Cantilevered Beam Supporting Column Has No Torsion?

flashyyc15 · May 4, 2023

Hi all,

I'd just like to get your thoughts about the structural framing shown below. Some information:
[ul]
[li]W-Beam at roof level is cantilevered over HSS column[/li]
[li]HSS column is in turn supported by another W-beam at second floor that happens to also be cantilevered over an HSS [/li]
[/ul]

For my questions:
[ul]
[li]Will the beam at the second floor take any torsion at all based because of the unbalanced moments at the top of the HSS column? The roof beam cantilevers over the column and has a very long back span.[/li]
[li]I was told by a colleague that we can get rid of torsion by either: 1) stiffness or 2) deliberate detailing of the connections. Can anyone provide thoughts on what this means?[/li]
[/ul]

Any thoughts/opinions will be very much appreciated.

Thank you.

1503-44 · May 4, 2023

There would be no primary torsional forces, however secondary torsional moments may result, if the column is permitted to bend and how that might be limited, or not, by sway bracing.

If there is torsion, detailing will not eliminate it, but the addition of stiffener plates would spread that across the entire section, rather than twisting only the top flange.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

BAretired · May 4, 2023

flashty15 said:
For my questions:

1. Will the beam at the second floor take any torsion at all based because of the unbalanced moments at the top of the HSS column? The roof beam cantilevers over the column and has a very long back span.
2. I was told by a colleague that we can get rid of torsion by either: 1) stiffness or 2) deliberate detailing of the connections. Can anyone provide thoughts on what this means?

1. Sure it will! If the top of the HSS column rotates, the bottom will rotate nearly half as much. But the torsion in the 2nd floor beam can be ignored, because it is not required structurally. The torsional moment may be taken as zero.

2. If a beam is loaded in torsion, you may be able to reinforce to accommodate it, but you can't get rid of it unless you remove the torsion.

flashyyc15 · May 4, 2023

Thanks for the thoughts so far fellas...

BAretired said:
1. Sure it will! If the top of the HSS column rotates, the bottom will rotate nearly half as much. But the torsion in the 2nd floor beam can be ignored, because it is not required structurally. The torsional moment may be taken as zero.

2. If a beam is loaded in torsion, you may be able to reinforce to accommodate it, but you can't get rid of it unless you remove the torsion.

If the beam is taking torsion as you say in #1, could you explain why it can be ignored and is not required structurally? Doesn't the torsion need to be resolved somehow by the torsional resistance of the supporting beam either on its own or with the help of stiffeners &/or other members?

HTURKAK · May 5, 2023

1- The beam at 2nd floor will take negligible torsion and for modelling , the conn. shall be assigned pin for transversal direction of the beam. In order to mobilize the full torsional strength of the beam, web stiffeners shall be used.

2- Or do nothing.. since the small rotation of the top flange will balance .. and IMO , it is not reasonable to use very flexible HSS section or use real hinge to ged rid of the torsion..

My opinion ...

I cannot give you the formula for success, but I can give you the formula for failure..It is: Try to please everybody.

BAretired · May 5, 2023

flashy c15 said:
If the beam is taking torsion as you say in #1, could you explain why it can be ignored and is not required structurally? Doesn't the torsion need to be resolved somehow by the torsional resistance of the supporting beam either on its own or with the help of stiffeners &/or other members?

It is not necessary to resolve torsional moment if a slight twisting of the beam is acceptable. Consider two examples:

1. A beam is loaded eccentrically, the beam rotates slightly, but other framing is engaged to resist the moment.

2. A beam is loaded eccentrically, the beam starts to rotate, but there is no framing to resist the torsion, so the beam continues to rotate until it collapses.

In your case, there is a slight rotation, but the column resists further rotation, so stability is preserved.

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