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

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...


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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.