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Re: bar spacing

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Two things:

First, if a house fails, very few people die. That allows the use of "it's the way we've always done it" as an engineering method. I happen to deal in residential, and I get to see failures every day - all of them out of the statute of repose, and most of them outside the typical career life of a residential GC. If it were up to me, unreinforced masonry would be prohibited below grade, the tables would exclude GW/GP backfill (unless a chart showing how much was required was included), and the earth pressures would be per ASCE Table 3-1. Does the PCA use ASCE7's Table 3-1? If it does, that means the ML-CL column should be designed for 100pcf, and the GW-SM are all combined at 60pcf for walls over 8' tall.

Second - the Virginia Homebuilder's Association managed to strip the anchor bolt provisions from the VA version of the code. I don't know how many others have. Still, I've seen a couple of houses around my area in which the top of the wall has moved inward, unfettered by the floor system above, by up to 2 inches (the entire band, in those cases, was over empty space outside the wall).


Ehrlich, Gary wrote:

It should be noted that the basement wall tables in the IRC have been consistently maintained by either PCA or ACI (I’m forgetting which at the moment), so there is a solid technical basis to them. Ditto the ACI 332 tables. The principal differences are that the IRC & ACI 332 tables use a smaller minimum reinforcement ratio than and a higher limit on reinforcing tensile strength than ACI 318. The relaxations were deemed appropriate based on the observed performance of residential walls, as opposed to the limitations of the more general ACI 318, which has to work for everything from small residential and light commercial structures up to high-rise buildings and industrial/essential facilities.


It’s also worth noting, since the discussion started with ICF’s, that PCA has a new consensus standard (PCA 100-07) for prescriptive design of residential concrete walls, which is referenced in the 2009 IRC. Covers both below-grade and above-grade concrete walls, both traditional walls and ICFs. Complies fully with ACI 332 (or ACI 318 where necessary) and ASCE 7-05. I think they even used ACI 318 Appendix D to calculate ledger attachments and top-of-wall support. (Not without much grumbling, mind you.)




Gary J. Ehrlich, PE
Program Manager, Structural Codes & Standards
National Association of Home Builders (NAHB)
1201 15th Street, NW, Washington, DC 20005
ph: 202-266-8545  or 800-368-5242 x8545
fax: 202-266-8369

Attend the 2010 International Builders' Show
January 19-22, 2010, Las Vegas, NV

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