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Variable Shear Force (Not a variable section)

Variable Shear Force (Not a variable section)

Variable Shear Force (Not a variable section)

(OP)
I have had to model a wall panel in FE software, and I'm looking at the shear stress maps. At the lintel over an opening, the FE shear force map gives a reasonable looking output, but the shear force in the cut through the lintel is not constant --- it varies by about 20% from bottom to top (29kN/m to 36 kN/m). So I am wondering if I need to / how I would properly calculate the shear stress in that section. The lintel is 500mm deep, so I've gone ahead and calculated the shear force in the cut as 36*0.5, but I suspect that's conservative; and I wonder if taking an average is a bad idea because part of the lintel could be over stressed.

RE: Variable Shear Force (Not a variable section)

If you have the ability to integrate the shear stress across a section, I'd use values obtained that way which should be closer to an average. What material are we talking about here? Concrete?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Variable Shear Force (Not a variable section)

(OP)
I'm still getting into Robot, so I'm not sure how I'd integrate it properly. I suppose I can get a cut through the section to plot the forces; or I could just assume linearity from 29 to 36 kN/m. But is an average okay then? Checking that the section is okay for 33kN/m shear is okay, even if a part of that is going to be larger? (granted, I understand I won't be designing for such efficiency that it should fail near these loads...)

The material is CLT -- which I appreciate is a challenge for modeling in FEA software like Robot.

RE: Variable Shear Force (Not a variable section)

I don't understand the approach of taking the trouble to model something like this in Finite Elements in order to better understand the unique stress patterns, only to then take an average over a section, thus ignoring those unique stress patterns.

Why don't you just model the lintel with stick (2-node) members and use that?

Using the average shear stress might ignore a critical hot spot where the material might "unzip" and progressively fail.

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RE: Variable Shear Force (Not a variable section)

CLT? Fun. I'd think that an average would be fine. The vertical laminations should serve to drag the shear force out fairly uniformly over the section. Regardless of the software that you're using, surely you can get the support reaction at the end of the lintel. Why not just divide that value over the area of the section to get an average shear stress?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Variable Shear Force (Not a variable section)

(OP)
Thanks for the feedback. I'll take it into consideration as I go forward.

The model is a bit more complex than this, including a full panel with cutouts, and lifting points for a crane, hence the Fea model rather than stick and node.

RE: Variable Shear Force (Not a variable section)

You wouldn't expect shear force to be uniformly distributed over the cross section, and most codes account for this. For example, in calculation shear resistance of the web of a W section for the steel, the shear stress is limited to 0.66Fy. This is because the max shear stress on a rectangle like the web is 1.5x higher than the average. I'd feel fine just averaging it like koot suggests if there is a similar derating of shear capacity in whatever code you are using.

RE: Variable Shear Force (Not a variable section)

Quote (canwesteng)

For example, in calculation shear resistance of the web of a W section for the steel, the shear stress is limited to 0.66Fy. This is because the max shear stress on a rectangle like the web is 1.5x higher than the average.

Actually no. 0.66Fy reflects the expected shear yield stress of the material (Von Mises / Tresca formulation). We do, in fact assume that shear stress is uniform on the webs of steel wide flange beams.

That said, I had the same question regarding the 1.5, peak stress factor here. Here's what I know:

1) You use the 1.5 factor when designing for out of plane bending.

2) You don't use the 1.5 factor when designing diaphragms for in-plane bending.

I can't say that I've actually seen an example where a CLT header has been designed for in plane shear. Even the CLT handbook seems to be silent on that particular issue.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Variable Shear Force (Not a variable section)

Isn't it 0.6 from Von mises (or sqrt(3) which is almost .6)

RE: Variable Shear Force (Not a variable section)

Yep. 0.5 to 0.577 I think depending on your yield criterion of choice.

I've decided to cast my vote for applying the 1.5 factor to the lintel shear stress. With no "chords" and flexural resistance distributed throughout the height, that makes sense to me. I figured looking at shear wall panels would yield the answer as they typically don't have discrete chords either. Couldn't find a damn thing. My Googling must be getting weak.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Variable Shear Force (Not a variable section)

Yikes. Guess I need to brush up on my steel stuff

RE: Variable Shear Force (Not a variable section)

The maximum shear stress for a rectangular wood beam (which is 1.5 times the average shear) is supposed to be used when comparing it to the allowable shear stress. The derivation of the 1.5 factor for a rectangular beam is in basic strengths of materials books.

If you are using FE, I would use the maximum stress from the analysis to design it but then I would check it with basic line elements with the 1.5 factor to see how far off the stress from the FE analysis was.

RE: Variable Shear Force (Not a variable section)

(OP)
One little bit I have since learned: you can use the Panel cuts option in Robot to cut through the beam, and the Cuts (tab)> Diagram Analysis (button) to see the integral value of the shear stress over the cut region.

Interesting enough, when I had FE meshing set to 0.20m, and used the maximum shear value to calc the shear force over the interface, I got the same result as the integral value when I reduced the mesh size to 0.10m, even though reducing the FE mesh size results in larger peak shear values! This is slightly reassuring...

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