Maintaining Column Stability
Maintaining Column Stability
(OP)
I am working on demo plans for an existing structure. There is an interior mezzanine that the building owner would like to remove. The demo is simple for the most part, but I do have a few areas of concern. There are several locations where a continuous steel beam spans across columns running from the foundation level up to the mezz level. Directly above some of the foundation level columns are columns as well that bear on the continuous steel beam and go up to support the main roof structure above. So that the contract does not have to shore up and remove the columns above the continuous beam, I'm thinking about having the contractor cut the continuous steel beam (supports mezz only) on each side of the column and leave the beam section between the existing two columns providing some type of column splice detail that can be fabricated in the field. The columns are gravity only columns and are not part of the lateral system. Has anyone done anything like this in the past, if so, would you be willing to share a detail or describe the column splice detail?
The columns and beams are both W sections.
The columns and beams are both W sections.






RE: Maintaining Column Stability
assuming of course that you're confident your new KL/r will work,
there are two items you might want to get an evaluation on prior to fully designing your retrofitted column.. 1) does the contractor prefer this method, and 2) is the owner happy with the appearance. You could achieve this by sketching a rough and overstated idea of the connection for their consideration.
RE: Maintaining Column Stability
Currently there isn't a demo contractor on board, as the CDs will be sent out to bid once completed. If I were the contractor, I think I would prefer keeping the existing columns in place vs the option of shoring up 30+ feet of roof above, removing two columns and trying to retrofit a new steel section in place.
The final appearance is not an issue since the project architect plans to wrap all the interior columns within the space once the mezz has been demoed out.
RE: Maintaining Column Stability
RE: Maintaining Column Stability
BA
RE: Maintaining Column Stability
This is what is confusing me. I have no idea how to create an unbraced point that acts like a braced point. And if it's unbraced I think your L is from grade to roof. Your profile indicates registration in multiple states, so quite likely I may be misunderstanding what you're saying. Perhaps a sketch? Others here are brighter than me.
I am a contractor in addition to a registered engineer, and honestly, I think I'd rather pull the two columns and install a new one. But that's just me and, again, I think I may have a confused idea of what you're trying to do.
RE: Maintaining Column Stability
Triangled, I agree in that I would prefer to install a new column vs retrofitting the existing, but I'm not sure that's an option with the existing conditions. The new column would need to be 30+ feet. The only way in the building is a double door, so no room for large pieces of equipment for lifting into place. Also the owner would prefer not to have to shore up the roof framing above for the sake of three columns. The remainder of the mezz is free standing, so it can be removed with minimal impact to the rest of the structure. Just trying to think outside the box on this one vs a more traditional approach.
RE: Maintaining Column Stability
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin
RE: Maintaining Column Stability
You do not show the cutting of the beam and you don't show any new bracing at mezzanine level. If the beam is cut each side of the column, the effective length for both major and minor axis of the column will be the height from foundation to roof. Make sure the column is adequate to carry roof load over a two story height. If not, you may have to plate the column each side to improve its radius of gyration.
BA
RE: Maintaining Column Stability
In any case, what I would start looking at is splicing on a pair of large channels on either side (near and far sides in your sketch) that would lap the existing columns above and below some distance - say 3 feet each way....maybe much longer if needed for hour new KL/r values. The channels could be welded onto the existing column flange tips to create a sort of box shape with extended channel flanges pointing outward. These could possibly be installed prior to any demolition other than some holes in the mezzanine floor.
I would prior to this also check on the ACTUAL alignment of the upper and lower columns - they very well could be mis-aligned a bit in which case you would have some possibly serious out-of-plane second order effects developing.
RE: Maintaining Column Stability
Otherwise I agree with the last couple of posts about basically creating a moment connection to take care of the stability issue.
RE: Maintaining Column Stability
I can't think of any mode in the existing structure where column buckling would occur at that base plate location. Therefore I don't think you can make any deduction from the shear strength of those base plate bolts that leads to your solution. The bolts restrain the base plate because they're connected to a rigid floor which is presumably rigidly connected to the planet, although I think you mentioned that the continuous beam is not a part of the lateral system and that the beam is part of a free standing mezzanine. There must be some lateral stability to that mezzanine.......
The bolts at the beam line which are a pin connection for the bottom of the second floor column and the top of the first floor column, become a hinge when the remainder of the second floor is removed. The task then becomes to unhinge the hinge.
I could imagine the rx and ry of the first floor column, having been designed to support floor plus roof loads, could be sufficient to support a roof load only with the L increased. If the rx and ry of both story columns are happily both the same and also sufficient to support the roof load, then I would see the solution as being along the lines of creating continuity across the floor beam section as BA is saying.
If rx and ry are different between floors or insufficient at the revised two story L, then you've got a column with varying KL/r along the length, and things are getting tricky. You'll be beefing up the section as described above... "some distance - say 3 feet each way....maybe much longer if needed for hour new KL/r values. " That "some distance" item..... I'm not sure how to calc that. I'd love to hear a remark on that item.