Question about designing and detailing bundled reinforcing steel in a shear wall

[cliff234]

I have a question regarding how to lap splice bundled reinforcing bars. We always try to avoid using bundled bars, but we were asked to investigate whether using #11 bars in 4-bar bundles will be less expensive than mechanically spliced #14 and #18 bars. (The bars are the chord steel in a shear wall. The large area is required for tension - so increasing the concrete strength won't reduce the required area of steel.)

Here's what ACI 318-08 says about bundled bars,

Bundles are limited to 4 bars (Section 7.6.6)
Development length has to be increased by 33% for 4-bar bundles (12.4.1)
Lap splice lengths are determined based on the individual bar size and no more than one bar in the bundle may be lap spliced at any one location. (12.14.2.2). This means that I have to stagger the splices.

If I have a 4-bar bundle and I stagger the splices one bar at a time, I will essentially have a 5-bar bundle (with four bars being effective in tension). Am I interpreting this correctly? Or am I limited to a 4-bar bundle with three of the bars being effective (due to the staggered splices)?

 

[TXStructural]

Even the PCA notes are skimpy on bundled bars. As I understand it, the lap splice would result in 5 bars for a 4-bar bundle (or 4 bars for a 3-bar bundle). This is not exactly practical for many applications, since you could end up with 5 continuous bars along most of the length of the bundle in order to achieve a 4-bar bundle. If the lap is about 8 feet long, increased by the 1.33 factor to 10-1/2 feet. The bars are delivered in a maximum of 60 feet lengths, so only about 8 feet of bar is unlapped in each 60 feet. This is why mechanical splices/couplers are so popular, if not cost effective.

As an aside, I really think we need to reexamine lap lengths and design strategies, since the difference between a 113 and 118 inch lap is really unsupportable, given the variability in all of the factors involved.

 

[hokie66]

I doubt the economy of lapped bundled bars in tension, as you will always lose the area of a least one bar, sometimes more depending on the lap lengths required. If not mechanically spliced, I think bundles should be reserved for compression steel.

 

[yakpol]

Hokie, If using bar couplers, since coupler diameter is larger than of the bar, you end up with the gap between bundled bars. Is it acceptable? For #11 bars the gap is about half inch.

Yakpol