THE DANGERS OF LIGHTWEIGHT STEEL CONSTRUCTION

BY KARL K. THOMPSON

This catastrophic collapse of a three-story, single-family residential structure (photo 1) is just one more example of the dangers lightweight construction poses for firefighters. Tactical time frames may need to be reevaluated when fighting fires in lightweight steel-framed buildings. This L-shaped home catastrophically collapsed without warning in an early-morning fire about 10 to 15 minutes after the fire extended into the second floor. The cause of this collapse was a complete failure of the structural lightweight cold-formed steel studs, joists, and trusses. As with most “new” construction methods, the fire service is behind the power curve until we have had a few actual fires and use them as case studies to evaluate the performance of building materials and methods.


(1) This is what remains of a lightweight steel-frame, three-story, single-family residential structure after a catastrophic collapse. [Photos by J. Shepherd, district chief, Brevard County (FL) Fire Rescue.]

Lightweight construction has been a curse to the fire service, but this problem is amplified when the bearing structural members are made of lightweight cold-formed steel. The steel members are the same size as traditional wood framing in nominal widths of 2 feet x 4 feet or 2 feet × 6 feet and made of dime-thick (14 gauge) U-shaped channels of galvanized steel (photos 7, 8). For the construction industry and the homeowner, there are several advantages, including cost and resistance to rot and termites. Steel studs have been used for many years for nonload-bearing interior partitions in commercial buildings.


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(7, 8) Typical framing connections.

The materials used and the methods employed in lightweight steel frame construction pose a tremendous risk to firefighters. The structural elements are co-dependent on each other, as they are in all buildings. In these structures, it appears there is a small safety design margin for the assault from fire. Once the steel loses its strength from the heat, collapse occurs quickly. There does not appear to be any grace or forgiveness when these steel members fail.


(3) It will be all but impossible to identify the construction methods used when arriving at this house. This house is being built among other residential structures. It is the only unit constructed of lightweight steel framing. All the other structures in this neighborhood employ more conventional construction methods.

Such was the case we experienced. Compared with wood, for example, which has a somewhat predictable rate of burn or char-about one inch in 45 minutes-these steel members seemed to fail without warning. When failure occurs, it appears that all the elements fail at one time. Again, comparing this to wood-frame collapses, the loads usually shift and form a lean-to type of collapse. When this steel structure failed, it acted more like a beer can being pressed in from both ends in a pancake-type collapse.


(2) The pancake collapse of a lightweight steel-frame, single-family residential structure.

There has always been a risk when using tactical time frames to determine potential collapse. Many department procedures call for a defensive attack after 20 minutes of firefighting that has yielded little or no progress. The “20-minute rule” establishes that dimensional lumber used in wood-framing systems starts to become weak after 20 minutes of fire exposure. In addition to time, most collapses give some warning, such as cracking and moaning sounds and shifting loads. This is not the case with the lightweight-steel framing systems. It appears that once the compartment, the room or void space, is heated to the steel’s point of failure, all the walls in the area simultaneously fail. There is little or no warning, and time is not a reliable method of determining when interior operations must be halted.

The structure was an L-shaped single-family home, approximately 7,000 square feet in size built on a concrete slab. The stem of the L was a one-story type V (wood frame) construction, about 1,200 square feet, with a small finished cockloft attic apartment over half of this section. The fire started in electrical equipment in this single-story portion of the house. The base of the L was a three-story structure (1,500 square feet per floor) constructed of lightweight steel studs, joists, and trusses. Interior walls were covered with 1/2-inch gypsum board. The exterior was covered with oriented strand board (OSB), a vapor barrier, and clapboard siding. The exterior wall stud, spaced 16 inches on center, was insulated.


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(9, 10) A lightweight steel truss cockloft (attic). Note that the exposed plywood sheathing will allow fire to spread over the complete underside of the roof decking.

Two stairs extended through the three-story portion of the home. In the kitchen, there was a spiral stair that extended and stopped on the second floor. The main stairway was close to the front door and turned back on itself to access the third floor. The owner indicated there were no doors to separate the stairs from the floors below. It was obvious that the open stairs allowed for rapid fire extension to the upper floors. This house was located in a remote area where response times are approximately 10 minutes.


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(12, 13) Consider the structural integrity of the second floor and stairs. In this home, a second-floor bedroom is accessed and supported by lightweight steel framing.

The fire occurred early in the morning (around 0230 hours). The owner reported being aroused by the radio intercom’s suddenly playing music. As he awoke, he smelled a slight odor of smoke. His bedroom was on the third floor. He was able to look out the back window of his bedroom to the stem portion of the L and see that the section was on fire but had not yet autovented. He descended the stairs and ensured that the family had exited. He indicated that smoke and heat conditions were minimal as he escaped. He was reporting the fire by cellular telephone to 911 as he exited the structure.

Shortly after exiting the structure, he noted fire extending onto the second floor; it was visible in the front windows, where the base joined the stem portion of the L. He explained that the fire rapidly advanced through the second floor and into the third floor.

As fire units arrived, they reported the house was heavily involved in fire. The company officer ensured that the occupants were safe and out of the structure and ordered the first line to protect an exposure on the D side. As this line was being stretched, the second floor suddenly buckled and failed, causing a complete collapse of the entire three-story portion of the building.

Obviously, the magnitude and extent of the fire were severe, and collapse would have occurred regardless of the construction methodology. The question becomes, how much time can this type of building endure? Based on previous experiences of comparing “regular” lightweight wood frame (2-inch x 4-inch or 2-inch x 6-inch studs), these buildings collapse in less than half the time. It appears that as soon as a significant percentage of the floor is involved, a complete collapse is probable.

