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RE: Masonry development lengths

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>Where can I find information on the recent history and near future of
>the ACI development length equations for masonry reinforcement?

>The Florida Building Code references ACI 530-05 equation 3-15 for
>strength design but for allowable stress design they have replaced ACI
>530-05 equation 2-9 with ld=0.002*db*fs.


>Christopher Banbury, PE

The ACI530-99 code had only ASD included with the ld = 0.002*db*fs equation. However, the 2000 IBC over-ruled this equation and used the more cumbersome equation ld = 0.16*db^2*fy*gamma/(K*fm'^1/2). In addition, the 2000 IBC included a procedure for strength design that had a similar equation for ld. According to one ACI530-99 committee member it was a mistake to use the latter equation for ASD, it was only applicable to strength design. I agree with Bill Sherman that this does not make sense.

The ACI530-05 uses the equation ld = 0.13*db^2*fy*gamma/(K*fm'^1/2) for both ASD and strength design. Now, the 2006 IBC overrules this equation for ASD and reverts to ld = 0.002*db*fs with the important exception that when the calculated stress in the bar is greater than 80% of the allowable stress then ld shall be increased by 50%. I don't know about you, but almost all of my masonry designs result in steel controlled sections when using ASD, thus ld must be increases by 50%.

The IBC uses the equation ld = 0.13*db^2*fy*gamma/(K*fm'^1/2)for strength design. This results in shorter lap lengths (most of the time). Therefore, I have now begun using strength design for masonry. This has resulted in much more economical designs in overall rebar use as well as lap lengths.


Adam Vakiener, P.E.
Structural Engineer
Southern A&E, LLC

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