Brick Relief Angle 1

[dylansdad]

There are probably posts in the forum that answer this ?, but my search has been for naught.
The attached sketch shows a typical relief angle for brick veneer. The question is how is the
tension calculated for the screw bolt? Is is simply done by resolving moments about the tip
of the 5x5 angle, using a moment arm at say 1/2 the outstanding leg? Should prying be considered?
Thanks in advance.

 

[phamENG]

It won't be about the tip. What is it anchored into? Concrete? You need to determine what your bearing stress can be, and then either set up a system of equations or use trial and error to to balance the distributed bearing stress in the concrete with a tension force in the bolt.

Prying is a good question. I don't think I've directly considered that before in this kind of connection. The SCM equations in section 9 wouldn't help you all that much - I think there was an article in Modern Steel some time ago that gave alternative equations based on bending stresses in the fixture. Google should bring it up pretty easily. "A Quick Look at Prying Action" or something like that.

 

[dylansdad]

phamENG,
Thanks for the link.
It is 5000 psi concrete.

 

[phamENG]

Ok. Don't use 5000psi. Use the bearing stress equations in ACI 318.

 

[masonrygeek]

You might be having some problems locating information on this since they are termed "shelf angles" and not relieving angles. See this article on terminology:

https://www.structuremag.org/wp-content/uploads/2014/08/C-StructDesign-Trimble-May091.pdf.

And here is a recent paper on the subject:

http://canadamasonrydesigncentre.com/download/13th_symposium/085-Dillon-Paper-1.pdf.

This provides a better model for analyzing shelf angles than is currently out there.

 

[phamENG]

Thanks for posting that, masonrygeek. I was playing with the same idea for my thesis last year. Glad I'm going another route.

 

[cliff234]

Be sure to take into account the 1.75" edge distance when you compute the allowable tension strength of the bolt. That's a pretty small edge distance. Also, you might want to consider lowering the bolt a little more in order to avoid hitting what I assume will be a top bar in the slab running parallel the slab edge.

 

[JLNJ]

Work your way through the load path. Each element needs to work at each step of the way. The horizontal angle leg bends, so you need to check the bending stress. It is concentrated where the angle fastens to the concrete.

Once the force is collected at the anchors, the brick weight and its eccentricity creates a moment. That moment is resolved by the couple of the angle leg in compression against the concrete and the tension at the screw. The distance for the force couple depends on the bearing stress at the tip of the downturned leg. The compression will distribute over a fairly long length of the angle. The distribution of two-way stress in the angle is quite complicated. There is no prying per se.

My first impression (without knowing the brick height) is that your angles is pretty thin and you anchors are too small and too close to the top (free) edge.

 

[r13]

I suggest to use equations for reinforced concrete design. Do it both ways - ASD (if you know how), and USD. Both should yield similar result.

 

[RobertHale]

I agree with JLNJ. I generally size the leg of the angle based on cantilever bending of the outstanding leg. Then I back into an anchor spacing based on a preliminary spacing assumption (in most cases 4'-0"). I calc the anchor shear based on the assumed spacing and use the interaction equation to find a maximum ultimate tension for the anchor. From there I assume there is an effective bearing width that spreads at 30 degrees from the anchor in each direction. I use that width with the maximum ultimate tension and the bearing strength to find the moment arm. Then I use that moment arm to calc a phi*Mn of the anchor. You can then use the ultimate moment per linear foot from the design of the outstanding leg to find a maximum spacing of the anchor. So long as it is larger than the assumed spacing I move on and check the bending of the angle between the anchors. The bending between anchors tends to be proforma and my gut says that this behavior is only seen during a partial construction since arching action should occur once the veneer gets higher. I will be honest and say that I pulled the 30 degrees out of thin air. I would think it is a conservative assumption especially considering I tend to put the root of the angle at the bottom.

Robert Hale, PE

 

[dhengr]

Dylansdad:
RobertHale has pretty well spelled out the design process and thinking for a shelf angle. Follow the load each step of the way into the structure and to the ground. The horiz. leg of the angle should not project out beyond the face of the brick. Rather, that joint should be caulked and flashed for waterproofing, and there should be a vert. moisture barrier on the outer face of the backing wall structure. Why not add a course or two of brick and turn the angle with the vert. leg up so as to hide the angle and the edge of the conc. slab?