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Does tension tie force translate to horizontal shear at the top of column
2

Does tension tie force translate to horizontal shear at the top of column

RE: Does tension tie force translate to horizontal shear at the top of column

That is a classic truss analogy, or strut and tie problem.  Trusses have axial forces, and the horizontal force is taken by the tension tie / transfer girder.

RE: Does tension tie force translate to horizontal shear at the top of column

Will it be L2 tie-in that keeps this thing happy for overturning?




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

RE: Does tension tie force translate to horizontal shear at the top of column

(OP)
Because of the cold joint, does the joint need to be checked to allow for adequate transfer of forces?

RE: Does tension tie force translate to horizontal shear at the top of column

The answer to your question is yes.

The "Y" shaped support will do an upside down leg split without some means of attaching the top of the "Y" to the horizontal transfer beam.

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RE: Does tension tie force translate to horizontal shear at the top of column

See attached. The way you show it, shear has to be checked along the cold joint. Roughening the surface helps. To avoid the shear check, you could cut off the columns normal to the axial force either as shown or if preferred, recessed into the beam.

BA

RE: Does tension tie force translate to horizontal shear at the top of column

I agree that the cold joint is no good oriented that way. I would "tooth" the struts into the tie beam, so that the struts bear normal to their axes.

RE: Does tension tie force translate to horizontal shear at the top of column

For reasons of constructibility, I think that the cold joint should be exactly where slick deals showed it. That's unless it can't be made to work by that method that shall not be named.

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

RE: Does tension tie force translate to horizontal shear at the top of column

Drawn to scale, I suspect that accidental moment transfer between beam and wishbone will create a diagonal tension shear demand in the columns that will need attention regardless.

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

RE: Does tension tie force translate to horizontal shear at the top of column

(OP)
The original construction joint was shown perpendicular to the axis of the column. Our contractor friends thought we were crazy and put it horizontally.

RE: Does tension tie force translate to horizontal shear at the top of column

The dream was much greater but the level of preaching (by me) is about the same. Last digression Slick, I promise. I'm in real danger here of inadvertently comparing my achievements to the champion of civil rights.

Anyhow, as you can see, I side with your contractor on this one.

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

RE: Does tension tie force translate to horizontal shear at the top of column

If there's shear at this interface, what if we move one foot down into the brace? Is there shear along that horizontal plane that we should be designing for?

RE: Does tension tie force translate to horizontal shear at the top of column

@Mark: no, not if we're sticking with an axial load only truss model. The vector sum of the shear across the cold joint and the vertical beam reaction would yield a resultant force parallel to the wishbone columns.

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

RE: Does tension tie force translate to horizontal shear at the top of column

So there's shear at the interface, but no shear 1" away from the interface? How does that work?

In this instance what we're calling a 'shear', isn't actually a shear in my opinion. It's not a shearing action. It's just the horizontal component of our inclined axial load in the brace. Say we had a load directly over the waypoint instead, so we'll take out any sort of beam action in the tension tie. Load goes applied straight down and then converted into an axial load in the brace and a tie force in the tension tie. There's no shear between tie and brace there.

RE: Does tension tie force translate to horizontal shear at the top of column

slickdeals,
I don't think you are crazy. I think those who allow inclined joints in columns/struts are crazy. Unless the clamping force is a reliable prestressed force, I don't buy it.

RE: Does tension tie force translate to horizontal shear at the top of column

Any column in pure axial compression will have shear stresses acting on every plane except the plane normal to the axial compression. This is true throughout the height of column. The Mohr's circle indicates maximum shearing stress at an angle of 45 degrees to the column axis.

If a cold joint is formed anywhere along the length of the column at an angle other than normal to the column axis, the shear stress must be considered at that joint. Otherwise, with adequate ties, the column will fail when it reaches its ultimate strength (including the contribution of concrete and longitudinal steel).

BA

RE: Does tension tie force translate to horizontal shear at the top of column

Quote (MarkHirschi)

So there's shear at the interface, but no shear 1" away from the interface? How does that work?