Designers, engineers, and contractors are marketing steel framing as fire resistant and noncombustible. Compared with wood, steel will not contribute to the fire load. The industry points to testing conducted on walls and partitions, touting an impressive fire resistive performance. The combustible properties, however, are not the issues. The assault on the structure and the structure’s ability to resist collapse are of critical concern for firefighter safety. The primary considerations in evaluating the potential for collapse are the weight the structural members are supporting and the amount of fire impingement.

There is no argument about cold-formed lightweight steel framing’s being an effective and useful building material. According to the Steel Framing Alliance™, it is used in about two percent of the residential market. Steel framing has gained significant acceptance in Hawaii, California, Florida, and Texas. Nonresidential buildings have a higher market share. The Steel Framing Alliance™ reported that in 2002, 81 percent of interior walls, 47 percent of exterior walls, 13 percent of floors, and four percent of roofs were constructed using steel framing in commercial buildings.

Houses using this method of construction are not readily identifiable from the outside when approached. There are few indicators of the construction since exterior and interior finishes cover the steel framing.

PROPERTIES OF STEEL

Lightweight steel obviously will lose its ability to support loads early. To my knowledge, the amount of heat needed to cause failure and collapse has not been reported or well documented, although observations indicate that when heat conditions have extended throughout the majority of the area (in this case, the second floor) and these lightweight steel members are the only means of support, failure will occur, and it could be catastrophic.


(4) The obvious deformity of these steel structural members shows that the collapse of this building resulted from the load applied. It is also obvious that the steel framing did not contribute to the fire load. However, if anyone were trapped in this building, rescue would be impossible.

The properties of structural steel construction are well documented. Steel is greatly affected by heat. At approximately 400°F, steel begins to lose strength; at 1,200°F, it will have lost more than 60 percent of its strength. Steel can be expected to fail when temperatures are in the proximity of 1,000°F. Steel elongates when heated; a 100-foot steel beam heated to 1,000°F will expand 9 1/2 inches.

The failure rate of steel is affected by the following: the size of the steel member, the load supported by the framing system, the amount of heat exposure (this will vary according to the amount and type of interior finish used to cover the steel framing), and the distance from the steel unit to the fire.

The only protection for the steel is the gypsum board wall covering. The thermal insulation priorities of 1/2-inch gypsum board will not greatly delay the transfer of heat to the framing members. Once the wall stud space inside the wall reaches 500°F, failure is possible. These structural members do not need a great deal of time to be heated to the failure temperature. They lack mass!

The cold-formed steel members have periodic holes punched through the member. The holes, like all penetrations, will allow fire and heat to travel in concealed spaces. Although they may not be directly related to the collapse problem, the penetrations are of concern when checking for extension. These holes will also allow hot fire gases to weaken other steel members. In looking at one of these homes under construction, voids that extended throughout the walls were noted. Fire can easily travel to the attic once it enters a concealed space.

WORKMANSHIP

As with all types of construction, the workmanship is extremely important. Several of the connections, which are screwed together with sheet metal fasteners, did not appear to have structural integrity. At several locations, the screw barely had a bite on the steel. Clearly, this is well outside the manufacturer’s design for connections. The Steel Framing Alliance™ has on its Web site reference material on proper construction methods. But the reality of poor installation is a major factor for firefighter safety. The weakest part of the building is the connections.


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(5, 6) These photos reflect poor workmanship: The sheet metal screw is already close to failure. In photo 6, the nut is missing from the anchor bolt. The anchor nut that holds the frame to the foundation is missing.

The National Fire Protection Association’s Fire Protection Handbook reports that open-web joists can collapse after five to 10 minutes of exposure. The predictability of lightweight truss failure is not an exact science; however, lightweight steel truss assemblies will most likely fail under the weight of a firefighter. This again raises the issue of roof operations on truss-constructed roofs.

These structures gain part of their stability by the sheathing applied to the exterior walls and roof. This also includes the interior that gains rigidity from the gypsum wallboard. The ability of gypsum board and lightweight plywood to resist sudden load shifts is minimal.


(11) The structure gains rigidity from the exterior sheathing applied.

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INSPECTION HOLES ARE NECESSARY

After evaluating this fire, make inspection holes to evaluate the type of structural material employed. The first should be made in the bearing wall near the point of entry. The inspection hole would be similar to the one made in the ceiling to confirm the absence of fire overhead and to determine the construction method. Make additional inspection holes in an internal partition to confirm the type of structural member employed. If lightweight steel structural members are present, Command must adjust the interior attack timetable. If crews encounter high-heat or well-involved fire conditions, Command must reduce the time the crews can operate, if at all, inside the building.


(14) This house, under construction, depicts the construction method used in building these homes.

These conclusions are based on a limited number of actual fires. If you have any fire experiences, please share them. Some will argue that lightweight steel construction is the wave of the future and point to the advantages. The same type of argument has been made by the truss designers and manufacturers for years. To them, I say, “Prove me wrong. Provide full-scale testing under load conditions for building materials.” We no longer should sacrifice firefighters for economic building construction considerations.

KARL K. THOMPSON, a 33-year veteran of the fire service, spent 25 years as an officer with Brevard County (FL) Fire Rescue, serving as a lieutenant, fire inspector, training captain, and district chief. He is currently the assistant fire marshal for the county. He has a master’s of public administration from the University of Central Florida and is an adjunct faculty member in the fire science and technology programs at Florida State Fire College, Brevard Community College, and Indian River Community College.

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