Braced Column? 1

[tclat]

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

 

[msquared48]

It looks like the slab/beam is integral with one of the long column edges.

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

 

[hokie66]

The columns are braced, as long as the slab and beam reinforcement is terminated in the columns per standard practice. Column C30 appears to have a band beam bracing it, while Column C50 would depend on the slab reinforcement extending through.

Why aren't the BM30's shown with hidden lines?

 

[tclat]

thanks Mike/Hokie

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.

 

[hokie66]

As I said, I consider the column braced...in both directions. You don't have to have contact with the short faces in order to brace the column in the strong direction.

 

[asixth]

I would say that the column is braced in both direction. I thought the ACI code gives specific criteria which limits the storey shear to drift ratios where a column can be considered braced or not.

Alternately, use a FE package to calculate the buckling effective lengths.

 

[PUEngineer]

The plan is a little hard to read without the appropriate lines being hidden, assuming the top of slab is flat. I know the question was only asked regarding column bracing. But column C50 looks a little suspicious if it's supporting the end of the BM30 that is running 'north-south'.

 

[tclat]

Thanks for all the replies.

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

 

[mijowe]

how many floors are you supporting on your 8" column?

 

[DST148]

@tclat: I agree with the posts above regarding bracing of the two columns on grid line "A".
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.

 

[hokie66]

tclat,

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.

 

[tclat]

Hi,

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.

 

[hokie66]

In that case, they are not beams. Your floor structure is a flat plate, and you should remove the "beam" lines to avoid confusion. A 250 flat plate may work with the short spans you have...you originally confused me by saying that the beams were to help with punching shear. As you haven't increased the depth, punching shear is not assisted. The limiting case may be shear at Column C30. Column C50 won't be heavily loaded, as Column C40 will pick up most of the load on that side.

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.

 

[ishvaaag]

With the moderate spans you have and residential loading, with 250 mm thick slab you shouldn't have any practical problem nor in deflection nor in punching loads. If some punching problem develop, embed an RC flat beam and make it appear in the model, the problem may magically disappear (it happens in CYPECAD models) because of the different treatment of reinforcement and structural scheme. Main concern should be with the thickness of the columns, disfavoured in practice for columns for above 3 decades now. Some resource to label the colums short walls to keep them at this low thickness.