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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.

>Thanks.

>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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