COLUMN SIZE INCREASE 5

[WiQi]

Hi,

RC column of size 18"x18" column height 11' have to be increased to size 30"x30"

how can we do this? building is under construction just columns are constructed upto 11' height on a raft 24" , design revision says column should be increased. code followed UBC 97, seismic zone 3. two more stories will be raised on these columns.

Regards

 

[Once20036]

How far has construction progressed?
I assume that you have two way concrete floor slabs/no beams at columns. Have they cast the first slab over the column(s) in question yet?

 

[KootK]

With an increase of 6" all around, you might be able to pour a new, reinforced, rectangular tube of concrete around the existing columns. You'll probably want to give some thought as to how to make the new and old concrete act together compositely. If the slab above has not yet been cast, then punching shear shouldn't be a problem at all which is good. Can I ask what led to such a drastic increase in column size?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[JAE]

If it is one column I'd be inclined to rip it out and build the 30" column new.

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[WiQi]

@once20036 just columns are cast just one story height of 11', beams and slab will be cast on the increased column sections.
@view in plan,building is irregular triangular with 50' span in center and than decreasing on both sides ending with base of 24' x 111' rectangle. I couldn't upload here, i d try uploading plan later. columns supporting beams 24 x 36 and beam column capacity ration exceeds so to increase the column sections. any idea about making old and new concrete behave together.

thanks all

 

[CELinOttawa]

I have done this similar to treating the condition as a later repair. Effectively you chip off the cover to a minimum of 3/8" roughness, lay and tie bars, and simply cast a new column around the old.

Often you can even consider the existing bars for shear transfer, if your finished column can be considered pin-pin.

Note that on the three occasions I've detailed this, only one was built. Once the builder sees the level of work, it is more likely that JAE's solution is going to be selected.

 

[KootK]

Unless you can position all of your new column verticals near the corners, you'll have to dowel new hooked bars into the existing column in order to provide buckling restraint to the new column verticals. It's probably a good argument for using fewer, larger verticals and greater than normal cover. The buckling restraint dowels would rightly need to be in addition to any dowels that you'd require for shear friction.

This all tips the scale further in favour of demolition.

The new ring of concrete that you're considering casting will likely be doing the bulk of the column's work when it's all said and done, particularly when it comes to satisfying the intent of the column/beam ratio. Perhaps the inner core of existing concrete could be ignored altogether.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[RHTPE]

If the beam & slab supported by this column have not yet been cast, I would recommend demolishing the smaller column and constructing the proper size column. The preparation necessary to "wrap" the existing column, combined with the cost of additional rebar & formwork, combined with the time needed to accomplish all of this, will undoubtedly be more than removing the smaller column. Just be sure to preserve the existing raft-to-column dowels in the process. You will no doubt need to increase the size/number of dowels coming out of the raft.


Ralph
Structures Consulting
Northeast USA

 

[WiQi]

it is going to be increasing the sizes of smaller columns, now how the extra bars will go into the raft. bars are of #6, raft has 3000psi concrete strength, 15" anchoring is proposed into the raft with SPIT EPOBAR, this anchoring length is taken procedure given in the SPIT fixing design guide. new bars will tied with the older bars by ties hooked around both bars, new bars will have ties #3@4"c/c.

 

[CELinOttawa]

Well now you've got a new problem: Verifying that you meet your Fire Code requirements.

If you are using vinyl epoxy, you will need to ensure you provide fire protection (and no, just encasing in concrete at typical clear cover does not suffice from what I've heard).

Think about it: If you place your bar correctly at the corner, the epoxy extends out from this and can melt/break down during a fire. Some do have reasonable fire tolerance, but check the product carefully - Better yet, call the manufacturer for support.

 

[WiQi]

hmmmmmmmm bars will be grouted into the raft and than flooring layer of about 3"-4" PCC and 1"-1-1/2" marbles will cover the holes from above I guess fire will not be the problem.

 

[hokie66]

Are the bars required in compression? If the concrete carries the load, you may not need additional dowels, or only a nominal amount. I don't see that setting these starter bars in epoxy would be a fire rating issue. Among other reasons, they are at the bottom of the column, while fire temperatures at the top are normally controlling.

 

[CELinOttawa]

Would you not require the dowels at the top? If they are not needed for bending, they aren't needed at the top nor bottom....

 

[hokie66]

The floor above is not cast, so he can continue the bars as required at that level.

 

[WiQi]

there is one column of 18x18 which has 6#8 longitudinal bars but ties are not sufficient as spacing b/w long bars exceeds 6" limit so the new increased sized column 30x30 should have lateral ties but do we drill them across the section or do something else? I have suggested drilling and making holes through the column to the opposite face.

 

[KootK]

This is what I was getting at in my last post. You should only need to develop <5% of the axial capacity of the rebar so partial depth hooked dowels should be adequate .

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[CELinOttawa]

Ah, missed it Hokie... My mistake; A star for you good Sir.

 

[WiQi]

client needs some other options than increasing the sections, could we have some other options to satisfy the 6/5 beam column capacity ratio?

 

[KootK]

Weaker beams? Just kidding, that was mean. Perhaps you could demo the columns and replace them with new concrete columns that are heavily reinforced and compositely encase wide flange sections inside.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[WiQi]

thanx KootK
actually they have also cast a reinforced basement wall all around the columns periphery when design was revised it was failing in beam capacity ratio I suggested increasing column sizes but contractor have to do lot of chipping and drilling, and anchoring chemical so the need more options. i have all span varying like structure is a triangular in about both sides of the central span. central span is 49' than on both sides spans are 41' than 35',27'.columns on central span are 24x24 with 24x36 beams,41' span has columns 24x24 with 24x36 beams than comes 35'span it has 18x24 beams, 27' span also have 18x24 beams.