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Suppose the following beam situation within a mundane building (viewed from above). Assume vibration and torsion aren't issues. Suppose we have a choice to span a gap in 2 different ways:

enter image description here

(Grey = supporting walls, black = beam/s. Not to scale)

Clearly (B) isn't usually valid for prestressed precast lintels, unless the manufacturer advises otherwise, because of the risk of breaking over the midpoint, because the tension will be at the top, but the tensioning wires at the bottom. But if the beam is a steel profile, or perhaps timber, which handle both compression and tension fairly well, is there any common reason to prefer (A) to (B), or any matters to take into consideration before adopting (B)? Or any reason (B) would be weaker than (A) if it used the same profile beam/s?

Or is (B) always likely to be the obvious preference because it provides more continuity hence stiffness, has fewer connections hence freedoms/modes of failure, and is likely simpler and cheaper to fabricate?

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    $\begingroup$ The main reason for the adoption of the two simple spans solution is for the convenience of construction, as a result, it potentially cost less to build. Note that in a confined place, the long beam will be difficult to set in place. $\endgroup$
    – r13
    Apr 11, 2022 at 0:11
  • $\begingroup$ But no engineering based reason not to?? Just practical considerations of size, handling and installation then? $\endgroup$
    – Stilez
    Apr 11, 2022 at 0:50
  • $\begingroup$ Yes. I could,t think of any good reason from design/engineering point of view, $\endgroup$
    – r13
    Apr 11, 2022 at 1:23
  • $\begingroup$ Thanks,want to make that an answer? $\endgroup$
    – Stilez
    Apr 11, 2022 at 4:05
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    $\begingroup$ I think @TigerGuy has already said it. Thanks. $\endgroup$
    – r13
    Apr 11, 2022 at 14:11

2 Answers 2

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The stress-strain diagrams will be different, but not enought to drive the design. The bottom would have lower deflection because the beam must be constant through the center support. Compare two supported ends with 1 suported and 1 fixed in Engineering Toolbox.

The considerations would be primarily cost and installation-based. I believe you will find that the type of supports will dictate the usage. For an industrial or warehouse building where the roof is usually held up by bay poles I think you will find that the bottom option is by far the most common, due to the complexity of building separate supports like your top option. I think most common would be I beams for your up-down member and then trusses for the left-right supports. Plus, the electricians would hate you if they had to add bends in their conduit for the offset beams in the top option.

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Most times, the short single span beams as shown in A are butt-jointed on the pillars to look like your diagram in B.

This means that the construction is easy and as a pillar is installed the next beam can be put into place to help hold it.

There are times a longer "beam" is used, like overhead cranes between two rails one on each side of the building.

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