Braced Column?
Braced Column?
(OP)
Hi,
I have a situation where the edge of a concrete slab is offset from the columns. Would you consider the concrete columns as braced effectively at the slab level? I'm considering embedding some beams within the thicknesss of the slab to help with punching shear but I'm also concerned about the effective length. Would these beams help brace the columns since they are only 8" away?
Please see attachment for clarification. Columns in question are on grid A and are 8"x24" and 8"x36"
Thanks
I have a situation where the edge of a concrete slab is offset from the columns. Would you consider the concrete columns as braced effectively at the slab level? I'm considering embedding some beams within the thicknesss of the slab to help with punching shear but I'm also concerned about the effective length. Would these beams help brace the columns since they are only 8" away?
Please see attachment for clarification. Columns in question are on grid A and are 8"x24" and 8"x36"
Thanks






RE: Braced Column?
If that's the case, then I would say yes in the weak direction. Possibly in the strong direction depending on the connection, if any, of the column to the slab/beam.
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Braced Column?
Why aren't the BM30's shown with hidden lines?
RE: Braced Column?
I'm pretty comfortable about the column being braced in the weak direction. There is no beam or slab framing into the column in the strong direction so based on your explanation the column is unbraced in that direction.
Since the column is not braced in the strong direction, is the effective length factor "k" equal to 2 as in steel members with a fixed base? I can't seem to find information for k for concrete members which are not restrained at one end.
The building is braced by shearwalls therefore nonsway. If k=2, the weak axis will govern since the moment about the strong axis is small and the radius of gyration about the strong axis is more than double that of the weak axis.
Alternatively, can I run bars diagonally from the slab in the column and consider it braced? This will also help cracks at the "re-entrant" corner.
Will fix the drafting error Hokie.
RE: Braced Column?
RE: Braced Column?
Alternately, use a FE package to calculate the buckling effective lengths.
RE: Braced Column?
RE: Braced Column?
Sorry I did not have the plot styles on my pc when I created the pdf.
I have cleaned up the drawing to make it easier to read.
The green line represents the extents of the slab. The dashed lines are beams embedded in the slab. I put those in tentatively to help brace the columns in the strong direction and to help with punching shear. I'm still working through the calcs.
thanks
RE: Braced Column?
RE: Braced Column?
Although not directly related to your OP........Column C30 is shown as 8" x 24". As per IBC-2003, the minimum dimension of the column for 2 hr. FRR using siliceous aggregates is 10"(I am making some assumptions here based on the conditions we normally encounter in concrete buildings). The minimum dimension is permitted to be reduced to 8 inches for rectangular columns with two parallel sides at least 36 inches in length. However, one can get away with this requirement assuming the column is located on the exterior of the building and thus likely to be exposed to fire from one side only.
8" thickness for walls is not very unusual since the verticals can be staggered. But keeping the verticals aligned in a 8" wide column may cause conjestion and other problems. More over, it is very difficult to anchor the beam bars with standard hooks in a 8" wide column. For column C30, go for a larger width if you can.
RE: Braced Column?
What do you mean by "beams embedded in the slab"? How deep are they? I assume they are band beams, not just to "help brace the columns...".
Agree with others...I don't like 8" columns.
RE: Braced Column?
Answers to questions.
mijowe: The column will be supporting two floors so similar size and orientation.
dst148: The structure is residential so I believe 1hr fire rating is sufficient. I take your point about congestion but since the loads are relatively low (column only supporting 2 floors) I am not expecting that I would need that much reinforcement where congestion would be an issue. Will review however. Thanks
hokie66: By embedded, I mean the beams are only as thick as the slab (10"). Architect wants a flat concrete soffit.
PEeingineer: I put the beam in as part of the original post where I was questioning if the beams helped brace the column in the strong direction. Based on most of the comments, it is not required.
RE: Braced Column?
The architect wants a flat soffit...watch out that he also wants the top to be flat. When he throws in a step or setdown, your whole scheme will be ruined.
RE: Braced Column?