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One can get values for load bearing of simple wood constructions and thus the right beam profile dimensions to choose from tables from wood supplies (e.g. http://www.merkleholz.de/en/Static-table-KVH-8082-192.html). Does it matter for such simple constructions in which sense to orientate the beam, i.e. does a 60mm x 100mm beam on two 100mm x 100mm posts support the same in this

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and in this

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orientation?

I read the wikipedia articles on beams and beam theorie and I guess it's in the equations, but I don't seem to figure it out.

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  • It's also in the real world. Find some oblong lumber a couple meters long and see if it bows more on flat or on edge. No offense but if you're going to be a structural engineer, best to try actually building things from time to time, it's a lot easier understanding theory when you know theory must explain what you've already observed!
    – Harper - Reinstate Monica
    Commented Apr 30, 2017 at 23:22
  • So when the table reads 60mm x 100mm, that means the 60mm is the horizontal direction...it's not written randomly...the first number is always the horizontal number...same with doors, windows, etc.
    – Lee Sam
    Commented May 1, 2017 at 1:06

2 Answers 2

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There are 3 factors to consider in the design of a beam:

  1. Section modulus, which is a factor of the "area" FROM the neutral axis (usually the centre of the beam). That is why a beam set in the vertical direction can support more than a beam set in the horizontal direction...because more of the area is further from the neutral axis.

  2. The extreme fibre in bending, which is the strength of the material of the beam (pine, fir, steel, etc.) from the neutral axis.

  3. Vertical shear, which is the area of the beam at the bearing point. This is important because if the area is too small, the beam will "crush" (compress) due to the load.

So, one of the above factors will "govern" in the design of each beam. When manufacturers of beams put "allowable" sizes of their beams in a table, they've calculated these factors into their tables. By the way, usually, 2. (Extreme Fiber in Bending) governs extra long spans and 3. (Vertical Shear) governs for short spans with extra heavy loads and 1. (Section Modulus) governs for normal residential construction with moderate spans and moderate loads.

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Absolutely. A beam oriented horizontally has a minor fraction of the rigidity and load-carrying ability as one oriented vertically. Beam strength is largely a function of vertical face height. Cross-sectional area is a minor factor.

To demonstrate this to yourself, try bending a sheet of paper laid flat on a table with respect to its width. It can't practically be done. Now try bending it with respect to its thickness. Floppy as can be, right? There's your answer.

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  • Thank you! I believe you, but the paper example is difficult because it has a ratio of thickness to width of > 100 and is extremely thin so that it's hard to make the connection to a beam. I'd change this to a 100 page book where the example applies as well and is easy to transform to a beam (very helpful :)).
    – Kalle Richter
    Commented Apr 30, 2017 at 22:27
  • I disagree. Actually, "cross-sectional area" has everything to do with the "strength " of a beam.
    – Lee Sam
    Commented Apr 30, 2017 at 22:46
  • @LeeSam but only with the logic of the argument, not the fact that beam orientation matters, right?
    – Kalle Richter
    Commented Apr 30, 2017 at 23:04
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    Yeah, let's keep the conversation in scope. Obviously a beam of 20x30 units is stronger than a beam of 2x3 units. However, withing a given cross-sectional area, face height is the critical variable.
    – isherwood
    Commented Apr 30, 2017 at 23:16
  • The 100-page book isn't a valid example because the pages can slide with respect to each other. The single sheet of paper is a near-perfect example of the phenomenon I describe.
    – isherwood
    Commented Apr 30, 2017 at 23:17

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