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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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.