Development Length Fresh Concrete Location Factor

[Quade999]

Hi Everyone,

When calculating the development length, you have to increase it by 1.3 when there is more than 12" of fresh concrete below the development length or splice. Does this mean directly below, as in taken completely vertical? If there is a local thickening like the one show below, is there a minimum distance that the splice should be located away from the local thickening so that the 1.3 factor does not apply?

Thanks

 

[r13]

1.3 factor is not required for your case. However, why provide splice at maximum tension zone?

 

[HTURKAK]

I copied and pasted the relevant section of ACI 318-08

In your case , ψt = 1.3 is not applicable. ψt = 1.3 is to account that , the full bond could not develop under the horizontal reinforcement if fresh concrete depth below the splice is more than 12 in.

You are expected that, you should provide bar splicings at points of minimum stress and stagger the splice locations where possible ..

 

[KootK]

1) I've simply never considered this before. It's an interesting and valid question.

2) I know of no recommendations for this.

3) Rationally, it makes sense that there would be some, non-zero value for the dimension that you've inquired about, as shown below. Nature usually dislikes an abrupt transition.

 

[Agent666]

This reduction is due to entrained air in the mix migrating upwards under the bars and excess water. It recognises that for top reinforcement the reduction in the quality of bond because the excess water used in the mix for workability and air entrapped during the mixing and placing operations may rise toward the top of the finished concrete before setting is complete.

Entrapped below bars, this water and air leaves the bar less bonded to the concrete on the underside. I'd always assumed this air and water migrates directly upwards in practice, so the reduction applied only in thicker regions as I understand it, and didn't apply out into the slab in your example.

There's obviously some merit in being conservative with lapping/splices in critical areas. Often though due to concrete cover or spacing you'll qualify for the substantial reduction to development length associated with these aspects, which often isn't utilised by designers in standard development length tables as part of standard notes, etc. So in practice if using project wide tables you usually have something up your sleeve.