Steel Bar Joist Trusses And Steel C-Beams - Part 2

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Many renovated buildings replace wood and masonry structural elements with steel. The replacement of heavy wooden roof and floor joists with the lighter steel open-web bar-truss joists is a common renovation change. A lightweight steel open-web bar-truss joist, similar to the floors of the World Trade Center and the roof of the Charleston, SC, sofa store, is a long steel bar (web) bent at 90-degree angles and welded to angle irons (chords) at the top and bottom of the bar bends. Classified as type II non-combustible construction, it is used in new commercial buildings. However, it is also used to replace old wooden joists in renovated ordinary-type, brick-and-joist buildings.

Buildings of non-combustible construction have masonry or steel non-bearing enclosure walls and steel column and girder framework, and use lightweight steel open-web bar-truss joists as roof and floor supports. Non-combustible construction using steel in place of wood for interior structural framework is well suited for large-area commercial buildings. Long-span steel truss beams create large open spaces.

The lightweight steel bar joist must be viewed by firefighters as extremely hazardous in roof and floor construction for two reasons: the failure characteristics of unprotected steel and the joist spacing. The Handbook of Fire Protection, published by the National Fire Protection Association (NFPA), states: "When unprotected, the steel bar joist may collapse after five or 10 minutes of fire exposure." Advancing a hoseline inside or operating on a roof of a burning building constructed with steel bar-joist trusses is a high risk. A rapid collapse may occur.

The average response time of an urban fire company is five minutes; a suburban or rural fire company is somewhat longer. If a fire occurring in an unoccupied building with a roof supported by unprotected lightweight steel bar joists or C-beams is so severe that roof venting is required to assist advancement of the first hoseline, then the roof must be considered too unstable to send firefighters there for ventilation and interior attack with a hoseline. Fire chiefs should instead consider horizontal ventilation and defensive firefighting attack after all occupants have been removed.

Some one-story, non-combustible buildings using lightweight steel joists are constructed with large rectangular windows at the upper portion of the masonry or corrugated steel enclosure walls. These windows are effective for smoke venting. Located at the top portion of the walls, where heated smoke would accumulate, these windows have a horizontal length that is greater than the vertical depth of the window area. Ventilating several of these windows can be accomplished faster and more safely than making a four-by-four-foot roof cut. Roof ventilation has been a very effective and relatively safe fire department operation when carried out on a wood-joist roof support system spaced 16 to 24 inches on center.

Roof construction that varies from this standard design is more dangerous. When a different roof construction, such as that using lightweight steel bar joists or C-beams, is introduced into a community in which standard wood construction has predominated, the different collapse characteristics present a new safety hazard to firefighters. A steel bar joist and C-beam will collapse more quickly. An experienced firefighter who has advanced hoselines into burning ordinary-constructed buildings or cut vent openings in a roof of solid-wood joists cannot transfer his or her sense of safe operating time to a building with a lightweight steel roof. Firefighters have developed a sense of how long they can work inside a burning building or on a roof over a fire, based on the type of roof construction used in ordinary construction, not noncombustible construction with steel trusses.

Over the years, fire officers develop the ability to size-up a fire. This size-up is correct 99% of the time; however, the 1% of the time when this size-up is incorrect a roof or floor collapses, killing or seriously injuring a firefighter. Today, this incorrect size-up is often the result of a change in construction. Lightweight construction using steel bar trusses or steel C-beams is often the cause of a size-up misjudgment.

Spacing of Lightweight Steel Bar Joists

An important design difference between a wood joist and a steel bar joist and a C-beam roof support system is the spacing of joists. Open-web steel bar joists can be spaced up to eight feet apart, depending on the size of the steel used and the roof load. This wide spacing creates several dangers to a firefighter cutting an opening in a roof deck. First, when the outline of the roof cut is near completion, and if the roof deck is not directly above one of the widely spaced steel bar joists, the cut roof deck may suddenly bend or hinge downward into the fire. A firefighter who has one foot placed inside the roof cut opening could lose his balance and fall, with the saw, into the fire below. The wide spacing of the steel bar joists allows a firefighter to fall through a roof opening when visibility is poor because of darkness or smoke.

