minimum steel reinforcement in retaining wall

huangyhg

minimum steel reinforcement in retaining wall
i'm looking for some clarification regarding the exemption for minimum reinforcement given in aci 318 10.5.3 (allowed where as provided is at least 1/3 greater than required) versus requirements for a minimum spacing of reinforcement steel of 18" in sections 7.6.5 and 14.3.5.  this came up in a cast-in-place retaining wall design (relatively small, 10' high).  specifically, i was wondering if you can use the exemption of 10.5.3 to allow for cutting off alternating bars or are you stuck with the 18" max spacing?
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for a retaining wall the minimum reinforcement would be according to 10.5.4 of aci 318, which says that the minimum reinforcement in the direction of the span is the same as that required by 7.12 of aci.  section 10.5.4 also states that the spacing of this reinforcement is not to exceed 18".  this is also what crsi uses.

and i believe there are additional checks required to cut-off bars, that may reduce you cost saving to the point that they are not worth pursuing.  if i recall you will always have to use 18 inches or twice the thickness of the   
bjb - i think 10.5.4 is only applicable to slabs and footings.  but 14.3.5 is for walls - albeit not specifically for retaining walls.  i've seen the exemption of 10.5.3 cited in design examples (and in design software) for retaining walls - i've just not seen any documentation why it was applicable over the 14.3.5 which leaves me a little nervous about the application.  however, the exemption in 10.5.3, as i can pick out from the commentary, is to give some leeway on reinforcement where the concrete section is larger than it would need to be from a strength standpoint (e.g., an 8" thick wall section could handle the loads for a 10' high retaining wall, but the 10" wall is easier for most contractors to build and get the reinforcement/concrete placement right) - that would be the only justification i can figure out.
dinosaur - there are requirements for development that necessarily extend the rebar beyond the point where they're needed, but it's relatively small (6" for #4) so if you truly exceed the 18" limitation, there is some economy in cutting of alternating bars.
still, i would think the 18" max is there to ensure the reinforcement behaves properly in the system.  otherwise it might be that you have to look at the wall as a structural plain concrete design?
thanks for your input.
retaining wall is essentially a structural cantilever beam, it behaves differently than a typical wall that is usually subjected to greater gravity load than lateral forces.
to me, the 18" requirement is a "good practice", rather than mandatory, to prevent wide cracks. actually, for certain cases, you may need more bars in a closer spacing to achieve that purpose (crack control).  
i missed the "gravity retaining wall". for which, i prefer to provide temp & shrinkage steel in 6"-12" spacing, if steel is required at all.
the stem of a retaining wall is basically a vertical slab.   if you have the 2002 crsi handbook, look at page 14-7 where they discuss minimum main steel.  they reference aci 318 10.5.4.
i would never have less primary reinforcement in a slab or wall than was required for shrinkage and temperature.  no matter what the minimum area of steel is, you can't exceed a bar spacing of 18" or 3 times the thickness.
i believe that the intent of 10.5.2 and 10.5.3 is for beams, where you could have a concrete section much bigger than needed.  a grade beam could be an example of this.
this diverges from this thread a bit, but i noticed the prescriptive design table for concrete foundation walls in ibc 2006 (table 1805.5(5)) and there's nothing remotely approaching an 18" spacing of rebar.  would a pe be within their standard of care for a client if they specified a wall based on this table, since it violates requirements of aci 318?
dear sir,we are construct?ng a group of retainning walls in hillside please do assist me about reinforcment placement in foundat?on and wall, these wall protect the road ,from coming big rocks on the mountains.
thanks alot
regards
we use to have a similar clause in as3600 where the minimum steel requirements were waived when the minimum capacity exceeds 1.33 x m*.