horizontal rebar in foundation wall

huangyhg

horizontal rebar in foundation wall
i often see rebar for foundation walls in residential construction spec'd at a typical spacing (i.e. #5 @ 24 ctr.) and often the only absolute dimension is given as 4"-6" from the top of footing for the 1st bar.  
my question is why does the spacing always begin at 4"-6" above the footing?  i used to tie bar and always put a bar at the top and a bar at the bottom, then put the spacing between, but since becoming an engineer, i've never heard an explanation of why.  it seems as though placing the 1st bar anywhere in the 1st 12" for a 24" spacing would be adequate to meet bar area requirements.
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i always make my reinforcement simple so less error can happen in the field.  i have a feeling if you start the horizontal reinforcement 4'6 from the top of footing, the designer probably specify more rebars at the corners (at negative moment).  
i usually put the same amount of reinforcement at the corners and the middle of the wall to make it easier to understand.
cracking most often starts at the side of the face, so the further away from the side the first bar is the less it is going to prevent cracking.
csd
the crack will begin at the top of the wall and proceed downward. corresponding to this, the size of the crack width is larger at the top then at the bottom. i would be more concerned about placement beginning within 2" of the top of the wall and preceeding downward at the specified spacing.
jike explained it better, but thats what i meant.
csd
rstewart,
i would argue that jike is completely wrong.  the reason that you want a bar or bars near the bottom of the wall is that restraint by the footing is what causes the wall to crack, and reinforcing to control that cracking needs to be where the crack starts.  i have usually specified a larger bar at the bottom to attempt to control this cracking.
there is no reason for cracking to start at the top of a wall, because it is not restrained.
are we talking about foundation wall with basement here? i am a little loss.
shrinkage cracks are normally vertical in a foundation wall with the crack being more open at the top than at the bottom. the restraint of the footing keeps the crack from opening up at the base of the wall. the width of the crack is an indication of the amount of strain at that level and it seems logical to locate the  rebar where the strain is the most as this will be most effective.
i have always had good results with this approach.
jike,
i respectfully suggest that you rethink.  from many years of observing concrete walls cast on concrete footings, the cracks propagate from the bottom.  some may eventually be wider at the top for other reasons.  so i would agree with you that additional reinforcement at the top is good practice also.  the next time you have a chance, suggest you observe a wall for a few days after the forms are removed.  cracks will start to form at the bottom, at generally about 10 ft centres.  thus the need to control the cracking by additional reinforcement near the bottom.
coengineer,
the function of the op wall doesn't matter.  any wall cast on a previously cast element, whether footing, beam, or slab will behave in a similar manner.
op and others,
the key thing in considering where we put bars like this is recognizing that shrinkage of concrete does not in itself cause cracking.  it is restraint that causes cracking, and i think we should call them restraint cracks rather than shrinkage cracks.
agree with hokie, as i've observed cracks propogating up the wall from the base mat of concrete tanks. i've always considered these to be caused by the restraint of the base mat on the wall.
i have seen cracks at the top of foundation walls, but these were the result of foundation settlement at the end of the wall.
i'm beginning to wonder if the horizontal reinforcing isn't contributing to the cracking, by providing additional resistance to shrinkage...
i do agree that the restraint to wall shrinkage (provided by the footing) is the cause of the cracking and it is likely that this is where the cracking begins. i have not seen this with a foundation wall and footing but i have seen this with a new slab doweled to an existing slab. perpendicular cracks will form off of the joints at regular intervals.
if you take an unreinforced piece of concrete wrapped in 2 layers of visqueen, it will shrink unrestrained without any cracking. but if you take the same piece and restrain it at the bottom, it will form perpendicular cracks at regular intervals.
the reason the crack is larger at the top, is the concrete wall thruout its height is wanting to shrink at the same rate but cannot at the bottom since it is already restrained by the footing but it is completely free to do so at the top. the footing is already acting as the reinforcing to restrain shrinkage at the bottom of the wall. this is why the cracks are very small at the bottom and much wider at the top of the wall.
you seem to agree that "some may eventually be wider at the top" but then claim that it is for "other reasons".  it is caused by the same mechanism: shrinkage.
it seems logical and most efficient to add rebar across the location where the crack is anticipated to be the largest or evenly distribute reinforcing across the entire crack width.
we have the same objective but have two different approaches to the solution. the proof of the effectiveness of the approach is whether the placement of the reinforcing actually works to limit the cracks. if each approach works to accomplish the objective, then we have both been successful. after all we desire to solve the problem, not to have everyone necessarily use the same solution. it sometimes is easy to fall into the mindset of there is only one solution.