indications of imminent collapse such as floors or roofs “softening,” water flowing through bricks, smoke pushing out of mortar joints, “strange noises,” or whatever are, at best, grossly inadequate, even for the buildings built in past years. If these warning signs are relied on solely to alert us to collapse in today’s lighter buildings, bloody disasters will be the certain result.
Most fires are fought by firefighters positioned under or upon wooden members. These buildings of wood or ordinary (masonry walls) construction are not required by law to have any rated fire resistance. They are classified as “non-fire resistive.” In other words, they are not required to have any resistance to collapse. To the extent that they resist collapse in a fire, we are simply lucky. Firecollapse resistance is not really part of the design criteria.
Some jurisdictions have a “protected combustible” classification in which combustible structures are “protected” with gypsum board. The many construction deficiencies of these structures do not give the firefighter any assurance that they will resist collapse for longer than an unprotected structure.
When a combustible structure is involved in fire, no code provision, however well written and however well meaning, provides real personal safety for the firefighter. We must know that the building is the enemy; and we must know the enemy.
TRUSS FAILURE
The truss can fail in a variety of ways. All parts, all connections of a truss are vital to stability. The failure of one element of a truss “entitles” the entire truss to fail. If some undesigned feature inhibits failure, we are, as with wood and masonry buildings, just lucky. Multiple truss failures are the rule, not the exception, and the failure of one truss can cause serious problems to other parts of the structure, even parts far from the initial failure point. As a matter of fact, trusses are tied together for stability, and when one truss fails, undesigned stresses can be passed to other trusses by way of the ties and cause multiple failures.
Many mechanics are not aware of the vital importance of all parts of a truss and blithely cut away “unnecessary” members to make room for a duct, for instance. Some trusses are delivered from the manufacturer with warning labels attached.
A YMCA in Los Angeles, CA, was built of concrete trusses concealed in partition walls. Workmen, cutting through a wall, cut a truss member. It cost over $100,000 to repair the damage.
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Photos by Frank Brannigan
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The economy of the truss is found in the fact that it separates compression and tension. All loads must be delivered to the earth in compression. Any tensile load must be converted to a compressive load. The load changing system must involve connections, which are logical failure points.
In sketches of trusses, compression members are shown as thicker lines, while tension members are shown as thin lines. (Note how compression and tension are reversed in the pratt trusses.) This illustrates the fact that compressive loads are best resisted by columns whose material is as far as possible from the center (e.g., a cylinder), while tensile loads need only the required strength of material, and shape is unimportant.
What has this to do with the safety of the firefighter?
The bottom chord of a truss is under tension. Think of it as a rope. How many places must a rope be cut to fail?
Tubular steel and wood trusses burn through at panel points, where there is little wood and a steel pin that carries heat into the wood causing pyrolytic decomposition. Long span gusset plate or sanford trusses usually have a bottom chord made of two or more pieces of wood (2X4 or 2X6) held together with metal gusset plates or gang nails. The gusset plate heats up, the wood fibers holding the plate are destroyed by pyrolysis, and the gusset plate falls out. Of course, wooden members can burn away and fall out.
Some writers distinguish between trusses built of large timbers and trusses built of lightweight material. I am afraid that this is a distinction without a difference. A truss is a truss is a truss. The failure of any part of a truss is a failure of the truss. Heavy timber trusses have metal connections that can fail early. A group of heavy timber trusses in a Florida country club rest on an unprotected steel girder. The twisting of the girder, attempting to elongate, could drop all of the trusses.
Some large wood trusses are lattice trusses. The web is composed of thin boards (such as 1 X6s). Such a truss obviously has little fire resistance.
In Witchita, KS, as firefighters turned out for a fire in an auto dealership, a second alarm was ordered because of the visible “loomup.” Upon arrival, a line was stretched into the building. The steel bowstring trusses collapsed and four firefighters died. A lightweight steel truss building has almost no inherent fire resistance. If there is enough fire to justify a second alarm, it is almost a certainty that the building is unsafe to enter.
