Shear friction - shear in two directions
Shear friction - shear in two directions
(OP)
When calculating the shear friction demand for an interface with shear both parallel and perpendicular to the shear plane, do you just take the resultant of the two forces ( sqrt(Vperpendicular^2 + Vparallel^2) ) and use that value to calculate the required area of the steel?






RE: Shear friction - shear in two directions
RE: Shear friction - shear in two directions
RE: Shear friction - shear in two directions
So, Hokie is asking a very good question: Which is it? Otherwise it is not Structural, but Aerospace.
RE: Shear friction - shear in two directions
RE: Shear friction - shear in two directions
RE: Shear friction - shear in two directions
SMWK3: You're likely overthinking the problem. You should design the wall to not need the dowel shear force for the retaining wall component. It is not wise to design a retaining wall with V* > 0.5 Phi Vc. The shear demand should be one of the parameters that sets the thickness. IF for whatever reason you cannot prevent this then make sure you are using an appropriate load combination. You won't be expected to combine full live, etc, with your EQ load. In that case again, yes, you can combine the shears and address the total shear. Just bear in mind this isn't all that realistic, as you won't be retaining soil which is also undergoing EQ actions; The soil will be moving, greatly increasing your load against the wall where the wall is perpendicular to the EQ, and virtually eliminating the load where the wall is parallel. That's the best of my understanding of the situation, but I'm no Geotech.
RE: Shear friction - shear in two directions
RE: Shear friction - shear in two directions
RE: Shear friction - shear in two directions
You're 100% correct, you will have earth retention loads acting perpendicular to your wall simultaneously with the lateral loads that will deliver in-plane shear to that same wall. Hence the bi-directional shear friction issue at the connection to the slab. I've considered this myself and, to my knowledge, there is no directly applicable code guidance.
Technically, I agree with the vector sum approach for determining Avf and Ac_min. In practice, I go with a straight sum.
One thing that you may want to consider with this connection is that, per code, you can only use bars for shear friction if they are fully developed on either side of the shear plane. Pro-rated, partial development is not allowed. And, depending on your slab thickness, full development may be hard to achieve. I don't personally agree with this provision. As a workaround of sorts, I limit myself to #5 bars or smaller and place at least one continuous longitudinal reinforcing bar in the knuckle of the hook.
Kudos to you for investigating this. In my experience, most designers do not.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Shear friction - shear in two directions
RE: Shear friction - shear in two directions
How Harmful Is Cold Bending/Straightening of Reinforcing Bars?
Document: CI2104Restrepo
Author(s): J. I. Restrepo, F. J. Crisafulli
Publication: Concrete International
Volume: 21
Issue: 4
Keywords: embrittlement; reinforced concrete; reinforcing steel; strain rate; temperature; tension tests
Date: April 1, 1999
Does that sound familiar? If not, do you remember anything of the title?
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Shear friction - shear in two directions
Are you saying that paper shows that stirrup hooks around longitudinal bars are not effective? Hard to believe, as it goes against conventional practice and codes.
RE: Shear friction - shear in two directions
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Shear friction - shear in two directions
Effectively this issue affects starter bars and other bars of short embedments. Here's the thread it was posted into: http://eng-tips.com/viewthread.cfm?qid=357585 Direct link: http://files.engineering.com/getfile.aspx?folder=c...
Kootk: You couldn't find it because I didn't mention Restrepo by name in that thread. Naughty CEL.
RE: Shear friction - shear in two directions
We do have new research underway about hook lengths, since this has not been reevaluated thoroughly in the modern age... since we went from grade 33 or 40 to the more typical grade 60. There is also some examination of the development and hook issues associated with higher strength bars (grades 80 and up.)
RE: Shear friction - shear in two directions
RE: Shear friction - shear in two directions
@CEL: I couldn't find any suggestion that a transverse bar reduced pullout capacity. Can you direct me to it? All that I found was a blurb on page 32 where they mention that, in Jirsa's research, they didn't find any increase in pullout capacity as a result of transverse bars.
@TX: This paper seems to have been written after Jirsa's work. It makes reference to Jirsa's work and specifically tests Jirsa's compression restraint hypothesis, to no avail. Refer to the last paragraph on the 19th page of the PDF.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Shear friction - shear in two directions
RE: Shear friction - shear in two directions
Section 3.4.2 - Page 19 at bottom: "No major difference in strength or load-displacement behaviour were noticed between the specimens in which the failure surfaces were restrained or not restrained by external compression reaction []"
Page 20, at the top: "This recorded behaviour contradicts Johnson and Jirsa's observations. The main reason appears to be that any enhancement of the load carrying capacity should be given as a proportion of the ratio between the effective embedment length and the distance between the origin of the pull-out cone and the restraining plane, instead as a proportion of lever arm distance alone as they proposed."
Page 25, at the top: " The good post-elastic behaviour of these tests is due to the effect of the transverse bar in contact with the inside of the hook. [] this bar kinked and prevented the hooked bar from totally pulling out. Similar behaviour was also reported by Johnson and Jirsa (1981). They also pointed out that the transverse rod had no enhancing effects on the cone pull-out strength."
Note that post-elastic behaviour IS improved by the presence of the transverse bar, but this is not of any design use - Simply a good thing to keep doing for tying the steel anyway. No LOAD increase, but I owe you all a profuse apology for saying that there is an occasional reduction - That's what I remembered, but reading the paper shows my recollection was quite wrong indeed. Sorry all! *blush*
RE: Shear friction - shear in two directions
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Shear friction - shear in two directions
RE: Shear friction - shear in two directions
Quando Omni Flunkus Moritati
RE: Shear friction - shear in two directions
One of my favourite references is Ketchum! lol...