Masonry Development Length 2

[gr8gonzo0]

The IBC 06 (or in my case the 2007 CBC) refers to ACI 530 for masonry design. ACI 530 gives a formula for development of bars in tension (2.1.10.3) that seems to yield some extremely long development lengths. Has anyone else ran into this issue? Any thoughts?

 

[CTSeng]

Yeah, these relatively recent changes suck. I assume it's a result of some new testing. I hate having to argue these type of code changes with contractors.

 

[Lion06]

Actually, I thought masonry had some rather short development lengths. The last time I looked was ACI 530-05, but they seemed shorter than a comparable bar for concrete.
For a #4 bar centered in an 8" wall and f'm=2500 psi, the development length is only 15.6". I think that is pretty short, not too long.

 

[cap4000]

IBC 2006 section 2107.5 was in fact revised and shorter splice lenghts are now shown. Their is however a stipulation indicating 50% more length than required by the calculations depending on the stress in the splice.

 

[SteveGregory]

NCMA has a nice short paper with charts for IBC 2003 and 2006 editions. It is titled TEK 12-6.

 

[Lion06]

Nice post Steve.

That TEK note is showing an Ld of 21" for a #4 bar in 1500 psi masonry., this also translates into the 15.6" Ld for 2500 psi masonry that I mentioned above. The best case (shortest) Ld for a #4 bar in 4000 psi concrete is 19".
As I said earlier, the development lengths for masonry seem too short, not too long (when compared with a comparable concrete assemblage).

 

[gr8gonzo0]

Thanks Steve for the info.

I agree that for smaller diameter bars (i.e. #4 or #5) centered in a masonry wall have reasonable development lengths. However, we design a lot of retaining walls where we end up with larger bars (#6 or #8) located closer to the edge for moment capacity. This results in VERY long development lengths that are even longer than the required lap splice lengths. The equation seems to be very sensitive to the K value.

One thing a co-worker brought up was the use of fy in the equation. Why do we use fy (60 ksi) when ACI 530 puts a maximum limit on the allowable tensile stress at 24ksi. It seems like the development length equation should have an Fs term instead of fy. This is just a discussion we had recently. I could be missing something obvious.