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RE: Maximum steel spacing in concrete walls

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Harold Sprague wrote:

There is no disconnect. The concrete in residental construction is assumed unreinforced. The rebar is placed just for crack control. The IRC still allows you the latitude of designing a wall as reinforced. In that case, you will go back to the ACI 318. The vertical rebar is intended to just control the cracks so that you can have a relatively uncracked concrete element spanning in the vertical direction. The key is to expect uncontroled vertical cracks in an IRC design, but they can be sealed.

This is where I'm perplexed. The 72" spacing allowable (and tabulated spacings of 16" to 48" for 8" walls, 32" to 72" for 12" walls) with no horizontal reinforcement is termed "Reinforced Concrete" in the IRC (tables 404.1.1(2) and (3)). The rebar is specified as tension side, and the walls do not "work" as unreinforced sections per section 22 of ACI318 - ie Mn=5S(fc')^0.5. They are clearly relying on the steel to create a T/C couple with the soil-side concrete, but are not compliant with ACI318.

There IS a table for unreinforced concrete, and the values work with liberal interpretations of backfill pressures for the soil types specified (iirc, 23pcf for GW/GP/SW/SP).

The reason I asked is to get some insight as to why the ICC would ignore the recommendations of ACI318 for residential, while incorporating it by reference in the IBC. I'm doing more residential work that I used to (60% now vs 0% two years ago), so I'm running into more of these issues. I don't mind telling owners/builders that their #4's 24"o/c each way are useless (for my calculations), but it would be nice if someone had a logical reason for this spacing/steel "exception" in the IRC.


Jordan Truesdell, PE



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