More precisely, there would be no shear about a section taken at a right angle to the longitudinal axis of the column. For sections taken at all other orientations, there would be shear. The vector sum business that I mentioned previous is important here too.

Quote (MarkHirshi)

Say we had a load directly over the waypoint instead, so we'll take out any sort of beam action in the tension tie. Load goes applied straight down and then converted into an axial load in the brace and a tie force in the tension tie. There's no shear between tie and brace there.

There is shear between tie and strut, at least according to most engineers' definition of shear: a force tending to cause slip across two parallel planes.

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

RE: Does tension tie force translate to horizontal shear at the top of column

@BA:
Given that, then shouldn't we be checking 'shear friction' across those exact same planes when poured monolithic? We just get a stronger value of shear friction based on the larger cross section and higher friction coefficient?

You wouldn't expect it to govern because you have a larger surface area to consider, plus an induced normal force to add compression. But if you apply the shear friction limits provided in the code, you're still going to fail every time.

RE: Does tension tie force translate to horizontal shear at the top of column

@KootK: Okay, so a shear at some plan other than normal to the axial member's axis. If this were a column oriented vertically with only axial load, no shears, no moments, no slenderness concerns, would we be checking that?

RE: Does tension tie force translate to horizontal shear at the top of column

If you can stomach the read Mark, I believe that all of your questions will be answered by these two threads that I initiated earlier this month:

Link
Link

Quote (MarkHirschi)

But if you apply the shear friction limits provided in the code, you're still going to fail every time.

This statement, in particular, you will find to be in error. The exact opposite appears to be true.

Quote (MarkHirschi)

Okay, so a shear at some plan other than normal to the axial member's axis. If this were a column oriented vertically with only axial load, no shears, no moments, no slenderness concerns, would we be checking that?

Only if there were a diagonal cold joint across said vertical column, like there is at the top of the wishbone struts in Slick's example.

@Slick: while I agree with your contractor about how this bent should be constructed, I certainly do not think that you're crazy. Hopefully my comments haven't read that way.

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

RE: Does tension tie force translate to horizontal shear at the top of column

A cold joint introduces a plane of weakness which is not present in a monolithic pour.

There have been a lot of compression tests performed on tied and spirally reinforced columns over the years. The mode of failure does not suggest a need to check shear-friction in monolithic columns.

BA

RE: Does tension tie force translate to horizontal shear at the top of column

Does nobody else object to the whole concept of shear friction? To me, it is like a passive rock anchor, in that for the clamping force to be activated, there must be movement/elongation. In a concrete joint, you don't want that to occur.

RE: Does tension tie force translate to horizontal shear at the top of column

I stand beside KootK in my pro shear friction stance. I am not quite as excited about it as he is, but I still think it is a valid approach. Different topic for a different post.

And yes, there is shear at the joint that needs to be handled.

RE: Does tension tie force translate to horizontal shear at the top of column

To some extent, I share hokie's objection to shear-friction in the usual case where the main reinforcement is normal to the cold joint or the angle is in the direction of the shearing force.

In this case, I have a stronger objection. The longitudinal column reinforcement is in the wrong direction to provide shear-friction across the cold joint. They will tend to break out of the column. In this case, I would not be willing to rely on shear-friction.

BA

RE: Does tension tie force translate to horizontal shear at the top of column

@KootK: Maybe I'm misunderstanding things if the opposite is true.

Point out where I'm going wrong here:

24"x24" tied axial member, 4000 psi
Ignore Phi factors for simplicity (the difference is large enough that they won't matter)

Compression strength, ignoring rebar:
0.8*0.85f'c*Ac = 0.8*0.85*4 ksi*24"x24" = 1567 kips

Translate that into a 'shear friction' along a 45 degree plane (or any other plane over 20-25 degrees from normal):
1567 sin 45 = 1108 kips

Calculate out shear friction capacity along that plane using the maximum allowed for shear friction under ACI 318-05 (11.7.5):
Max = 0.2f'c*Ac = 0.2*4 ksi*24"x24"/sin 45 = 652 kips
OR
Max = 800Ac = 800*24"*24"/sin 45 = 652 kips

Results in a major deficit, under 60% of the 'required' if the shear friction mechanism actually applies here. Way larger than the difference in Phi factors (0.65 compressions/0.75 shear = 87%) would suggest.

