Drilled Pier-Rebar Development 1

[newb]

I am designing a drilled shaft for the support of a transmission monopole. Threaded #18 bars act as anchors and longitudinal reinforcement. The pier is 9.5 ft in diameter and about 40 ft long. Soil layers and tower loading are such that a very large moment is developed in the drilled shaft (per L Pile analysis). For a lighter more economical cage I have extended 16 of the 32 bars the full length of the pier. The other 16 terminate about half way down. They alternate every other bar.

Mu is located in the shaft such that half of the bars are fully developed below Mu and half are about 55% developed. The nominal moment I've calculated assumes that all 32 bars are fully developed. What percentage of this nominal moment can I use?

Here is what I was thinking but I doubt it is this simple:

Mn1=((.5*Mn)*(L1/Ld))+((.5*Mn)*(L2/Ld)) where L1=Ld and L2=.55*Ld

Any thoughts or code references on this issue would be greatly appreciated. Thanks!

 

[TXStructural]

ACI 318 12.2.5 gives this as an acceptable ratio, but not for development length. Bars are either developed or not, but may be discontinued when not needed to resist stresses in the member... but ACI 318 12.10.3 requires that the bars extend "d" distance beyond the point where it is required. (Chapter 12 has other requirements, but this one extends your bars about 9 feet beyond the extent of the theoretical demand.)

Discontinuing the bars at 22 ft will save on the order of $1800, on a cage that will cost about $10,000. (steel rebar cost only). My figures are based on prices published ENR in October 2011. Your thread-deformed bar may be significantly more.

 

[paddingtongreen]

One comment: I stopped using the reinforcing for anchorage because it was, too often, out of place. I used a regular cage with the bolts with on an anchor ring sufficiently far down the hole for enough rebar development length above the ring. This allows precise placing of the projections.

Reinforcing what TXStructural said, there is a moment gradient below the max. At some point, it reaches a value that can be handled by only the long bars, this is the theoretical cut-off and the short bars must then extend a further distance "d".

Michael.
Timing has a lot to do with the outcome of a rain dance.