Split beam

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    Jenny Guzman

    I think how the beam reacts on forces pushing down on it (or even from the sides) at the middle support would be different than the two separate beams. You might get two different moment/overturning reactions. I hope someone does give you a more informed answer as this is an interesting question.

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    Michael Ermann

    Let's answer your question with a practice question. . . 

     

    Because of a local shortage of large delivery trucks, the steel fabricator would like to swap a floor beam that spans two bays with a pair of beams that each span one bay. She calls the general contractor offering a shorter lead time for the swap, and the general contractor, in turn, writes an RFI for the architect proposing the switch. The span length between supports will not change nor will the loading on the beam. Which best describes the state of this request?

     

    It can be approved with a beam weaker than W200 x 31 beam

    It should be approved with a W200 x 31 beam

    It must be approved with a beam stronger than a W200 x 31 beam

     Scroll down for the answer

     

     

     

     

     

     

     

     

     

     

     

     

     

    Answer: It must be approved with a beam heavier or stronger than a W200 x 31 beam

    Explanation:

    A W200 x 31 beam describes a wide-flange beam, 31 inches deep, weighing 200 pounds per linear foot. Deeper or heavier beams can span farther unsupported. It’s generally the length of the span and the thickness of the plates of steel in the wide flange that determine the beam’s strength, so a W310 x 31 weighs more, at 310 pounds per linear foot instead of 200 pounds per linear feet—and can span farther or support a heavier load—but remains 31 inches deep. And a W200 x 21 beam weighs the same per linear foot, but because it is shallower in its depth dimension, it will support less weight or span a shorter distance. The weight per linear foot serves as a proxy for the thickness of the steel plates.

    But the configuration of the structural system also has a meaningful effect on what a beam can carry, where a single long beam supported at midspan can support more load than were the same beam cut in half and reconfigured as two separate simply-supported beams.

    Picture three sawhorses, each spaced 10 feet apart in a line. Now picture a single continuous 1x8, laying flat across all three of them. The top of the board at the middle sawhorse will be in tension, reducing the maximum stress on the board at the midspan point between supports. In other words, relative to two boards, each spanning between two of the sawhorses, the one-long-continuous board would sag less.

     

    The maximum moment (arrows on the diagram) controls what the beam can support. --Michael Ermann, Amber Book creator

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    Jenny Guzman

    Thank you!

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