Maximum Wall Rebar Spacings: IBC vs ACI

[sundale]

Would anybody wish to comment on the bar spacings called out in 2003 IBC Tables 1805.5(2) to (4) as compared to ACI 318-02 Section 14.3.5?

Namely, the IBC prescribes bar spacings from 32" to 72" o.c. in their concrete foundation wall tables whereas ACI Chapter 14 says 18" o.c. maximum.

Since the IBC has more or less adopted the ACI, with some minor changes in IBC Chapter 19, where do they get these crazy (my subjective opinion) bar spacings?

 

[henri2]

A few points.

1. These prescriptive tables are for foundation walls that are laterally supported at the top and bottom and within certain parameters---IBC Sec 1808.5. Note that the first sentence of the paragraph states that foundation concrete walls shall be designed per Chapter 19...so this section is in a sense an exception. For significant axial, flexural and or shear loads, this section IMO would not be applicable.

2. Numerous restrictions---see footnotes to tables

3. Seismic requirements/limitations--- IBC Sec 1805.5.5 would limit this mainly to SDC A and B while for SDC C-F it may apply to plain concrete foundation walls.

4. These provisions would be applicable to most residential construction..a situation that can cut the structural engineer out of the action. In fact I think these tables are also in the IRC.

 

[sundale]

Good points and thanks henri2. These prescriptive spacings are most certainly an exception, by 72"/18"=4X, of the maximum bar spacings per ACI. The caveats contained in 1805.1 to 1805.5 are not real hard to meet if you are in a low seismicity area.

Most ALL foundation walls are laterally supported at the top and bottom. A typical full basement wall, with a simple span floor to floor, is what I think of here and is what these IBC rebar tables are describing.

For a walkout basement, with a large unbalanced dirt force, I usually design these as a cantilever retaining wall condition unless I have a real strong upper floor diaphragm (i.e. not wood). I hope (but doubt) that the contractor's can make this distinction, as that is to whom these prescriptive "monkey" tables are most likely directed. The IBC's definition of unbalanced dirt height is not real clear in regards to considering an overall story of unbalanced dirt force on a sloping site (walkout basement).

Anyway, the ACI 18" max rebar wall spacing has been in every edition of the ACI I ever owned and I presume this is to control cracking. ALL concrete foundation walls are subject to curing shrinkage and differential settlement, despite whatever seismic design category or caveats are placed on them. These IBC tables do not even come close to satisfying the min 0.0012 vertical reinforcement ratio.

This is a factitious anthropomorphism of reinforced concrete, but the question I would pose to the IBC code writers is how does the concrete KNOW it has met all the caveats of 1805.1 to 1805.5?