Omitting SOG Diamonds at Columns

[KootK]

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?

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.

 

[BUGGAR]

What I learned in STEM:
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.

 

[hokie66]

KootK,
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.

 

[KootK]

Thanks for the clarification hokie.

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.

 

[Ron]

KootK....exactly the reference.

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!

 

[KootK]

@Ron: I'm going to try it your way. I'll report back in the spring. With regard to the sub-grade flatness issue, the crux of that is unintended restraint, right?

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.

 

[hokie66]

My answer to your last question...yes, reducing subgrade restraint by whatever means is helpful. I've seen lots of things tried, but there will still be a degree of drag, whatever is done.

 

[OzEng80]

I'm with hokie. I always locate joints away from the 'restraint' that is generated by a column/footing. Not only is the joint more effective, but with this arrangement you also have the opportunity to engage the slab and potentially reduce footing size.

 

[hokie66]

Well, you are not exactly with me, OzEgn80. I would not contemplate engaging the slab to reduce footing size.

 

[XR250]

Funny, I did the checkerboard placement when I built my house (I had never heard of it before and thought I invented it :>). Worked extremely well.

 

[OzEng80]

Why not hokie? We typically detail our pad footings so they are more or less (horizontal construction break with 'Z' bars if not poured monolithically) act integrally with the slab. Footing and slab act together - the extra mass is especially useful for hold down.

 

[Ron]

OzEng80...I've designed hundreds of aluminum canopy footings using the walkway slab tied to or keyed over the footing for increased uplift resistance. I use a 5-foot influence area for the walkway slab....more if the footing is deeper and can engage more soil depth.

 

[hokie66]

The slab doesn't need to be tied to the footing to assist in resisting uplift.

 

[MrHershey]

@KootK- I wouldn't think so. A lot of offices will halt reinforcing at either side of the joint and provide dowels for force transfer. If you've stopping all reinforcing at either side of the control joint, the joint is still very much going to be your weakest plane, especially once you consider the presence of the saw cut for the crack to initiate from. Wouldn't think the presence of a couple diagonals through an otherwise unreinforced joint would chase your cracks into the remaining reinforced slab. If you're using enhanced aggregate interlock for force transfer instead (there's some good discussion on this in ACI 360), then you actually continue about 0.1% steel through the joint anyways and would think you could just chalk the corner bars up as being part of that 0.1%. Comes out to about #3 @ 18" in a 6" slab on grade.

 

[rowingengineer]

With hokie on this one, we detail our slabs like hokie describes and overall few issues. however we do place a compressable marerial around the column.

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."

 

[Agent666]

Something else to consider might be steel fibre reinforcment and get rid of all of the sawcuts. These days you can achieve very large crack free floors using steel fibre reinforcement.

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/concrete-reinforcement