Strut and tie - fanning strut within kern distance of pier

[canwesteng]

I've got a situation where I'm tying a deep concrete girder into some existing piers. It's a strut and tie type of situation - in my opinion as long as I develop the bar into the existing pier enough to get my node into the kern distance of the column, I can just use a a fanning strut to distribute the load into the pier, and from there just design the pier with interaction diagrams, but interested to hear if I'm making a huge mistake. The strut and tie design also seems to address the issue of interface shear between the girder and existing pier - I'm likely doweling bar in there anyway for peace of mind but don't see how it is strictly necessary. Sketch is below with a basic concept of the S+T diagram, forces OME as ~300 kips in the tie.

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[KootK]

1) Fundamentally, I don't disagree with your approach.

2) Keeping the beam reaction within the kern of the piers is a nice touch although, in my opinion, not strictly necessary.

3) As you've rightly identified, a key aspect of this is the shear friction transfer between the beam and piers. My thoughts on that:

a) In general, I feel that this is a pretty aggressive use of shear friction. I'd prefer to see some kind of keying action.

b) I agree, the compression in the struts enhances vertical shear friction at the struts, even without doweling.

c) Yes to doweling and roughening as you want the vertical interface to start shear grabbing before it starts shear slipping.

d) In my mind, you ideally want to get the shear friction job done across the width of the strut +/- per the sketch below.

 

[KootK]

I find something like this to have a visceral appeal:

1) Lots of keying action.

2) Minimally invasive to the piers.

3) Produces a lateral offset between the struts and the ties. Here, I think that is surmountable with the semi-circle u-bars and net benefit.

 

[Robbiee]

Devolving rebars into a pier using epoxy to resist 300kips is questionable, I think!
With the spacing and the edge distance reduction factors, it could be impossible, unless your beam and piers are really wide.
I wonder why you are not bringing the beam down to the footing! do you need the full gap in between? Are you bridging over something?

 

[canwesteng]

Good feedback,

I'm thinking within the strut itself, I'm allowed phi*1.0*(compressive force across the plane), which works out to not needing reinforcement as long as the strut is 45 degrees or shallower. I just need to make sure the strut is detailed so that max shear across the joint per ACI isn't exceeded. In any case I'll add dowels for some percent of the force.