Omitting SOG Diamonds at Columns
Omitting SOG Diamonds at Columns
(OP)
I've got a client that wants to omit the diamond buck outs that we typically place in the slab on grade at columns to reduce unsightly slab cracking. They don't want to have to do a second pour. I've warned the client that there will be an increased risk of unsightly cracking and it seems that they're okay with that:
Questions:
1) Are there any truly structural implications? Will the bottoms of my columns get sheared off from their foundations?
2) Anyone have a sense for how bad the cracking could potentially be and the risk of it occurring?
3) Any ideas for one pour solutions that wouldn't add a lot of cost? Larger compressible joints? Low shrinkage concrete?
Questions:
1) Are there any truly structural implications? Will the bottoms of my columns get sheared off from their foundations?
2) Anyone have a sense for how bad the cracking could potentially be and the risk of it occurring?
3) Any ideas for one pour solutions that wouldn't add a lot of cost? Larger compressible joints? Low shrinkage concrete?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.






RE: Omitting SOG Diamonds at Columns
RE: Omitting SOG Diamonds at Columns
Will there be a floor covering applied after the concrete has had a chance to shrink? If so, the cracking won't matter.
DaveAtkins
RE: Omitting SOG Diamonds at Columns
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Omitting SOG Diamonds at Columns
2. If the columns are square or rectangular, cracks starting at the reentrant corners of the floor. Length of the cracks depends on several factors: floor reinforcing steel, concrete mix design, concrete curing method and duration.
If the columns are circular, possibly no problems at all.
3. One or two short rebar (say, 24" long) close to each reentrant corner. Position the rebar perpendicular to the expected path of a crack. This will help limit extent of cracking.
Wet curing the concrete... but that could add to cost compared to curing compound or other short cuts.
www.SlideRuleEra.net
www.VacuumTubeEra.net
RE: Omitting SOG Diamonds at Columns
RE: Omitting SOG Diamonds at Columns
I'm still curious about saw cutting versus tooling. If I take the control joints right up to the column, it would not be possible to saw cut them then, would it? They would need to be tooled?
Also, we've been mostly focused on on horizontal slab movement. There's also a vertical movement issue I believe. As weight is added to the columns and they settle, they will tend to drag the SOG downwards near the columns and create tension cracks in the top of the SOG near the columns. Are the compressible joints, by themselves, generally considered to be an adequate form of vertical isolation?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Omitting SOG Diamonds at Columns
Expansion material will be fine for both horizontal and limited vertical isolation. A call to a manufacturer's representative will help select the best product: Link
www.SlideRuleEra.net
www.VacuumTubeEra.net
RE: Omitting SOG Diamonds at Columns
most of the time in this condition i've seen the slab is very very thick at the diamond compared to the slab and the column gets encased in concrete to slab elev. suppose you could backfill over the ftgs which hardly ever gets done to the degree of the wide open areas and have soil/stone against the column steel coated or not... diamond concrete is more/less part of the footing than the slab... having diamonds lets the contractor do better finishing technique on small areas... surely this could get bundled in with another concrete misc pour. will the steel be finished with no need to plumb when the slab is done?
RE: Omitting SOG Diamonds at Columns
RE: Omitting SOG Diamonds at Columns
Perhaps. I wonder if the transition would be aesthetically unappealing however.
Concrete columns this time around.
I pushed back a little regarding risk of cracking and the client caved. The diamonds are back in.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Omitting SOG Diamonds at Columns
Cracks in a slab on grade in a parking structure are just aesthetic. The joints are just straight cracks, and even then, sometimes you get crooked cracks.
RE: Omitting SOG Diamonds at Columns
The diamond is there to prevent re-entrant corners, obviously. It is also a "stand-off" termination of the saw-cut joints to allow the sawing operation to finish at the apex of the joint rather than getting close to the column and allowing the shrinkage crack to migrate to the column through the surface of the slab.
The owner will complain about the "random" cracking later, without regard to what he says now. Further, when he sells the building (which he will at some point), the cracks will cost him money in a reduced value of the building....not much, but something and it creates a negotiating point that doesn't need to be there.
RE: Omitting SOG Diamonds at Columns
What a radical departure from standard practice in my area. It's eye opening to see how things taken as inviolate in one market are no big deal in another. One nice feature of your method, I think, would be that your CJ layout could be much more regular than your column layout.
What? I thought that a major reason for the diamond was to allow the slab to shrink without getting hung up on the columns. Every firm that I've worked with has had verbiage about not allowing the diamond to be cast for a certain number of days which I've assumed was for this reason. Is your understanding different?
With regard to the standoff sawcut terminations, I'm now starting to wonder: how is this done where control joints abut walls?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Omitting SOG Diamonds at Columns
Where joints abut walls, the joint is sawn as close as practicable and allowed to crack on its own for the last few inches.
RE: Omitting SOG Diamonds at Columns
Have typically done what hokie advocated. Provide corner bars (especially if you have control joints going to faces of column), and frame it out just like you would a drain or even like an opening in a suspended slab. Just make sure to provide building paper or expansion material or whatever you would normally provide around the columns for the differential movement.
RE: Omitting SOG Diamonds at Columns
RE: Omitting SOG Diamonds at Columns
RE: Omitting SOG Diamonds at Columns
RE: Omitting SOG Diamonds at Columns
In this regard, would your method not be nearly identical to Ron's in that they both rely upon vertical slip and compressible joint around the column? If anything, I would think that Ron's scheme might have a slight leg up in that you might luck out and see some of your circumferential cracking right at the diamond.
No doubt about that.
I sketched this up while I was at a meeting. For any reasonable corner bar length, the bars would actually intersect across the control joints coming into the column. In a sense, isn't that kind of defeating the purpose of the control joints?
I presume that you're referring to these statement from ACI 224.3/302:
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Omitting SOG Diamonds at Columns
When you cast a concrete slab monolithic around say, a manhole frame (or column), the concrete shrinks to grip the manhole frame. If you place radial joints in the concrete projecting from the manhole ring, the concrete pulls away from the ring.
RE: Omitting SOG Diamonds at Columns
To clarify, the vertical movement which I have found objectionable was the diamond going down with the column, causing a depressed area around the column. I can also recall the diamond ending up higher than the rest of the slab. This was in heavy industrial work, not in a carpark.
RE: Omitting SOG Diamonds at Columns
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Omitting SOG Diamonds at Columns
I like hokie66's method in theory. It makes sense; however, most "odd" or random cracking is caused by improper timing of sawcuts and/or improper subgrade flatness control. Until we get subcontractors to understand the importance of quickly placing control joints, what we do in design makes little difference!
RE: Omitting SOG Diamonds at Columns
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Omitting SOG Diamonds at Columns
RE: Omitting SOG Diamonds at Columns
RE: Omitting SOG Diamonds at Columns
RE: Omitting SOG Diamonds at Columns
RE: Omitting SOG Diamonds at Columns
RE: Omitting SOG Diamonds at Columns
RE: Omitting SOG Diamonds at Columns
RE: Omitting SOG Diamonds at Columns
RE: Omitting SOG Diamonds at Columns
http://www.nceng.com.au/
"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."
RE: Omitting SOG Diamonds at Columns
The key is to use a contractor who is familiar with placing the concrete with fibres to avoid issues with fibres being visible and the like.
I believe Dramix is available in Australia, they are usually very helpful in offering design services/advice if you are unfamiliar with the design procedures involved. They can achieve some very large pours with the 4D & 5D fibres as the anchorage can actually yield the high strength steel fibres.
http://www.bekaert.com/en/products/construction/co...