Behold the beam, an amazing structural element that bends when loaded — but one that must not bend too much. A fallen tree spanning the banks of a river was perhaps the first beam used by primitive man for a specific purpose: to see what's on the other side. That fallen tree was an...
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Behold the beam, an amazing structural element that bends when loaded — but one that must not bend too much. A fallen tree spanning the banks of a river was perhaps the first beam used by primitive man for a specific purpose: to see what's on the other side. That fallen tree was an accidental beam.
Let's consider the six basic beam configurations that you are likely to encounter during pre-incident planning:
1. Simple beam — As mentioned previously, a simple beam is a single-span beam supported — but not restrained — at each end. Because its ends are not restrained, the entire length of the beam can deflect when loaded. Due to compression and tension within the beam, the material at each beam end is free to rotate. The beam shown in Figure 1 and Figure 2 in part one of this article (July 2010) is a single-span, simply supported beam.
When heated, an unrestrained steel beam is free to elongate, elongation generates lateral thrust. Axial column/beam assemblies are not designed to resist thrust. In this context, think of thrust as a lateral outward push.
2. Restrained beam — At first glance, a restrained beam looks like a single-span simple beam. However, because each end is rigidly fixed (Photo 1), it cannot move; clockwise rotation is restrained. A restrained beam is also referred to as a fixed-end beam.
Like the column described previously, should a 40-foot unprotected steel girder be heated to 1,000 degrees Fahrenheit, the girder will want to elongate about four-inches. If the girder is rigidly restrained, it will not be able to elongate. The restrained steel girder will release the elongation energy by twisting (torsion). Girders support other beams (purlins and/or joists). These beams often support the floor above or the roof. As the girder twists, these beams are compromised; loads are transferred, axial becomes eccentric and factors of safety disappear. Heat-induced structural shifts can produce seismic-like results.
3. Continuous beam — A single-span beam extending over three or more supports is a continuous beam (Photo 2). Because the single-span beam continues over the middle support, multiple-curvature is generated. There is simple beam sag between the supports and an upward curve reaction as the beam passes over the middle support.
This becomes strategically significant should a continuous beam become a simple beam during a fire or because a forklift accidentally backs into and buckles a column. This can happen if a middle support fails. If the middle support is a 40-foot unprotected steel column that heats to 1,000°F, the column will want to elongate four inches. (Imagine the amount of energy required to stretch a 40-foot steel column by four inches.) If compressive loading prevents the column from elongating, it will release that energy by buckling (deflecting). When a column deflects (bends), it is no longer a column; it has become a vertical beam. Beams bend, columns don't bend.
With the loss of the center column, the continuous beam has just become a simple beam. This means that the load carried by the center column has been transferred to the end supports. This load redistribution arrives at the end supports as a dynamic impact. Can the columns or bearing walls support the extra load? You do not want to own this situation if you are the incident commander.
4. Cantilever beam — A cantilever beam is also a single-span beam, but only one end is supported. To maintain equilibrium, the end support must be rigid enough to resist rotation (similar to a lever). The supported end is "restrained," which means it cannot move (thrust) or rotate (lever) when the cantilever is loaded.
Try this: Stand up and extend one arm perpendicular to your body. One end of your arm is supported by your shoulder assembly and the opposite end is unsupported. You have just created an anatomic cantilever. In a cantilever, the top portion of the beam lengthens; thus, the stress is tension; the bottom portion of the beam shortens, thus the stress is compression.