Cantilevered Ground Beam - Force transfer into CIP Pile

[rscassar]

Hi

I have a quick questions regarding concrete detailing. I have a cantilevered ground beam which is supporting a column load of 100kips. I am designing by strut-and-tie method.

I have resolved my free body diagram so that I need to transfer a tension force of 63kips into a cast-in-place pile.

To detail this joint, I was thinking to cog the top reinforcement down full length of the ground beam and provide 30 bar diameters extension of the pile reinforcement into the ground beam. These two bars are not in a direct contact lap splice however, would this be sufficient to transfer the force in the tension pile or is this detailing in-sufficient.

All help will be appreciated.

Regards

 

[ishvaaag]

Normally you would want to get the tensile force at the axis of the tension pile (Except you really know what tensile stress each particular bar has, of course). Then you apply the splice length, typically.

The spanish code allows for up to 4 diameters between bars being spliced; then, the separation between splices themselves, and the amount of the required capacity being spliced at some point has a say in the required splice length and all the common variables.

Since it is unlikely that the bars being spliced fall all between 5 diameters between axes with the reinforcement at the pile you might need an special detail. You may need to add some 1" bent bars going deeper into the pile or whatever for a proper splice according to such specification.

In any case 30 diameters seems not quite long by present standards of splices, but should work in fact; for everyone is accustomed to harsher introduction of the tensile stresses by adhered anchors that won't have half the embedment of the said 30 diameters and codes are not going -as of now- very fussy about this. It is clear that the passive rebar taking any tensile action of any of such anchors is not typically following the stringences of splicing rebars, and it seems such details are not becoming particularly critical, or at least have not become yet famous "enough" for repeated bad behaviour.

 

[BAretired]

Requirements for a lapped splice should be covered in your code. In CSA A23.3, 30 bar diameters would not be enough to fully develop the bars.

It would be better if you could extend the pile reinforcement a bit higher and provide hooks or mechanical anchorage above the top steel of the beam.

Also, the top steel of the beam needs to be adequately anchored at the bend point at each end.

BA

 

[rscassar]

Thank you guys, I thought the detailing was not adequate because there is no splice and the bars are not developed. As such, I have decided to increase the length of the backspan so the piles are under no tension for working loads.

It has now created the dilemma whether I can design the pile cap by strut-tie method because the span depth is approaching 3.5. If it were to be designed by traditional beam flexure approach, I should be including shear ties.

Anyway, I am going to purchase ACI document SP-208 to see if that can unlock any secrets.

Thanks and regards.