Masonry Arch 2

[XR250]

I

could use some input....
These brick arches support an exterior deck. The joists cantilever over the arch about 7 ft. The arch span is about 7'-4"
My service load on the on the arch is about 850 PLF not including the self-weight of the masonry. The masonry wall can be any thickness up to 16”. The Architect wants to see brick on both sides of the wall and wants to avoid seeing a rolled lintel if possible. I think I can talk him out of the flat portions at the base of the arch and using soldier courses above the opening (a necessity). I was considering reinforcing the piers and typing them together above the arch with a reinforced masonry beam that would support the gravity load and take some of the demand off the arch. This would also allow the piers to resist the arch thrust. Looks like NCMA has a TEK article about designing these things.

Do you guys have any better ideas?

Thanks

 

[phamENG]

I think the column and beam idea is the best bet. Be careful about expansion joint locations in there. I did a similar design recently (but we had precast arches) and we had to do several more joints than usual due to the small horizontal aspect ratio of the wall above the arches. It can really screw up your thrust load paths if you try to make modern brick perform as a real arch - especially when you have a long series of them.

 

[Celt83]

I'd tend to shay away from using the brick in any structural capacity and provide either an underlying steel or concrete frame and use a concealed lintel product similar to this one by Halfen: Link

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[phamENG]

Celt83 - those things are neat, but every time I've suggested them I've been laughed at - they have a pretty hefty price tag.

And it all depends on the kind of brick. The building I did used Thru-Wall Structural Brick. Specifically designed to be the primary structural system (grouted and reinforced like CMU) and the exterior finish (clay brick). Granted: that was really expensive, too. But the architect picked that, so it was ok...

 

[nackra]

a brick arch will be constructed different than shown in typical CAD details:

http://www.gobrick.com/docs/default...calnotes/31-brick-masonry-arches.pdf?sfvrsn=0

 

[RPMG]

Nice reference nackra

 

[North2South]

I used a system on a low spring line arch about a year ago. It was only for dead load though so I’m not sure how useful it could be as a structural support.

1. Form the shape of the arch with shoring.
2. Put 2 tapcons into the back of the brick and lay them face down into the shoring/form to make up the soffit.
3. Size rebar for a concrete beam width of the arch and the depth minus about 8” and install rebar.
4. Lay bricks with 2 tapcons up the sides of the arch on each sides up to at least the height of concrete you will need.
5. Pour concrete the depth of your beam.
6. Finish laying bricks up to the top of the arch.

I used CMU columns where the concrete poured into the top course of the cmu column.

Not sure how well it will work for you with your load and application.

 

[XR250]

Thanks for everyone's responses.
I think I should talk to the GC to assess his comfort level with a steel beam supporting a rolled plate or going full reinf. masonry. The wall is 25 ft. long. If I run a cont. concrete beam the length of the wall on top of the arch (using the brick as the form), can I get away w/o using control joints? I am worried that the concrete will shrink and the brick will grow and cause something to explode! (ok, not really, but you get the idea)

 

[phamENG]

I think you'd be better off using grout and designing it as a masonry beam. The wall would have to be really thick to ignore your permanent brick "form" and use just the concrete core as your beam width. And you can be more comfortable about material compatibility within a given cross section of the beam.

You're still going to need expansion joints in the wall. You have a short, long wall sitting on top of whatever you use for the arches. It's going to be prone cracking more often (tighter spacing) than a masonry wall of normal height.

 

[XR250]

Thanks Pham. I should have been more clear, When I said concrete, I was referring to a reinforced masonry beam but ignoring the masonry in the calc.
Was thinking - construct the brick portion of the wall, add ladder wire and rebar as they go up - then grout it solid. Where would you place the joints in this instance?
Would the joints to go thru the grout as well?

 

[jayrod12]

Are there ever expansion joints in a string of masonry Arches? I didn't think so, but maybe I'm wrong. I couldn't tell you where I would want them to ensure the arch performed as intended, maybe directly over the peak?

 

[phamENG]

Sorry - forgot these are 16" thick. 8" wide concrete beam...you might be able to do it. Though I think using the full masonry section will give you a better result.

As for the joints, yes. Everything breaks. You can use a keyed joint or smooth dowels to transfer out of plane loads between panels. I've used this arrangement with keyed joints. I think the architect was able to find an expansion joint material died to match the color of the mortar:

 

[phamENG]

You may not need them at every arch - just have to do the math to figure that out.

 

[jayrod12]

At that location, isn't gravity and the associated thrust going to close those joints long before thermal expansion happens?

 

[phamENG]

Depends on how it's designed. Those arches are pretty shallow, so if they're designed as beams you're probably looking at something closer to pure flexure with minimal arching and therefore minimal thrust.

 

[jayrod12]

Fair point

 

[XR250]

Though I think using the full masonry section will give you a better result.

Pham, what do you mean by that?

 

[phamENG]

I'm not sure I completely understand what you mean by designing a reinforced masonry beam but ignoring the masonry. Build two faces of veneer 8" apart and fill it with concrete? You can do that, but you have to be careful - efflorescence could wreck the brickwork. Or do you mean using ACI 530 but limiting the section to that area that is only grout? I suppose that could work, but seems really conservative.

If you take the whole section, and detail it and analyze it per ACI 530 as a beam, I think'll you get a better picture of how it will behave as opposed to just looking at the core of the wall.

 

[XR250]

Build two faces of veneer 8" apart and fill it with concrete? You can do that, but you have to be careful - efflorescence could wreck the brickwork. Or do you mean using ACI 530 but limiting the section to that area that is only grout? I suppose that could work, but seems really conservative.

Pham,

That is correct - Build two faces of veneer with ladder wire 8" apart and fill it with concrete. I certainly could design it either way using 530 or 318. My demand is small enough and my available depth is large enough that an R/C beam would be easy. Wouldn't efflorescence be an issue regardless of the design methodology. Couldn't it be cleaned?

Thanks for your support.

 

[phamENG]

It can be. There are salts in Portland Cement used in some grouts that can cause it. But - standard specifications calling for low alkaline cements and mix water free of deleterious salts are more common when looking at masonry grout. It's usually not much of an issue in CIP concrete. There's typically going to be a gap between the concrete and a veneer, so there's no direct path for the salts to migrate to the surface. If the concrete is the finished surface, is usually easier to clean. Yes, you can clean it from brick, but depending on the style of brick it may be difficult to remove it from the surface without damaging the brick. If it's bad enough, hazard materials (acids) are required to remove it.

So I think you can do what you're suggesting, but be careful in how you specify the materials and pay close attention to the concrete mix design.