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Maximum steel spacing in concrete walls

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Does anyone have a link/reference for additional commentary on the Minimum reinforcement provisions of ACI318 ch 14?

14.3.5 indicates that
"Vertical and horizontal reinforcing in walls shall not be spaced father apart than three times the wall thickness, nor 18in."

Along with the requirements for small vertical bars (As/Ag=0.0012) and small horizontal bars (As/Ag=0.0020), I read this as an 8" wall requires .115 in^2/ft vertical (#3@11 or #4@18) and 0.192 in^2/ft horizontal (#4@12 or #5@18) as bare minimums. Also, the way I read it, any spacing greater than 18" in either direction results in the wall being analyzed as plain concrete.

The reason I ask is that the IRC allows spacings of up to 72" for vertical, and does not appear to require ANY horizontal steel, save in D1/D2 seismic zones, where a single #4 bar needs to be located in the top 12" of wall.

This appears to be a significant disconnect. Are we over-specifying everything else, or is prescriptive Residential assuming these are throw-away buildings with no life-safety concerns?

Jordan Truesdell, PE

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