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RE: Maximum steel spacing in concrete walls

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I will add my responses below.

Regards,
Harold Sprague


From: "Jordan Truesdell, PE" <seaint(--nospam--at)truesdellengineering.com>
Reply-To: <seaint(--nospam--at)seaint.org>
To: seaint(--nospam--at)seaint.org
Subject: Maximum steel spacing in concrete walls
Date: Mon, 16 Aug 2004 10:33:59 -0400

Does anyone have a link/reference for additional commentary on the Minimum reinforcement provisions of ACI318 ch 14?
I would suggest that you go back to the ACI 318-63. It was one code that covered both reinforced and unreinforced. It defined reinforced concrete as steel and concrete designed to act together. Take a look at ACI 318-02, Ch 22, and compare it to the IRC. I don't think that there are any inconsistancies. I believe that the ACI 318 allows for no reinforcement at all. The IRC is more restrictive and added the rebar to provide some assurance of a relatively uncracked vertical span.


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.
This is correct.


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?
No, we are not over-specifying. And no we are not designing throw-away buildings. There are many concrete and masonry structures constructed in the past centuries that are unreinforced, and are performing quite well. Steel reinforced concrete and masonry structures are relatively new, and have only been used for about one century.

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.

I rarely do residential, but when I do, I design the walls as reinforced. I have become involved in way too many excessively cracked basement walls. The rebar is worth pennies compared to the expense of constructing a house. The risk is just not worth the minimal savings.



Jordan Truesdell, PE

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