Odd Column Framing

And this column does not continue up to the roof or anything like that?If the column is braced at the top and the bottom without that strut, and works with an unbraced length in the strong direction equal to the full height then I don't see the brace necessary.

If it does not continue up to another lateral bracing point, then I don't see an issue with your proposal.

What's the lateral system? Where does that brace load go when it hits the W24x55?

Thank you for the reply.

This is a new mezzanine inside of an existing building, so it the column stops at the new mezzanine elevation. LFRS is a system of W18 columns with moment connected steel joists in every bay. The thought process has the two structures not touching each other so we don't have to bring the existing structure up to current code requirements.

Seems to me like there would be a torsion issue, (even though there is no real bending on the column).

If you connect the HSS brace through the centroid of the column and beam, then that would eliminate the torsional aspect, like a truss. Would put some minor tension/compression into that beam, but I can't imagine it being too drastic that you couldn't handle it.

Make sure wherever the brace connects at the other end there's a way for the load to get into the lateral system. Whether that be plan braces, or some bridging/blocking to get it into the diaphragm.

Otherwise I don't see an issue here. I've done similar things to stiffen up wobbly gantry frames.

We are proposing to to brace the column back to the new mezzanine with a HSS "strut".

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The thought process has the two structures not touching each other so we don't have to bring the existing structure up to current code requirements.

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If the WF column is existing, I see conflict in above statements. You should have your own framing columns for the mezzanine, and placed with consideration of future expansion. I don't understand the need to tie into this column.

This column is not existing. All framing shown on the plan is new. Existing framing is not shown.

Can you brace the mezz any other way? can floor be used as a diaphragm? to transfer loads to other bracing?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

If this is the new column, then if lateral stability is a concern, you maybe better off to provide vertical bracing, or horizontal braces below the mezzanine floor.

If I'm understanding the intent of the strut correctly, it's to prevent having to treat that column as cantilevered off the floor. Therefore he's just locaikng at providing a lateral restraint to the top of the column. Vertical bracing below the beam will do nothing for him. And horizontal braces would be in place of any diaphragm action.

SteelPE said:

Seems to me like there would be a torsion issue, (even though there is no real bending on the column).

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Is the source of your concern:

1) the torsion that the brace would induce in the column when the floor deck attempts to move laterally and take the brace with it, or;

2) providing torsional restraint to the top of the column to prevent torsional buckling?

Either way, see the sketch below for a proposed setup that, in my mind, would rectify both issues. I don't perfectly trust "concentric" connections to behave perfectly concentrically in situations like this.

If pushed, I think that you could even omit the brace and, rather, get you strong axis column bracing from the beam cantilevering out weak axis from the floor deck. If not pushed, however, I'd do exactly as you've done as it's more robust and less computationally intensive. As shown in the sketch, torsional bracing at the top of the column would again come from an end plate or double angle connection between the beam and column.

jayrod,

I see.

The column is tied to the interior column by floor beam in the E-W direction, if the stability concern is the N-S direction, why not to add a beam in that direction, which is needed for later expansion anyway. I wouldn't consider the inclined strut is an unquestionable brace for this column. Also in question is how this column is connected on top, free, or to the upper framing?

Lost track of this thread, sorry,

"Can we brace the mezzanine any other way". We are not using the column to brace the column but rather we are using the mezzanine to provide stability to the column in the strong axis. We are using a system of brace frames in the E-W direction and moment frames in the N-S direction to resist lateral loads. Eventually, the mezzanine will be expanded and this column will become part of the lateral force resisting system for the mezzanine.

The mezzanine has a concrete floor, so we plan on using the floor as a diaphragm to transfer only loads to the lateral force resisting system.

The column runs from the slab on grade to the mezzanine.... not up to the roof

"Why can't we just add a beam in the N/S direction".... the column opposite this column is 70 +/- feet away, and has the same strut configuration. Due to constriction issues, the interior columns do not align with the exterior columns. So rather than fabricate and install a 70' beam for column bracing concerns, we decided to use this strut instead.

Are we being "pushed by the client or anyone". At this point, no but I imagine that will come once they install the strut.

So I assume that column is being used as part of a pin-pin moment frame in the final configuration once the mezzanine is expanded?

Is there the possiblity of considering it fixed base? If so, I can't imagine the strong axis buckling at this construction stage being overly onerous. It's only seeing 30%+\- of the axial load it would in that scenario.

Well, the columns were designed with partial fixity (I think we used 10% of a fully fixed condition), so, no the base of the column can not be completely fixed. That being said, I am sure we have more fixity than what we assumed as we have W18 columns (to use in the future moment frame) which I am sure the base condition will be more fixed than we assumed.

I would likely be looking at the current baseplate and anchorage configuration to determine how much fixity you should realistically expect from it and see if that would meet your needs to brace the column out of plane

How do the moment frames in the N-S direction work? It looks like one end of the beam has a rigid connection to the column, but the other end has a pin connection to the girder. Is this what an elevation looks like?

Chris,

Yes, something similar to what you are showing. We had originally talked to the joist manufacturer about making a moment connection through the girder. They said that was OK, but upon further analysis that wasn't necessary, so we just ended up with a pinned connection at the girder.

It's not the best solution, but we were told where the column could go in the building. We had to work around existing conditions/structure. What you see a small portion of is what we came up with to solve the problem. Is it the solution I wanted..... no, does the solution work..... it should.

It's an interesting solution - I haven't come across something like that before. Not to get too sidetracked, but did you use R for a steel OMF, steel cantilevered column, something else?

As far as bracing the column, I think I like KootK's suggestion. You wouldn't really be able to connect the brace concentrically to the column unless it was attached to the very end of the flange. Plus, by connecting the brace to the beam, they won't have to remove it as cleanly when they install the rest of the mezzanine.

Kootk said:

If not pushed, however, I'd do exactly as you've done as it's more robust and less computationally intensive. As shown in the sketch, torsional bracing at the top of the column would again come from an end plate or double angle connection between the beam and column.

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I agree with this.

like has been said I consider carefully about what assumptions are being made about the column's lateral and twist restraints. It 'looks' OK. But the degree but the degree that it is laterally restrained in the 'north-south' direction is somewhat questionable. Likewise for twist restraint. Though if your column is being designed for future expansion the upsize column will help things.