If my math is correct above, we should be seeing this failure all over the place. But we don't. Why?

RE: Does tension tie force translate to horizontal shear at the top of column

@ Mark,

I don't see anything wrong with your assessment; it's an interesting case study that I hadn't considered explicitly in the other threads. Here's my critique of your stuff:

1) By choosing to examine a member loaded to it's squash load, you've narrowed the range of members over which your example would be representative. Most members will be subject to considerably less axial load and, as a result, the limits on maximum shear stress will become less important.

2) We're several miles off the reservation here now that we're discussing hypothetical, monolithic shear planes. In this context, I don't feel that the maximum shear stress provisions apply. The limits given in ACI are only there because, beyond those limits, the shear friction equations "may become un-conservative in some cases". It's not as though there isn't more capacity to be had; it's simply not predicted accurately by the design equations.

If you lift the limits on maximum shear stress, ignore the fact that the capacity equations may be bunk, and consider only the clamping force provided by the compression imposed on the column, things work out like this:

P = axial force in column.
V = P x sin(45) = 0.71P = shear on 45 degree plane.
A = V = P x sin(45) = axial force on 45 degree plane.
Vsf = 1.4 x A = 1.4 x V = 1.4 x P x sin(45) = 0.99P = unfactored shear friction capacity.

So Vsf / V = 0.99P/0.71P = 1.40. Always and forever okay. And this doesn't even account for any contribution accruing from the presence of reinforcing steel.

Keep in mind that I truly do not know what I'm doing with this. If you read my other two threads, you'll see that in spades. I've given it a lot of thought, and I have some strong opinions on the matter, but I do not have the answers. I've been inadvertently tagged a shear friction "fan" because I whine about it endlessly. I'm not a big fan of shear friction. Rather, I'm a big fan of trying figure out what heck I'm supposed to be doing with shear friction and why.

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

RE: Does tension tie force translate to horizontal shear at the top of column

@BA/Hokie: I think that we're in agreement that shear friction reinforcing in compression is questionable. ACI 318 agrees. R11.6.4.2 reads:

Quote (ACI 318-11)

...should only be used when the shear force component parallel to the reinforcement produces tension in the reinforcment...when alpha is greater than 90 degrees, the relative movement of the surfaces tends to compress the bar and equation [whatever] is not valid
.

That being said, a substantial portion of the clamping force in this situation will come from the load itself. And that contribution is 100% reliable because, without the load, there's no shear demand in the joint. That, in part, is why I am comfortable using shear fiction here.

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

RE: Does tension tie force translate to horizontal shear at the top of column

Quote (slickdeals/OP)

The original construction joint was shown perpendicular to the axis of the column. Our contractor friends thought we were crazy and put it horizontally.

Your contractor friends had no business making such a change without consulting you. If there is any doubt about the capacity of the cold joint to resist shear, remedial measures may be required. Cost to be borne by the contractor.

BA

RE: Does tension tie force translate to horizontal shear at the top of column

Do agree that the shear friction limits provided in 11.7.5 of 318-05 appear to be extremely conservative for members with any significant compression. Until your angle gets pretty extreme, your clamping force is fairly huge. Even before you look at the effects of any longitudinal reinforcement or ties crossing the plane.

There's been a few articles in the ACI journal that note this. ACI 318-08 increased the limits a bit, but still fall short of some of the recommended limits I've seen in journal articles (including one of the articles specifically referenced by the commentary on shear friction limits, which is odd). Additionally, the increases appear to be directed more at increasing the upper limit based on research with high-strength concrete, but don't really address further increases you can get when you've got a significant clamping/compressive force as well.

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