In the past, wood joists spaced 16 inches on center could prevent a firefighter from falling through the roof openings if he lost his balance. The mask cylinder and backplate could snag on the closely spaced joists, or the joist could be grabbed by the falling firefighter. When several large roof vent cuts are planned over a fire, a firefighter cutting the roof will overcut the initial roof vent opening. On a fluted steel roof deck with steel joists spaced several feet apart, the roof deck area around the initial roof vent hole near these overcuts is extremely unstable; the edge of the roof deck may bend downward and drop a firefighter through the roof opening.

The blade of a power saw cutting a roof deck sinks several inches into the roof. If the blade is drawn across the steel bar joist, it can slice the top chord completely in two. In some instances, the top chord is the main load-carrying member of the steel joist. Over the past hundred years, training for roof operations has been carried out under the assumption that a firefighter is working on a wood-joist roof two-by-10 or two-by-12 inches in size and spaced 16 to 24 inches on center. This assumption is no longer true. A firefighter must know the type of roof on which he is operating above or below and know its collapse potential.

An important, 10-year study conducted by the NFPA on building collapse revealed that of 56 firefighters killed by collapses, 21 died in floor collapses, 19 in roof collapses, 14 in wall collapses and two in ceiling collapses. Of the 19 firefighters killed in roof collapses, 15 were operating inside buildings below the collapsing roofs; four firefighters were operating above the collapsing roofs. This study reveals it is more dangerous to operate below an unstable roof. Most firefighters killed by roof collapse are operating inside the building when the roof fails. Remember, most firefighters killed by roof collapse are inside the burning structure , as well as operating on the roof when it caves in. The Charleston, SC, steel bar joist collapse that occurred 22 minutes after arrival killed nine firefighters operating inside the burning building.

Causes of Failure Of Unprotected Steel

Four factors determine the speed with which unprotected steel will fail during a fire: temperature of the fire, the load stress, the steel thickness and the fire size.

Temperature of the fire. Fire load, the amount of combustible material that can burn, includes combustible content and combustible structure. Unprotected steel used in non-combustible building is safe where there is a low combustible content rating. If the non-combustible steel building contains content in the amount, or combustibility, considered as high fuel load, fire protection systems such as automatic sprinklers and fire partitions must be added — sprinklers with the backup of compartmentation in case the sprinklers fail. In some buildings, such as those of heavy-timber construction, more fuel may be supplied to a fire by the wood structure (timber columns and wood floors) than by the content. In a building classified as non-combustible, featuring lightweight steel bar joists instead of wood joists, a considerable amount of fire loading is eliminated.

Unprotected steel has no fire resistance, so a steel building can be quickly destroyed by fire. It will collapse when heated to temperatures that are easily attained at a fire. When heated, steel will bend, sag, warp and twist unless it is encased, cut off or covered in some type of insulating material. The principal danger to a firefighter in a burning non-combustible structure that contains unprotected steel is its potential for early collapse. The fire service considers 1,100 degrees Fahrenheit to be the failure temperature of steel for at this temperature steel will lose 40% of its load-carrying capacity. This temperature, however, is not the temperature of flame and heat of the fire. The temperature within the steel itself, not just the surrounding temperature, must be raised to 1,100F before it will fail. Because steel is a good conductor of heat, there is a time lag between the time the fire area reaches 1,100F and the time the steel itself reaches this temperature.

The high temperature of a fire can also cause steel to expand. During fires, masonry walls have been moved to the point of collapse by expanding steel girders and beams. A 50-foot-long steel beam that is heated uniformly over its length from 72F to 972F can increase in length by three feet nine inches. This increase in size will either push out an enclosing wall or cause the steel beam to buckle.

Firefighters should have some idea of the heat that can be generated by a fire. The standard time-temperature test fire gives us some idea of how rapidly temperature can rise during a fire. The two important facts of the time-temperature curve are that:

  1. Within the first five minutes, the temperature of the fire will rise to 1,000F.
  2. After 10 minutes, the temperature reaches over half the total temperature rise (1,300F) attained after eight hours.

Load stress of steel. The second factor that determines steel failure during a fire is the load supported by the structural member. The greater the supported load, the faster a structural steel member can fail. In modern non-combustible buildings, roofs are not designed to support the same load as the floors below. A floor must be capable of supporting contents and people, but a roof is designed to support only the weight of rain or snow accumulation. (In the South, a roof may be designed only to resist a wind load.)