McCormack Place, a giant exhibit hall in Chicago, IL, with massive unprotected steel trusses, was destroyed in less than an hour by a huge fire that started on the exhibit floor. In tests conducted at Underwriters Laboratories (UL) after the fire, 1,500°F temperatures had reached trusses 30 feet above the floor in less than five minutes.
The top chord of a truss is under compression. The top chord of all trusses is the equivalent of a braced, long, slender column that would fail by buckling except that it is braced every few feet. In effect, it is a series of short columns one atop the other. Consider that a fire distorts the steel of a tie or strut, pulling the rivet out of the panel point. Two short “columns” are now united into one twice as long. The load carrying capacity is reduced to 25% of what it was. Few structures have enough reserve to stand such an increase in loading. The truss buckles.
Note that when a truss is cantilevered out, the situation reverses. The tension is in the top chord and the compression is in the bottom chord.
It is not uncommon for all or a substantial part of a trussed roof to fail simultaneously. Firefighters have been knocked down by the blast of air pushed from doors and windows when the roof collapsed. Collapses are often accompanied by huge bursts of fire, so it is not sufficient to withdraw personnel from the immediate collapse zone. Firefighters operating outside lines close to windows or doors are in serious danger of heat bursts and wall collapse, triggered by the roof collapse.
The steel bar joist (a parallel chord truss) has been with us for some time, both for roofs and floors. Steel bar joists, when tested in a standard fire, failed before nominal two-inch wooden beams that lasted only 10 minutes. The steel is so light that it reaches elongation temperatures of over 1,000°F very rapidly. At this temperature, it can exert a sufficient lateral thrust to push a masonry wall to the point of collapse. At somewhat higher temperatures the steel fails.
While wood trusses fail suddenly, steel trusses are inclined to sag, thus giving some warning to fire forces. However, when sagging, the roof may be hot enough to liquify the tar and the firefighter may find himself sliding into the fire.
Photos by Frank Brannigan
Insulated metal deck roofs
Steel bar joints are often used in conjunction with insulated metal deck roofs. Looking up from the underside, the construction appears to be entirely noncombustible. The hidden fuel is the tar adhesive-vapor seal on top of the steel decking. When heated, the gases cannot escape upward so they blow down through the openings in the decking, providing a rapidly moving, self-sustained roof fire, independent of the fire in the contents.
Without sprinklers to cool the steel, the only way to control such a fire is to cool the steel with hose streams constantly from a safe location. This will not only stop the gas supply, but will cause the steel to draw back to its original dimensions. If it is already failing, it will freeze in place. Keep cooling the steel even after the flame disappears. If the cooling is stopped prematurely, the possibility of failure will return.
Such a metal deck roof fire can heat up bar joists far from the fire, elongate them, and push down a masonry wall. In a Dallas, TX, fire, elongating bar joists apparently pushed down a masonry block wall, causing the roof to collapse and six firefighters to be injured. (Note that Factory Mutual Type 1 and UL Classified roofs are designed not to produce a self-sustaining metal deck roof fire.)
Fire resistance-rated floor and ceiling assemblies
Prior to World War II, the steel industry had a monopoly of high-rise construction, and the heavy weight of concrete or tile fireproofing was immaterial with steel construction. When reinforced concrete became a serious competitor of structural steel for high-rise construction, lighter fireproofing was sought to reduce the “economic penalty” that the fireproofing imposed on steel. Membrane, rather than individual fireproofing came into prominence. The entire floor (or roof) and ceiling assembly is rated as a unit.
Steel bar joists are used in some rated fire resistive floor and ceiling assemblies. Such assemblies must be installed exactly as they are in the test laboratory, a requirement difficult to meet. In addition, the removal of one ceiling tile can subject the entire assembly to destructive fire temperatures.
Some building departments permit fireproofing to be omitted from columns passing through the plenum space (the truss void). A ceiling failure in such a building might well mean a column failure and a catastrophic loss.
Such rated floors are tested for passage of fire upivard through the floor, and for collapse. They are not tested for the problem of providing a cockloft or trussloft in every floor of the building. This void can and has extended fire over partitions into other occupancies on the fire floor.
