Tuesday, May 24th, 2016

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Structural Steel Rules of Thumb

by Avery Morris

Structural steel has been an integral part of the building industry since the development of the Bessemer process in the mid-1800s and its popularity as a construction material continues to be strong. Its strength, stiffness, and ductility make it an appealing choice for structural engineers, especially when challenged to span further while limiting member depths. When planning for a new building framed with structural steel, here are a few tips.

Beam Depths

For determining beam depths, a reasonable estimate is that the depth of the beams will be equal to the span divided by 24 (L/24). Another way to state it is ½” of depth is required for every additional foot of span. Most structural steel systems also have girders. A girder is just a type of beam that supports other beams and typically spans between columns. Since girders support more weight than beams, they are typically deeper. An approximate depth of L/15 can be assumed for girders.1 If limiting the overall depth of the structure is the goal, span the girders in the short direction and the beams in the long direction of a rectangular bay.

Deeper is Cheaper

You’ve likely heard this phrase repeatedly from your structural engineer. Of course, it’s somewhat of a myopic standpoint since deeper floor-to-floor heights can add cost elsewhere. However, if saving on structure cost is a goal, allowing more space for structure is the way to go. See the previous point for guidance.

Cantilevers

While some people may think structural engineers always recommend avoiding cantilevers, that’s not the case. In fact, cantilevers oftentimes make beams more efficient by reducing demands on the back-span (the portion of the beam that isn’t cantilevered). If a cantilever is desired, the key is to have a well-proportioned member which is typically a cantilever length equal to 1/3 the length of the back-span. Another key consideration is whether the cantilever results in costly moment connections (a splice connection with added plates, bolts and/or welds). Moment connections can be avoided if the beam is able to run over the top of a girder or column.

Repetition

Where possible, plan for a structure that includes a high degree of repetition. Repetition isn’t an engineer just being lazy; it’s a way he or she can save the project money. The repetition of steel beams and columns can save on detailing, fabrication, and erection. One simple way to do this is by providing equal grid spacing.

 

1Modern Steel Construction, February 2000.

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