Unbraced Length of Column 2

[bpstruct]

Having an issue analyzing an existing column as part of a moment frame. Eave height is 35 feet. Kicker angles from flange back to wind girts at 8 feet +/-. Can't justify this as bracing for the weak axis...no info on the wind girts. But the column is way off for the full height...design doesn't work for existing conditions, and the owner is wanting to add a jib crane. Plans showed that the column of interest was intended to be added on to in the future.

Any thoughts on that method of bracing the weak axis? I don't regularly design metal buildings, but I haven't noticed that kind of bracing before.

 

[JAE]

Metal buildings use those kickers as braces all the time. The girts, plus the diagonal kickers, provide for weak axis bracing of the columns.

The paper thin columns of PEMB's can't work without them.

 

[ToadJones]

not likely that a jib of any substantial size can be added to a PEMB.
You may consider adding bracing between columns or, better yet, a freestanding jib.

 

[bpstruct]

It's a fairly big column (W24x68). Was originally intended to be a center column (building was intended to expand). It was designed to support a bridge crane, too. But it was never installed and the building was never expanded. The jib crane is a 1-ton capacity and 23-foot max arm. My analysis indicates that it will work assuming that the kickers continue to work and brace the weak axis against column buckling. Mu isn't even close to phiMn...so that shouldn't be an issue either.

Thanks for the responses.

 

[ToadJones]

I'd suggest adding bracing between your column and one on either side, if they indeed exist....especially if the jib rotates.

 

[bpstruct]

What would be the purpose of the bracing? Adding buckling restraint? The way the jib is supported, it seems like all I'm adding in the grand scheme of things is more moment...which still isn't close to strength. I may be missing something though.

 

[hokie66]

Agree with JAE that the kickers to the girts, along with the girts themselves, will brace the column. I call those kickers "fly braces".

 

[ToadJones]

The point of the bracing?
It would be for bracing the column weak-axis as you were describing in the OP. If you were not confident in the kickers.
I have heard many debates on whether or not to count on those kickers for bracing, but one would assume that is why they are there.

 

[bpstruct]

Thanks again for all the replies.

 

[csd72]

If you dont usually design metal buildings then a jib crane can be a more demanding design issue than it may seem at first.

There are not just the vertical loads to consider, there are allso the accidental loads from sideways movement or out of straightness.

There are also probably deflection criteria.