Shear friction - shear in two directions

[smvk3]

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?

 

[CELinOttawa]

Yes.

 

[hokie66]

How do you get shear perpendicular to a shear plane?

 

[CELinOttawa]

I can name only three: EQ, Cantilever loading (under some circumstances), rail bridges (under only some rail loads, particularly braking).

So, Hokie is asking a very good question: Which is it? Otherwise it is not Structural, but Aerospace.

 

[smvk3]

I meant two shear forces, in the same plane, that are perpendicular to each other. I have #5 bent dowels at the top of a basement wall. I am trying to resist two forces; one shear force perpendicular to the wall from the soil back fill against the wall. And another shear force parallel to the wall (from the diaphragm shear force from seismic).

 

[hokie66]

I don't understand your examples, CEL. You can have tension and compression perpendicular to a shear plane, but shear? Maybe the OP is talking about diagonal tension, but that is not handled by the clamping action on which the shear-friction theory is based. Maybe I'm just thick, but I still don't understand the question.

 

[CELinOttawa]

The two forces act on shear within a common plane once applied; They simply appear to be two distinct shearing forces (as in the EQ load on retaining wall here). You're absolutely correct that if the second force's line of action is perpendicular to the PLANE of the shear force, the non-shear force is either tension or compression. That's not the case here.

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.

 

[smvk3]

So when calculating the required area of steel for ACI shear friction method, and you have two shear forces acting simultaneously (and perpendicular to each other), don't you simply take the resultant of these two shear forces to calculate how much area of steel you need?

 

[hokie66]

That would seem somewhat logical to me, but as I don't use shear friction or believe in the concept, I will defer to others.

 

[KootK]

I believe that you're asking about a common basement perimeter wall to ground floor slab connection SMVK3. If I'm wrong, please forgive me and ignore what follows.

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.

 

[CELinOttawa]

Kootk: The bar in the "knuckle" of the hook has been shown to add nothing to the strength of the connection, and has in fact on occasion reduced experimental test ultimate loads. I've posted the paper by Restrepo et al. on Eng-Tips before. It IS very useful for tying, but does nothing to improve the strength of the connection.

 

[KootK]

Yikes, if that's true, it's going to be very unfortunate for me. Searching here turned up one reference to Restrepo:

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.

 

[hokie66]

CEL,
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.

 

[KootK]

I had the same thought about the stirrup code provisions Hokie. Ditto for the strut and tie provisions. Of course, this wouldn't be the first time that testing made a mockery of my structural intuition.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[CELinOttawa]

No, No... Don't over think this... Not for stirrups around longitudinal bars (which are a 90 deg hook and very tight), but for hooks into concrete (J-hooks, L-hooks, etc) which go from one concrete mass into another (typically with a "loose" condition where the steel is not tight other than by ties).

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/getfil...ile=TensileCapacityofShortEmbedConnectors.pdf

Kootk: You couldn't find it because I didn't mention Restrepo by name in that thread. Naughty CEL.

 

[TXStructural]

The cited NZ research appears to have missed Jirsa's observation. On a T or L configuration, the hook is IN the compression zone created when, say, a cantilever retaining wall tries to bend and pull out the hooked bar. But in any case, it is the inadequate development of the hook that is the culprit. Other research into hooked bar development finds that splitting along the plan of the hook is the typical failure mode, so restraining such a crack with a crossing bar inside the hook is good practice.

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.)

 

[CELinOttawa]

Interesting TX! Do you have an appropriate reference to the Jirsa paper? Can you post a copy? Well keen to read it!

 

[KootK]

Thanks for your supplemental comments here TX & CEL. I finally got around to reading the research paper.

@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.

 

[CELinOttawa]

It is not a reduction, just that it did not aid in increasing the failure load. I'll have to re-read the paper tomorrow and follow up.

 

[CELinOttawa]

Okay, so here it goes (note: This discussion had induced a touch of insomnia, so I looked it up):

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*