Punching Shear 1

[rlmengineer]

I am looking for information on calculating punching shear on a non-reinforced masonry wall. In calculating the load capacity of a soil anchor used to tie back a basement wall, the capacity of the wall due to the anchor plate punching through the wall is what I need. I have scoured the internet and research papers, but have not found anything. Any information would be appreciated.

 

[KootK]

Ugh. I don't envy you this one. I might treat it as one way shear over a couple of cells. I'd be hesitant to rely on two way distribution much as I don't have a ton of faith in head joints. Hopefully the anchor plate can be made to span over at least two vertical webs. Otherwise, there would be potential for punching shear on just the face shell.

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.

 

[rowingengineer]

Normally a wailer spreading the lkad would be used

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[dhengr]

Rlmengineer:
I agree with KootK, the anchor plates have to be at least large enough so you can’t punch through a face shell. And, they should be located in such a way that they bring several cross shells into play, and even better several blocks both in terms of the block run length and in terms of several courses high. Otherwise, you have to use some more significant load spreading device such a Rowingengineer suggests. A light stl. channel, 3-4' high can be hidden nicely in an exterior found. wall finishing scheme. Then, you are left with making some engineering judgement about the gravity load on the wall (bed joint friction) and bond strength of the joints; so that you don’t just pull one or two blocks in a course, or three blocks in two courses out of the wall. It probably won’t be quite as clean a break as that, but that’s conservative. I would likely only consider the two bed joints on each block.

 

[CANPRO]

You might want to look at the allowable tension values for Hilti's HY-70 (ICC report linked below). They have values for pullout in unreinforced masonry. Not exactly the same thing as you're looking for, but a similar application and might be a good start to get a feel for the expected values.

http://www.icc-es.org/Reports/pdf_files/ESR-2682.pdf

 

[hokie66]

With an unreinforced masonry wall, I can't see punching shear as a realistic mode of failure. There is very little flexural strength, so flexural failures would predominate.

 

[rlmengineer]

My thought is the blocks would not necessarily crush, but the mortar joints of the blocks supporting the plate would shear along the red line as depicted in the attached graphic.

 

[jayrod12]

I'm with hokie. It's likely that you would have bending failure of the wall before you popped out a chunk of 8 or so blocks.

 

[KootK]

@OP: Can you turn that plate 90 deg? As shown, you'll be delivering load between pairs of side by side blocks and relying on the shared head joints in flexure.

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.

 

[WARose]

The 1991 edition of '246 Solved Structural Engineering Problems' (by Buckner) has a (somewhat) similar problem (i.e. Masonry-problem #7). Except in that case, it is a vertical load that is (eccentric to the wall) that is being transferred by a steel bracket connected by embedded anchors. The "punching shear" aspect of the problem comes in when the analogy is made between the bracket moment transfer and a slab moment transfer at the edge of a flat plate: part of the transfer will be through the eccentricity of shear.

In your case, this may not be applicable (as it may all transfer as a shear). The critical perimeter for the shear is the bracket size + d (per side). So that assumes the bracket is stiff enough to do/engage that (you should be sure your plate or whatever is). The allowable shear stress used is the normal allowable for flexural stress in masonry. The problem doesn't really check the anchorages (in masonry), but that is simple enough (per current codes). (Another difference is: the problem involves a (vertically) reinforced wall.)

 

[BAretired]

I would not count on any punching shear strength for an unreinforced masonry basement wall. Use a waler full height inside the wall.

BA