In older brick-and-joist buildings, the flat roofs could often support the same load as the floors below. This result was achieved by accident, not by design, because the builders were not as cost conscious as they are today. The roof joists were the same size and spaced the same distance apart as the floor joists below. Today, when designing a roof-support system, builders do not consider the weight of firefighters and their equipment on a roof or inside the building when it is burning. In a non-combustible building, the open-web steel-bar joists or C-beams used in roof construction will either be spaced farther apart than the floor bar joists or the roof joists will be of steel of a smaller dimension. A roof designed only to support a snow load may have a load capacity of 20 pounds per square foot.

The roof deck is also a load factor that can influence the collapse of a steel joist. The heavier the supported roof deck, the faster the collapse of heated steel roof beams. Two common types of roof decks are used above the fluted-steel deck of a bar-joist roof or C-beam support system: a lightweight insulation or a pre-cast concrete plank. The heavier concrete plank roof deck may reduce a firefighter's safe operating time on top of or below the unprotected steel roof of a structure.

Thickness of the steel. The size of the structural element is another factor that determines steel failure. A heavy, thick section of steel has greater resistance to fire than a lightweight section. A large, solid steel I-beam can absorb heat and take a relatively long time to reach its failure temperature, while a lightweight steel beam, such as an open-web bar joist or C-beam can be heated to its failure temperature much faster. By increasing the mass of a steel structural element, we can actually increase its fire resistance to a limited degree. An unprotected, built-up steel column of sufficient mass could even be given a one-hour fire-resistance rating if tested in a furnace by means of the time-temperature curve and if the unprotected steel absorbed sufficient heat to prevent the column's cross-section area from reaching an average of 1,100F within that time.

Unfortunately, the trend is toward lightweight-steel construction, and the costly "overbuilding" found in older steel buildings has ended. The "heat-sink property" of large-size steel is its capacity to absorb heat from a fire and its ability to conduct or transfer heat of a localized fire away from the point of flame contact to the cooler interior regions of the steel. This heat-sink property can lengthen the time required for the temperature of the steel to reach the collapse temperature; however, even large-size steel will eventually collapse in a fire. There is little heat-sink capacity in thin, lightweight steel structural supports.

Fire size. The size of the fire is the final factor that can affect steel failure. If a small-area fire comes in contact with a portion of a large steel beam, the steel will absorb heat and transfer it away from the flaming area to cooler parts of the structural element. A fire could burn for some time before it heats the entire steel beam to its failure temperature. On the other hand, a large-area fire in which flames involve much of the steel beam in a short time will heat the steel beam to its critical temperature more quickly. A "flash fire," suddenly involving a large area with flame, can heat steel to its failure temperature rapidly.

Lessons to Be Learned

There is a difference between the terms "non-combustible" and "fire resistance." Steel is non-combustible, not fire resistive. Non-combustible steel will not add fuel to a fire, but steel cannot resist fire. It will collapse from the effects of fire. The heat of a fire destroys the load-bearing qualities of steel. If you want to make steel fire resistive, you must protect it. You can make steel fire resistive by covering it with insulation. Heat-resistant materials such as concrete or spray-on insulation can give steel fire resistance for one, two or three hours, depending on the type of insulation, or you separate it from a potential fire area with a membrane ceiling.

Unprotected lightweight-steel bar-joist trusses and steel C-beams are not fire resistive. They can collapse after five to 10 minutes of fire exposure. Lightweight steel beams are designed for low-hazard occupancies with contents that do not experience severe fires. Unfortunately, the design of a non-combustible type II building with floors or a roof built with steel bar joist trusses or C-beams does not anticipate a possible occupancy change from low-hazard content to a high-hazard content or flammable-liquid use. When an occupancy hazard increases from low hazard to a moderate or high hazard, additional fire protection should be mandated.

During the renovation or a building where there is a change in roof construction from wood to unprotected lightweight steel bar joist or steel C-beam, there may be an advantage of reducing the fuel in the roof or floor construction that would contribute to a fire. There will, however, be an increased potential for firefighter death and injury from building collapse.