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Which is better to use in contructing ceiling frame? RHS or SHS? And why?

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    $\begingroup$ This doesn't seem like something that would have a blanket answer. I'm not a structural engineer or interior designer; but if you mean some sort of structural ceiling frame, I don't think we can easily answer that. Generally structural elements have codes and regulations they have to comply with. $\endgroup$ – JMac Jan 5 '17 at 11:49
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Obviously this depends on the relative size of the sections.

Assuming that the rectangular section's smallest dimension is equal to (or greater than) the square section's side and their thicknesses are equal, then the rectangular section will always be stronger than the square, for any load in any orientation. This is because the rectangular section's geometric properties (namely area and moment of inertia) will always be greater.

For the same reasons, if the rectangular section's largest dimension is equal to (or lesser than) the square section's side, then the square section will always be stronger than the rectangular section for any load in any orientation.

If, however, the rectangular section's smallest dimension is lesser than the square section's side but it's largest dimension is greater, then you need to calculate the section properties and see which is best:

$$\begin{align} A &= BH - bh \\ I &= \dfrac{BH^3 - bh^3}{12} \end{align}$$

where $B$ and $H$ are the section's width and height, respectively, and $b$ and $h$ are the hole's (so $b = B - 2t$, where $t$ is the section's thickness). The moment of inertia also depends on the rectangular section's orientation ("standing up" means $H>B$, "sideways" means $B>H$).

It's possible to get dimensions where the rectangular section has a greater moment of inertia but smaller area or vice-versa. In these cases, the best section also depends on the expected loading: moment of inertia is important for bending and torsion, while area is important for normal and shear loading.

Also, there may be case-specific height restrictions or architectural/aesthetic reasons to choose one over the other, but that's obviously not in the scope of this answer.

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    $\begingroup$ Sorry, but I think you've missed the key point. The choice between a specific RHS and a specific SHS depends on their relative sizes, but that's the same as the choice between a small RHS and a big RHS. The question is "in what circumstances would you choose RHS, and in which ones SHS"; for which the answer is whether you need significantly greater capacities in one axis compared to the other (i.e., my answer.) $\endgroup$ – AndyT Jan 6 '17 at 9:16
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    $\begingroup$ I have to agree with @AndyT. This seems less like "why would I chose one over the other" and more general mechanics of RHS and SHS beams (and just beams in general). You can definitely infer from this answer why you would choose one over the other; but I have to say AndyT's answer seems to answer it directly and in a more concise matter. A lot of this has nothing to do with choosing RHS over SHS. $\endgroup$ – JMac Jan 6 '17 at 13:35
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Use an RHS over an SHS when you want significantly more bending/shear capacity about/in one axis than the other.

The additional depth that an RHS has in one axis gives it greater shear capacity in this axis, and greater moment capacity about the perpendicular axis.

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  • $\begingroup$ Okay, this is a pretty good blanket answer; but it's also terribly dry (as it should be for this type of question). $\endgroup$ – JMac Jan 5 '17 at 11:56
  • $\begingroup$ @JMac - I was very tempted just to leave it as a comment. But we don't seem to do well enough on this site at posting actual answers, so decided in the end to leave it as one. I've now fleshed it out a tiny bit more. $\endgroup$ – AndyT Jan 5 '17 at 11:59

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