During a high-rise fire, the building’s plenum (truss void) was the return side of the air conditioning system. The system was set for automatic exhaust when alarmed, pulling the fire 200 feet through the plenum. The mineral tiles which lined the ceilings became incandescent and started new fires as they dropped.
Two buildings of equal fire resistance rating may have very different fire characteristics. A building with massive concrete floors will act as a huge heat sink. Every Btu absorbed by the concrete is one less to propagate the fire.
Wood truss floors
The now common wood truss floors are a real menace to firefighters. There is no warning that this void exists between each floor. Not only is there the hazard of early collapse, but the void can be a reservoir for explosive carbon monoxide gas waiting for the firefighter to “pull the ceiling,” possibly the last ceiling he will pull. Tactics based on sawn joist floors will kill firefighters if used on truss floor buildings.
The truss floor building gives no outward indication of its presence. The only solution is for the fire department to pre-plan, record, and be able to retrieve on the fireground the information on the construction.
The argument is offered that the floor is “protected” by a sheet of “fire rated” gypsum board. This is simply not true. The only UL listed truss floor requires two layers of gypsum board and a concrete topping on the floor. In addition, the test furnace is 14X17 feet (238 square feet) so the amount of oxygen available to burn the wooden trusses when they reach ignition temperature is limited. If the void is 1 foot deep and the area of the test furnace is 238 cubic feet, there will be about 48 cubic feet of oxygen.
Assume that 32 cubic feet of oxygen is available. The fuel burned by consuming 1 cubic foot of oxygen yields 537 Btus. Thus, only about 20,000 Btus are generated, about 40 ounces of wood.
The test assumes, incorrectly, that the fire will only burn upward. The fire can originate in the void or burn into it laterally. The test does not consider that the fire can burn downwards. Today’s fires can burn down into the floor void in as little as five minutes. In the test furnace, fire gases are removed by exhaust. There is no buildup of explosive carbon monoxide gas.
Contrast this with the real life situation. The voids will provide 2,000 cubic feet or more of air, and will hardly be as tightly sealed as the test structure. The floor as installed cannot have but a fraction of the fire resistance of a test structure. In Lake Benbrook, TX, an exterior fire burned laterally into the void through the fascia board and destroyed the trusses.
Firestopping is sometimes offered as an answer to the floor void problem. However, it doesn’t always work. A scrap of gypsum board “buttered” into place is supposed to resist a gas under pressure. Firestopping is removed or penetrated for all the utilities, including cable television and, most recently, the residential sprinkler piping (which does not cover the void).
There is no proof that typical firestopping would be effective. Even if the firestopping is effective, the firestopped area is as large as 1,500-2,000 square feet or more. Firestopping will not prevent collapse of the area.
Education of architects has been offered as a solution to many fire service problems such as those discussed here. Imagine an architect, well aware of the problem of wood truss construction, who decides to design safer, yet necessarily more costly buildings out of love for firefighters. He would starve as other architects demonstrate that they could design cheaper structures—that meet the building code. The fire department must take care of itself.
I examined a truss floor garden apartment protected by a residential sprinkler system. It is very likely that the system will control any fire originating in the contents of an apartment. However, if the fire originates in or penetrates the void, the sprinklers will not control the fire.
A recent fire in a “fully sprinklered” college dormitory destroyed the building. The fire originated in, or communicated to the unsprinklered voids. The floor trusses were cantilevered out to form the balcony. The “firestopping” at the front wall was pieces of gypsum board buttered into place with cement. A failure of one of these “barriers” would admit fire to the balcony truss void. It is not hard to envision a serious loss of life in such a structure. Unfortunately, such a loss of life might deal a serious or even fatal blow to the concept of residential sprinkler protection.
Truss voids or trusslofts
As with wood truss floors, truss voids in ceilings may harbor a well-concealed fire that’s ready to burst out with almost unbelievable fury when oxygen is admitted to a void containing heated toxic carbon monoxide. The reaction can range from a deflagration to a detonation, from a backdraft or flashover to an explosion capable of blowing a building apart. It is not an exaggeration to say that the opening of a void is the equivalent of disturbing a “suspicious package.” Most often nothing happens, but once in awhile—disaster.
