Factor used to modify development lenght

[magudfe]

Dear Colleagues, can anyone explain to me what is the meaning of the factor used to modify development lenght based on reinforcement location in 12.2.4.a (ACI 318-05) "Where horizontal reinforcement is placed such that more than 300mm of fresh concrete is cast below the development lenght or splice, ft=1,3. For other situations ft=1"?

Thanks

 

[hokie66]

It just means that you have to use longer development lengths for bars with 300 mm or more of fresh concrete cast below the bars. This is because the plastic concrete tends to consolidate and leave voids around the bars. When the bars are at the bottom, the settlement is negligible, so the development length doesn't need to be adjusted.

 

[WillisV]

Also see thread592-164055

 

[Lion06]

Also, when vibrating there is a larger volume of concrete below the bars which means more air bubbles that can get trapped under the bars and reduce the bond.

 

[magudfe]

Thanks to all for the clarification, this is a concept not included in the standard i usually follow for design.