Does tension tie force translate to horizontal shear at the top of column 1

[slickdeals]

The title of my post says it all. Please see attached sketch.

 

[hokie66]

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.

 

[KootK]

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.

 

[slickdeals]

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

 

[JAE]

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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[BAretired]

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

 

[hokie66]

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.

 

[KootK]

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.

 

[KootK]

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.

 

[slickdeals]

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

 

[KootK]

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.

 

[MrHershey]

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?

 

[KootK]

@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.

 

[MrHershey]

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.

 

[hokie66]

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.

 

[BAretired]

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

 

[KootK]

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.

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.

 

[MrHershey]

@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.

 

[MrHershey]

@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?

 

[KootK]

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

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.

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.