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...
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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.
Behold the most basic of structural elements, the beam. Because a beam can bend, it can support a load. However, if a beam is overloaded, and bends too much, it will fail. Every beam has a sort of bending sweet spot. As long as there is a neutral axis with compression on one side and tension on the other side, a beam can support a load. In this article, we will review basic beam behavior and offer the six beam configurations that you are likely to observe during pre-incident planning — or while waiting in line at your local big-box super-duper-mart.
For decades, the late, great Professor Frank Brannigan implored the fire service to "know your buildings." I would add that you should also know how your buildings work. Knowing how different structural systems function and behave will give you a better understanding of what to expect when a particular building is being assaulted by gravity, time and heat.
What You Need to Know About Beams
A beam made of wood or steel will bend when loaded. Anything that bends (deflects) experiences tension and compression. Here's the official description of the internal reaction of a beam to bending: The deflection of a beam at any point is its vertical displacement (strain) from the unstressed (neutral) position.
Here's what you need to know about the beams you'll behold as you meander from building to building from week to week: beams bend when loaded, but must not bend too much. What you also need to know is that not all beams bend when loaded. I know that is confusing, so allow me to explain. A beam is defined as "a rigid structural member designed to support and transfer transverse loads across space to supporting elements." (Say what?) Here's a simple translation: A beam is a horizontal structural element that resists a load by deflecting (bending), which sends the load sideways to a vertical compressive support element. Although beams may bend when loaded, a beam must not bend too much; just-right bending is good, too much bending is bad.
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. Fast forward a million or so years after prehistoric man crossed that river and we now have beams that can be pre-stressed so that there is no bending and thus no tension: behold the pre-tensioned, precast concrete beam. Technically, since a pre-stressed, pre-cast concrete horizontal member does no conventional beam deflection, there is no tension and thus it is not a true beam (think of it as pre-bent beam). Likewise, because each component is directly stressed in tension or compression, a truss is not a beam; think of a truss as a surrogate beam. Prehistoric man had no idea there was such a thing as geometry, let alone that you could use geometry to span a river. (More on pre-stressed, pre-cast concrete beams in upcoming articles.)
Consider the simply supported beam shown by Figure 1.
Long ago, Professor Brannigan explained that a simple beam is supported at two points near its ends. As shown in Figure 2, when a force (load) is applied, the simple unrestrained beam bends along its entire length into a smile shape. Because the ends of a simple beam are not restrained, they are free to rotate when the beam is loaded. This freedom of the beam ends to rotate allows the top of the beam to shorten (compression) and the bottom of the beam to lengthen (tension).