Does tension tie force translate to horizontal shear at the top of column
Does tension tie force translate to horizontal shear at the top of column
(OP)
The title of my post says it all. Please see attached sketch.
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Does tension tie force translate to horizontal shear at the top of column
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Does tension tie force translate to horizontal shear at the top of columnDoes tension tie force translate to horizontal shear at the top of column(OP)
The title of my post says it all. Please see attached sketch.
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RE: 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
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
RE: Does tension tie force translate to horizontal shear at the top of column
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
BA
RE: 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
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
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
RE: Does tension tie force translate to horizontal shear at the top of column
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
RE: Does tension tie force translate to horizontal shear at the top of column
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
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
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
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
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.
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
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
RE: Does tension tie force translate to horizontal shear at the top of column
Link
Link
This statement, in particular, you will find to be in error. The exact opposite appears to be true.
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
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
RE: Does tension tie force translate to horizontal shear at the top of column
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
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
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
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
.
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
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
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.