concrete dowels in shear with smooth slip plane

[samdamon]

Is it feasible to design a doweled connection between new and existing concrete, that is carrying shear, where the plane between the two placements is very smooth? I have an existing condition that is creating this issue. Shear friction concept would not apply since there is little or no amplitude in the face of the older concrete. I also do not want to roughen it for several reasons. In this case is it reasonable to rely just on shear resistance of the dowel steel, assuming that the dowel is embedded sufficiently? ACI 318 does not appear to mention this concept. Any one ever done this? Shear forces are not large, 1 to 2 kips of factored shear per dowel.

 

[hokie66]

Yes, you can do this if the forces are not large. Place the dowels at the centre of the member depth. Use deformed bars, as you don't want the joint to slip.

 

[dcarr82775]

Shear friction in ACI318 specifically includes surfaces not intentionally roughened. 0.6 factor applies (can't think of the Greek name for the symbol)

 

[Ron]

Agree with above...dowels do the work, not contact friction.

 

[msquared48]

Not true in all cases Ron.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Ron]

Mike...you're right, but I think in this case with a smooth face, the dowels will do the majority of the work.

 

[hokie66]

I still think shear friction is voodoo. Just me. I'll take good old bearing anytime.

 

[rscassar]

If you want the section to act compositely than you will need to provide drill and chemical epoxy pins into the concrete substrate. The calculations for shear friction possible will depend on what debelopment you can get across the shear plane. In this past I have provided both measures when adding to a concrete cross-section (dowels, scabbling the surface amd providing a bonding agent).

 

[dcarr82775]

Just my opinion, but I have always thought of the 0.6 factor in skin friction as more or less saying it is all dowel action in the steel. 0.6Fy is shear strength of steel per AISC manual. This may be a coincidence, but one that works well in my mind for describing what is actually happening with un-roughened surfaces. I have no problem using it per ACI318

 

[ishvaaag]

The simplification brougth by dcarr may be coincidence, because in a shear friction context the reduction would only mean that the bars need to take the higher brunt of less inclined -respect to the joint- compressive struts due to the -assumed- lesser available friction angle. It also would inmmediately bring to mind how the concrete could sustain in bearing the capacity of the dowel in shear, and that would remit us again to some kind of shear-friction concept.

Of course I would prefer like hokie some sound support, but there are bunchs of people out there promoting for the most variegated reasons that we sometimes proceed otherwise; in any case his observation is very well brought because it is too much a frequent cause of failure any kind of lack of bearing at supports.