Multi-storey steel column bracing

[EngDM]

Hey all,

I am sizing a steel column assembly that requires me to upsize my column at the lowest floor. Here is the situation:

2nd floor, column needs to fit within the wall assembly so it's size is restricted. At the lowest floor, the framing changes and this column now picks up a lot more load, and I need to increase it's size (the biggest column that would fit in my wall assembly above does not work at the lowest level).

For arguments sake, suppose we are going from a 5x5 HSS to a 6x6. I only have steel beams tying in in one plane at the 2nd floor, which is what I am relying on as bracing in that plane. In the out of plane direction, there are joists that frame and sit onto the aforementioned beams, so the bracing out of plane could be provided either by a wood tie joist, or by way of the joists sitting on the beam and sheathing fastened to the nailer, but this would mean the bracing load path would go thru the otherwise weak (in the out of plane direction) shear connection from beam to column.

If I am switching beam sizes, I assume it will be via some sort of end plate, or direct weld connection as opposed to sitting the upper column on the 2nd floor beam, and adding stiffener plates within the beam web to then bear on the column below. When I model these columns as seperate pieces in RISA, the unbraced length of the columns in the out of plane direction is limited to the length of the member since they are drawn seperately. My questions are:

• Should I be taking the unbraced length of the lower 6x6 column, as the full height of both columns together when sizing, since they are not effectively braced out of plane at the joint?
• Am I required to brace the transition point from 5x5 to 6x6? If I don't do this, is there a requirement for me to specify a full moment connection to develop the column (these columns are not apart of my lateral restraint system, but will go along for the ride during any building sway)?

I appreciate any insight y'all might have.

 

[JAE]

At the second floor don't you have a floor diaphragm that effectively braces the columns every which way?

 

[EngDM]

At the second floor don't you have a floor diaphragm that effectively braces the columns every which way?

Yes, but doesn't connect directly to the column. The diaphragm would go to the nailers, then to the beam. So the bracing would come from the connection of beam to column.

 

[JAE]

Wouldn't that be more than adequate?

 

[Retrograde]

opposed to sitting the upper column on the 2nd floor beam, and adding stiffener plates within the beam web to then bear on the column below.

That is probably how I would frame it out.

 

[EngDM]

Wouldn't that be more than adequate?

It may be, but in this case do I need to specify the lateral out of plane loading to the connections designer to ensure that the load path has enough capacity to get it into the sheathing?

 

[TRAK.Structural]

What will the connection look like where you change column sizes? Not sure what type of structure this is, but I've typically seen the column splices placed a few feet above the floor level.

If you think the out of plane shear in the beam/col shear connections is too much for that connection to handle than you could just detail a discreet brace that attaches directly to the diaphragm for the bracing direction of concern.

 

[JAE]

It may be, but in this case do I need to specify the lateral out of plane loading to the connections designer to ensure that the load path has enough capacity to get it into the sheathing?

I would certainly think so - as you stated the load path goes through the main beam connections into the column in a horizontal direction. Typical single plate shear connections wouldn't be capable, generally, of resisting any specific loads (i.e. wind shear) but simply lateral bracing you could check (against perhaps 2% of axial).