Behold the Beam

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...


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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Think of cantilever beam action as that of a first-class lever (like a pry bar) with one fixed end.

As with all structural systems, the connection is the weak link. True single-span cantilever beams are rare in building construction.

5. Overhang beam — Overhang beam reaction produces both simple and a cantilever beam action (or continuous and cantilever beam action). Between supports you've got simple beam behavior (or continuous). Instead of terminating at the supports, the beam extends over and beyond the supports — overhangs — thus becoming a cantilever. As the beam extends beyond the support, simple beam deflection is reversed and cantilever beam action is generated. A good example of an overhang beam is a diving board (photo 4).

When loaded, a single-span simple beam and a single-span cantilever beam deflect into a single curvature shape between their supports. The simple beam curve exhibits sag between the supports (the smile shape); the cantilever beam sags at the unsupported end. An overhang beam has at least two curvatures. (If there is overhang beyond both supports, there will be a cantilever on both ends.) The first is the simple-beam sag between the supports; the second is the cantilever sag at the unsupported end.

Overhang cantilevers (Photo 5) are much more common in building construction than true single-span, single-connection cantilevers. A "double-overhang" beam is simply a beam with overhangs at both ends, such as a classroom tabletop that extends at both ends beyond the legs. (If the tabletop terminates at the legs, it is a simple beam.)

6. Suspended beam — At first glance, a suspended beam looks like a cantilever: a single-span with one end supported and restrained. The key difference is at the unsupported end. Rather than hang free like a cantilever, it is supported by a member in tension. This member is often a hot-rolled steel rod or a cold-drawn steel cable.

Suspension systems are the reverse of the traditional structural hierarchy that includes beams, columns, and bearing walls. The traditional structural hierarchy sends load sideways and down. A suspension system thumbs its nose at gravity by sending the load upward. However, as the saying goes, what goes up must come down. Recall that all dead load and live load must eventually arrive at the earth as compression. At some point, the suspended load going upward must turn sideways (usually at a girder or purlin) and be sent down through a compressive member (column or bearing wall).

Do not confuse a suspended beam with the so-called suspended span. A suspended span is a variant of a cantilever bridge. With this bridge system, simple cantilever spans are formed by two cantilever arms extending from opposite sides of the span to be crossed, such as a river. Because the cantilever arms do not meet in the center, they support a mid-span truss section that is "suspended" by a connection at each end of the cantilever arms; thus, the middle span completes the main span. In this case, the tension travels sideways to the cantilever arms. (Note that the combination of anchor arm and cantilever arm exhibits characteristics similar to the overhang discussed previously.)

There are two important strategic considerations for structural suspension systems: their dependence on tension and their lack of mass. Size, rigidity and mass are required to support a given load in compression. The same load supported entirely in tension can be transferred through a component that is slender, flexible and comprised of much less mass (material). Less mass means less resistance to heat — not just fire, but heat.

Pure compression structural components send their load downward and pure tension components send their load upward (to other components that eventually send the load sideways and down). Up is also where the heat from a fire goes. Tensile structural components are the only structural systems that send load up before sending the load down to the earth.