Reading accounts of disasters, I am struck with the apparent nonchalance with which firefighters open up potentially lethal voids. It is very possible to have a raging fire overhead with only “light smoke showing,” a meaningless term when the source of the smoke cannot be seen.
Consider a night response to a fast food outlet. Light smoke is showing in the brightly lighted interior. A line is stretched to the interior. More or less at random, the ceiling is pulled or tiles are raised. In most cases, minor fire is found and extinguished. Once in awhile, however, the fire “blows.” In the worst case the ceiling falls.
Do not discount the hazard of the typical “fire rated” (meets flame spread requirements) suspended panel ceiling. If the grid system comes down, firefighters are trapped in a steel net. The area is pitch black. A firefighter, with no idea of what has happened, thrashes around until his air gives out. Train personnel to get down low and get below the net.
Wooden I-beams
Wooden I-beams are another method of lightening buildings. They consist of a 2X4 for the top and bottom chords, and a plywood web between. Contrasted with its predecessor, the sawn beam, the “fat” (structurally unnecessary wood along the sides) is no longer present. This fat is the basis of our interior firefighting tactics. Until it burns away, the beam is structurally sound. The gradual sagging of the exterior wood often gave warning of impending collapse.
The web of the I-beam is penetrated by utilities, so that any fire gets a grip on both sides of the Ibeam at the same time, guaranteeing early failure. I have pictures of cut off ends of I-beams used as “firestopping.” When I pointed to the fire gap through the “re-entrant space” (the space where the wood is missing as compared with a sawn beam), the inspector said, “We’ve always used the cut off ends for firestopping. You can’t ask the contractor to cut up good lumber for that.”
Photos by Frank Brannigan
Wooden I-beams or trusses are often cantilevered out to form the balcony. Fire inside may be destroying the interior connection and the balcony will collapse, possibly loaded with unwary firefighters. Never forget that a cantilever is truly a seesaw. When one end goes up, the other end goes down.
Recently I saw unique I-beams of 2X6 top and bottom chords and corrugated steel webs. The wood was in 8-foot lengths. Longer beams were spliced with gusset plates. The corrugations had knockouts for electrical wiring. The edges were sharp, possibly sharp enough to cut the insulation. These reduce the fuel load, of course, but if subjected to fire, the steel might move and separate the wood.
INCONSISTENT CONSTRUCTION
It is no longer possible to determine the construction of a building based upon what is seen at the first place opened for examination.
In a motel, wooden I-beams span from the exterior walls to the interior corridor walls. The corridor is about 6 feet wide. The corridor is spanned with 2X6 sawn beams.
The flat roof of a restaurant consists of wood and steel tubular trusses from the exterior walls to the interior corridor walls. In this case too the corridor is spanned with sawn 2X6s.
A steak restaurant has a hip roof. The sloping sections have sawn rafters. The flat top is deep parallel chord gusset plate trusses. The restaurant interior and all the voids are sprinklered.
Many older buildings are being rehabilitated. Extensions and additional stories may be built of trusses or wooden I-beams. Older buildings often had high ceilings for ventilation. Mezzanine floors, supported on trusses, may be inserted. It is unsafe to assume that a building is consistent in its elements.
When the fire involves the structure, as distinct from the contents, there is a radically different situation. Everyone should be advised, “This is now a structural fire,” the word “structural” being reserved for just such a situation.
Changes in building codes or better enforcement of existing codes will do little to reduce the firefighting hazard presented by these new (and not so new) construction techniques. There is little in the building code specifically concerned with firefighter safety. This is the responsibility of the fire department.
There is no substitute for the fire department developing a system of accumulating and organizing information for retrieval at the time of the fire. The situation is analagous to military intelligence. It is vital to know the disposition of the enemy. Many buildings have existed for a century or more, but as far as some departments are concerned, the structure might as well be a newly landed spaceship.
