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RE: ACI 318 section 12.2.4

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Previous commentary to the code indicated that the top bar factor is to
account for excess water and air which rise and collect on the bottom of
horizontal bars in deep concrete placements. This weakens the bond of the
bars. The commentary also noted that this effect is lessened in a deep
member with regularly spaced horizontal bars, such as in a wall.

This only applies to horizontal bars, not to vertical bars. The code does
not specifically state where this provision "does not apply", such as in
walls, but many engineers apply the factor where bond is critical due to
strength and neglect the top bar factor for minimum shrinkage reinforcement
in walls. It is best to note on the drawings that development and splice
lengths may be based on that required for "other bars" for horizontal bars
in walls if that is your intent, for clarity.

> -----Original Message-----
> From: Foy, Warren [mailto:Warren.Foy(--nospam--at)mhgrp.com]
> Sent: Friday, March 22, 2002 11:02 AM
> To: 'seaint(--nospam--at)seaint.org'
> Subject: ACI 318 section 12.2.4
>
>
> Can anyone provide information regarding the alpha factor used in
> determining development lengths?
>
> Why is development length increased for bars with 12" of
> concrete cast below
> the bar?
>
> What constitutes 12" of concrete cast below?  Is this
> applicable only to
> beams or does it apply to horizontal reinforcement in walls?
> If applicable
> to beams, is it also applicable to vertical tension bars in
> columns and
> walls with 12" or more to the compression face?
>
> TIA
>
> Warren.Foy(--nospam--at)confusedRI.com
>
> Warren S. Foy
> Structural Design Manager
> The Mason & Hanger Group Inc.
>
>
> 300 West Vine Street, Suite 1300
> Lexington, KY   40507-1814
> voice: 859.252.9980
> fax: 859.389.8870
> web: http://www.mhgrp.com
>
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