Shear friction dowel action

[PMR06]

Question: Per ACI R11.7.4.3, one can use mu=0.6 for shear resistance primarily due to dowel action of the reinforcement. Does the reinforcement still need to be developed on both sides? Is the 0.6 basically saying the dowels will be shearing (not in tension)?

Background: (See attached sketch) We have our typical housekeeping pad detail 6" tall concrete pad with #5 dowels w/ std hooks around the perimeter and a mat of #5's. We do have lateral load from equipment mounted above that needs to be transferred thru the pad into the foundation. My argument is that even if you used #3 dowels in our typical 6" pad, you cannot get ldh in the pad, thus you cannot develop the dowel, thus you cannot use shear friction. On the other hand, if you count on just the bars shearing (is that even valid), then you might get into side cover blowout appendix D issues...

Any thoughts, references, threads? Thanks in advance.

 

[dcarr82775]

I have always wondered about this myself. It seems that with mu=0.6 there is no need to develop the full tension force in the bar. I would think calculating an Appendix D type blowout if less than ldh is reasonable and defensible from an engineering standpoint.

You can always reduce the force by the actual development length / ldh

 

[JAE]

You might be correct that it is dowel action and therefore not so dependent upon the anchorage of the rebar on either side of the crack/plane but the shear friction method in 11.7 doesn't specifically recognize this and the commentary doesn't suggest you can do this.

So it appears that if you are designing within 11.7, you have to develop fy. Otherwise you head over to Appendix D.

 

[PMR06]

Thanks JAE, that was basically my thoughts too. "Otherwise yuo head over to Appendix D"... I cringe at the thought!

 

[BAretired]

Another option is to provide mechanical anchorage at the bend of the dowel, i.e. weld to a cross bar.

BA

 

[hokie66]

Why not place some more dowels internal to the plinth? I don't think this application is what is intended for "shear friction" design.