In the April 2006 issue of Fire Engineering, we discussed issues to be considered when confronting a fire in a one-story commercial building. Here, we review some of the major construction components that can affect your firefighting strategy.
Below, we use the example of a one-story steel building in various stages of construction to show how each of a building’s components can impact the structure’s stability (photos 1, 2).
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In the April 2006 issue of Fire Engineering, we discussed issues to be considered when confronting a fire in a one-story commercial building. Here, we review some of the major construction components that can affect your firefighting strategy.
Below, we use the example of a one-story steel building in various stages of construction to show how each of a building’s components can impact the structure’s stability (photos 1, 2).
Columns-upright structural members acting in compression. They transmit the weight of the roof and deliver it to the ground. The failure of a column may cause part or all of the building to suddenly collapse. In this example, the column is bolted to a concrete pad located at floor level and to an I-beam near roof level. In a fire situation, the steel beam at the ceiling or roof level can heat up and start to expand and twist. The expanding steel can pull the column off its vertical plane. Of all the building components, the failure of a column presents the greatest danger. If you see a column that looks to be leaning or that is not perfectly vertical, notify the incident commander (IC) immediately. The building must be evacuated immediately and a roll call conducted (photo 3).
Steel girder-a horizontal beam that supports other beams. Designed to carry heavy loads, girders rest on columns. As fire and heat start to attack the girders, the steel starts to absorb heat. At approximately 1,100 °F, steel will start to fail. At this temperature, the steel begins to expand and twist. A steel girder 100 feet long could expand about 10 inches. Once the steel starts to expand and twist, the columns holding up the steel girder also start to move. The expansion of the steel could cause the walls at both ends of the girder to push out (if the steel butts into a brick wall), possibly causing the wall to buckle or fail (photo 4).
Lightweight steel truss bar joists-a parallel array of light steel beams used to support a floor deck or low slope roof. Steel girders located in the front, center, and rear of this building support the lightweight trusses. The joists are welded to the steel girders. During a fire, the lightweight trusses absorb heat rapidly and can fail in five to 10 minutes. If the roof is heavily loaded with air-conditioning and other units, collapse may occur sooner. Do not attempt to cut a steel bar-joist roof. Doing so may sever the top chord of the trusses, the main load-bearing member, and could cause the entire truss structure and roof to collapse.
The spacing of the joists can be approximately four to eight feet apart. This wide spacing is one reason you don’t want to cut a roof that has lightweight steel joists and a Q-decking roof surface. In Collapse of Fire Buildings: A Guide to Fireground Safety (Fire Engineering Books & Videos, 1988) Vincent Dunn, deputy chief (ret.), Fire Department of New York, states: “An important design difference between a wood joist and a steel joist roof support system is the spacing of the joist. Open-web steel bar joists can be spaced up to eight feet apart, depending on the size of the steel and the roof load. This wide space between joists creates several dangers to a firefighter cutting an opening in the roof deck, even when there is no danger of collapse from the steel bar joist. First, when the outline of the 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. If a firefighter has one foot inside the roof cut, he may lose his balance and fall with the saw into the fire below (photo 5). (138)
Steel lintels-horizontal pieces of steel support that redistribute the weight of the brick above window openings and doorways. These pieces of steel usually can be found in an “L” shape for small openings or an I-beam for larger openings. Lintels are tied into the masonry wall on both sides of the opening. Just like other pieces of steel, once the lintel heats up, it starts to expand and twist. The failure of the steel lintel could cause the collapse of the wall above (photos 6, 7).
Façade-the exterior face of a building. Light-gauge steel members make up the framing for the facade. A water-resistant gypsum material is used to close in the cockloft. Lightweight-gauge steel can lose structural strength and stiffness rapidly in a fire. The ventilation of the cockloft may be accomplished by breaking through the gypsum sheathing instead of placing a firefighter on the roof. This exterior gypsum is similar in strength to gypsum board used on most interior walls of a home. Once the gypsum sheathing is in place, the builder used Styrofoam® over the gypsum followed by a coating of stucco (photos 8, 9).
Roof surface. The material used to make the roof surface in this building is simple to construct. First, steel Q-decking is tack welded to the bar joist. Then, foam insulation is placed over the Q-decking and fastened to the decking with screws. After the insulation is in place, a rubber membrane is glued to the foam insulation, completing the roof’s surface.
Another roof surface you may encounter, common to low slope roofs, is polystyrene foam insulation topped with a 3/8-inch layer of latex-modified concrete.
A third type of roofing surface consists of a layer of rigid insulation fastened to the roof deck. Asphalt felt paper is then glued to the insulation with hot asphalt. Stone is then spread over the roof surface to hold it all in place and protect the felt membrane.
With this type of construction, do not consider cutting the roof. With a five- to 10-minute collapse potential, there isn’t enough time to safely complete roof ventilation. Look to ventilate the cockloft by horizontal ventilation (breaking through the façade of the building) instead of placing members on the roof. Cutting any part of the truss could cause the entire roof surface to collapse. As mentioned above, the roof decking could hinge downward under the weight of the member cutting the roof, sending the individuals into the fire building. The industry has had enough experience with lightweight trusses to strongly recommend that you keep your members off the roof when they are present (photo 10).
Suspended ceiling-aluminum or steel grid system hung from roof supports with steel wire. This grid system will hold all the ceiling tiles to form the finished ceiling. The space above the suspended ceiling presents great danger for firefighters. Most commonly called the cockloft or truss void, it can hide fire and fire gases. Once this space is penetrated, explosive carbon monoxide may ignite, causing the entire grid system to collapse. You must examine the cockloft early in a fire and in a way that will allow all firefighters to escape the building should the fire suddenly explode out of the ceiling. Have a charged handline and all firefighters near a doorway in full turnout gear. Wires, HVAC system components, and gas lines are but a few of the building services that may be hidden in the truss void. Numerous natural gas lines can penetrate the roof to be used for heaters located on the top of the building (photo 11, 12).
Today, with steel and wood trusses in all types of buildings, from private homes to high-rise office buildings, the decision to withdraw firefighters may present itself much earlier in the fireground evolution. Truss construction has been around long enough so that all fireground commanders should know how buildings containing them react in a fire, and act accordingly.
For an IC to be properly prepared, he must start with a general idea of building construction. Francis L. Brannigan’s Building Construction for the Fire Service, Third Edition (National Fire Protection Association, 1992) and Dunn’s book have been available for some time and are must-reads for all members of the fire service.
Since we usually do not have time to consult with a building engineer at the fire scene, the responsibility falls on the IC to anticipate changes the building will undergo as it burns. If you are an officer or aspire to become one, you need to educate yourself on building construction.
JOHN MILES is a captain with the Fire Department of New York, assigned to Ladder 35. Previously, he served as a lieutenant with Ladder 35 and as a firefighter with Ladder 34 and Engine 82. He also served as a volunteer firefighter with the River Vale (NJ) Fire Department and the Spring Valley (NY) Fire Department and is an instructor with the Rockland County Fire Training Center in Pomona, New York.
JOHN TOBIN, a 33-year fire service veteran, is a former chief of the River Vale (NJ) Fire Department. He has a master’s degree in public administration and is a member of the advisory board of the Bergen County (NJ) Law and Public Safety Institute.