Brick Arch

[meca]

I am building a home, and have a brick arch over an entry way. Due to a mistake, the arch is wider and flatter than originally intended. I don't have any experience with masonry, and was looking for some guidance on how to confirm that this arch is adequate to support the weight of the brick and stone above it? Here is a picture:

http://www.mecaenterprises.com/downloads/arch.jpg

 

[kslee1000]

Analyze the arch with loads, and assume pinned on top of the columns. Watch out the thrust at support.

 

[hokie66]

The one over the upper storey window and the one over the entry doors both look in proportion. The one over the entryway, however, looks like there is not much to resist the thrust. I wouldn't accept it.

 

[meca]

I design steel frequently, but I'm not sure how to analyze this situation. I can model an Arch in StaadPro, but I'm not sure what material and section properties to use to simulate the arch? Once I determine the thrust load, I'm not sure how to determine if that is acceptable.

Hokie66, what are my options, to tear it out and start over?

 

[dik]

Do a web search for masonry arches and 'ring'. This is a program that was developed in England for checking historic masonry arches.

Dik

 

[kelowna]

It could be very messy, but if you could drill from the side of the arch, end to end through the wall and the ach, then you could put a rod, end plates, and tension it, eliminating the thrust problem.

It is the only solution I can think of appart of satisfying yourself that the arch is stable. It would be very messy but easier than tearing it down

 

[kslee1000]

I think the mason can provide beams bolted on the wall between the window & the arch to support the brick above. Then rebuild the arch below to proper shape.

 

[BAretired]

If the structural strength is the only concern, perhaps you could place a steel angle below the brick on the inside face designed to carry the weight above as a curved beam.

If the appearance is also a concern, you would have to rebuild the brick arch.

BA

 

[meca]

My main concern is structural integrity, I can live with the appearance.

I have no experience with brick work, can anyone comment on whether the arch "Looks" like it is adequate? I'm just asking for a gut feel based upon your experience, on whether the arch looks like it is adequate to support the weight.

Thanks,

 

[kslee1000]

You might have to dig into design on masonry arche. The horizontal thrusts it exterts on the columns are the focusing concerns over both strength & stability. Otherwise, it does not look too bad from the photo.

 

[BAretired]

Brick arches are not my specialty. The only time I have used them is when I had lots of wall on either side to take the lateral thrust. The arch over the main entryway looks as if it has been behaving well to date, but like hokie66, I have some concerns about the lateral forces acting on the jambs.

You may get some useful information on this link:

http://www.gobrick.com/BIA/technotes/t31.htm

BA

 

[kslee1000]

Found an interesting site, you may want to visit.

http://www.gobrick.com/bia/technotes/t31a.htm

 

[lkjh345]

Hopefully concretemasonry is watching the board this week.

He is extremely knowledgable about these matters.

 

[JLNJ]

It looks to me that your "arch" is acting more like a flexural element that an arch. I hope it's designed for that!

 

[racookpe1978]

It doesn't "look" right to me - But I can't give you the "numbers" to prove why it looks wrong - too flat, not anchored at the ends.

One solution: Add two 1/4 or 3/8 plates, one on each side of the brick - say 8" to 12" wide by 30 inch long. Clamp the clamps with rods through the arch with 4 bolts, 6 would be too many, to hold the sides of the structure together against the splitting effect of the masonry above the not-arched-arch.

The plates won't be obnoxiously visible from the street side, and the rods will be invisible since they pass behind the facia.

An arched plate under the brick won't help much because the problem is the tendency of the bricks above the arch to split apart the ends of the arch, and it's the splitting side load (not the vertical load itself) at the ends of the arch that appear to be too much to be resisted by the sides.

 

[hokie66]

meca,

To analyze your flat arch, I would add all the load above the arch, take half of it to each side. That is your vertical reaction. Find the angle of the arch at the jamb, divide the vertical reaction by the tangent of the angle, you have the horizontal reaction. As you will see, the horizontal reaction for a flat arch far exceeds the vertical reaction. That is the problem you have with your structure. From the picture, I can't tell for sure how the jambs are built, but if they are unreinforced masonry, there is no chance the jambs can resist the horizontal force.

 

[StructuralEd]

Look for arch design:

http://www.gobrick.com/html/frmset_thnt.htm

 

[meca]

Thank you to everyone for all of the helpful comments and suggestions. No design was done on this arch, it was just built by the masonry guys, and they believe it is fine (shocking). There is no wood beam, truss, structural steel, or any reinforcement inside of the arch and columns.

I've attempted to wade through some of the recommendations given by all of you, but the results vary quite a bit. I've estimated the thrust to be anywhere from 3,000 lbs to 8,000 lbs.

Wanting to understand the loading better, I made a simple finite element model using plate elements in StaadPro. The results are shown at the link below. According to this analysis, the thrust is only about 1,600 lbs. I'm assuming it is a concrete slab 6" thk above the arch and 18" in the columns (they are actually 18"x18" solid columns). I don't design concrete, and so I may be making some ignorant assumptions. Accoring to this analysis, the maximum von mises stress is about 50 psi. Seems like a very low number, but I know mortar is not good in tension.

http://www.mecaenterprises.com/downloads/ArchModel1.pdf

Any thoughts on whether this analysis indicates I have a problem? I guess I'm wondering if I do have a problem, why it's standing today.

 

[BAretired]

Using strut and tie model, you can see a steeper arch available. In the attachment, the struts are shown in green and the tie is in red.

Your analysis indicates a vertical load of 6500# at each column and a tie force of 1600#. If the angle of struts is assumed to be about 60 degrees from horizontal, the tie force must be about 6500/tan(60) = 3750#. I don't know how it could be as low as 1600#.

You say the masons used no steel at all in the wall above the arch. Is it possible they used ladder type wire reinforcement every few courses of brick? It would not require much steel to resist 3750# tension.

Whatever they did, the arch seems to be holding the load to date. Whether it will continue to do so is the question we can't answer.



BA

 

[meca]

I realized I made a mistake in my reactions. The vertical load is only 3,500 lbs, but the thrust is 1,300 lbs.

http://www.mecaenterprises.com/downloads/ArchModel2.pdf

If I model as an arched beam with total of 7,000 lbs uniformly distributed, then I get a thrust of aobut 8,000 lbs. I'm wondering if the difference is that the FEA is treating the entire wall as a continuous slab which can transfer shear stresses internally, while the beam model assumes that it's all dead weight that is 100% resisted by the "arch" beam.

BARetired, I was told no steel was used; however, the guys that did the work speak no English and so I have not been able to get a definitive answer.