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re: bar spacing in a wall

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Andrew,

You cannot achieve reasonable crack control with bar spacings greater than 18" according to ACI. Other codes have since reduced this to 12" (Eurocode and AS3600). Wider than that and the crack width between the bars is not controlled by the bar. BS8110 theoretically allows 750mm (30") as an absolute maximum but not if you want good crack control for which they give reduced values depending on certain factors I cannot remember (code is at work). So, yes, it has to do with the Z factor and its replacement method.

It has nothing to do with warm and fuzzies and little to do with strength, except that for a cantilever wall for strength I would not think you would want a spacing greater than the wall height, but probably significantly less than that, say about half wall height.


At 01:43 AM 5/09/2009, you wrote:
I should not post twice on the same subject, but I will be that guy because it is Friday and I am feeling chippy, hombre.

Riddle me this batman, why in one situation could I go with 8" CMU with #4@48" o.c. in solid grouted cells but ACI would require me to go 18" o.c. max for that same wall if it is 6" solid poured? I have done more than my fair share of CMU calcs so I understand the intricacies, I want a common sense type answer. Don't blind me with your science. I think the answer is somewhere between a longstanding ACI minimum that gives people warm and fuzzies, and something to do with bond stress or the old Z factor or something? I don't have my ACI around and I don't have it memorized like Scott, but I bet he knows the answer.

But Jeff says the IRC table says 48" o.c. for 30 psf, which makes sense from a CMU perspective, and we all know about CMU being of much less compressive strength than solid concrete. I have also heard people in the ICF business throw 48" o.c. out there also. There has to be some science or testing behind this, but it from a residential wall standpoint it would seem about right. (I still would pay the extra for my house from a crack control perspective).

I digress. Rebar, we talkin about rebar? REBAR? REBAR? (Allen Iverson talkin about practice). How much more, anyone anyone anyone, can this 860sf addition cost either way for say about 20-30 more bars? Have the GC tell you, then bill him for your extra time on this list and doing research and call it even. Add in our time as "consulting and research".

Its 100am where Conrad in Adelaide is, means we can all have a beer now right?


Andrew Kester, PE
Orlando, FL



Regards  Gil Brock
Prestressed Concrete Design Consultants Pty. Ltd. (ABN 84 003 163 586)
5 Cameron Street Beenleigh Qld 4207 Australia
Ph +61 7 3807 8022              Fax +61 7 3807 8422
email:          gil(--nospam--at)raptsoftware.com
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