CIP Pile Tension Connection to Base Slab 2

[waytsh]

When making a tension connection between a cast-in-place pile and the base slab with rebar dowels is there a recommended safety factor? ACI 543 seems to indicate using a factor of 0.5 when cacluating your tension steel. ACI 318 and 350 call for 0.9 for tension connections. I am also looking for some recommened details. I am considering bent rebar with the required emebedment and brought up past the bottom mat or a headed plate with a heavy plate washer. Does anyone know of any reasons to use one over the other or maybe a third type that I haven't mentioned?

Thanks!

 

[apriley]

Factors of safety depend on what kind of loads you're looking at. Seismic is different than service.

Typically in bridges we extend the reinforcement to the top mat in order to transfer the forces into the slab. Hooked or headed bars work well to develop this. It seems to me that a plate wouldn't be necessary.

 

[waytsh]

apriley,

Thank you for the quick response. I probably should not have used the term saftey factor. I am referring to the phi factor for calculating the required area of steel to resist tension. The controlling tension load by far is due to uplift from groundwater and is going to be around 43 kips.

It makes sense to bring the hooked bars up past the top mat as well as the bottom. For constructibility reasons the contractor has asked if they can go to the bottom side of the top mat. If the bar is adequately developed into the slab does it matter if it does not extend above the top mat? I know you will have a much lower pullout strength but is there anything in the codes that does not allow this?

Thanks,

waytsh

 

[apriley]

Ok, makes sense. I design using AASHTO so I won't be of much help for your fist question.

As far as development into the slab, we usually go to the bottom of the top mat as well. The Port of Los Angeles has developed a detail using short drop-in headed bars lapped with the pile reinforcement (takes care of constructability) to develop all the way to the top, but I think that's more important for seismic loads. I think developing as close as practicable to the top is sufficient. I don't know of anything in the codes you're using, though, that would validate this.

 

[KootK]

waytsh,

I think that 0.9 is the phi factor that you want to use.

As for whether or not you take your pile steel above the top Matt I think that it's all about developing those bars to resist pullout. In that regard, you've got three options:

1) Take the bars to the top and devlop the usual stut and tie model for something like this (pile cap as beam with top & bottom chord.

2) Terminate the bars at the underside of the top mat and count on concrete cone pullout resistance to keep the pile rebar anchored.

3) Develop a more sophisticated strut and tie model that takes advantage of rebar elsewhere in the pile cap (rebar other than the top mat wich you're not engaging directly). This can often be done. You just have to be meticulous in following your load path through. In my experience, this leads to some bottom reinforcementconcentrated over the piles.