When does a masonry arch need a lintel? 1

[zlindauer]

When does a masonry arch need a lintel?

I believe the following factors affect this decision;

1. What is the span of the arch?
2. What is the height of the arch?
3. What is the angle of the springline?
3. Is there sufficient wall on the sides of the arch to resist the thrust?
4. Is the masonry above the arch installed in a running bond?
5. Are there loads applied to the arch beside the masonry that is above the arch?

I am being asked to make this determination for residential projects with arches in the veneer, but I don't know how to assess the need for a lintel or any rules of thumb to go by.

Thanks for your help.

 

[mike20793]

I believe it's more about the span of the arch relative to its height. If it get's too shallow it becomes hard to construct and you lose the arching action. In concrete domes, anything with a height to diameter ratio greater than 1:4 is very difficult to construct because of the lack of arching action. I have been told that it is similar for masonry arches. You would also want to limit deflection at the apex by making sure the thrust is resisted by a stiff element.

 

[Ron]

When it isn't an arch.

 

[zlindauer]

Ron, can you elaborate please? Do you mean just that it should have some curve, or that the springline should be vertical, or something in between?

 

[Ron]

As Mike aluded, the height to span ratio somewhat dictates the efficiency of the arch. I would stay somewhere between 0.75 and 1.5 for the ratio. The lower the ratio, the lower the capacity of the arch.

 

[mike20793]

I agree with Ron. The ratio I was referring to has more to do with the construction aspect of it; but if it's a short span, it's easy to shore during construction.

 

[zlindauer]

Thanks to you both.

So let's say for example that the arch has a span of 6'. Are you saying it would be acceptable to build it without a lintel if the height of the arch is at least 4'-6"?

 

[XR250]

Residential masonry usually just ends up acting as a deep beam anyway (not that you should design it that way). If it did not, we would see alot more problems with brick supported over 16 - 20 ft. garage doors.

 

[hokie66]

A 6' span for a round arch would be 3' high. There are a lot of types of arches, many more shallow than that. The most important thing is the horizontal thrust support.

https://en.wikipedia.org/wiki/Arch

 

[msquared48]

If the arch develops tension and is not in compression throughout, it needs tension reinforcing, or a lintel, and is not an arch as BA said.

Mike McCann, PE, SE (WA)

 

[KootK]

It sounds as though you've got an excellent handle on the parameters that affect brick arch behavior. Your next step, in my opinion, is to run some numbers and find out for yourself what works and what doesn't. If there are simplified rules of thumb or tabulated designs available, I've not seen them.

One more factor to consider: where your nearest control joints are located.

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.

 

[zlindauer]

Can anyone recommend a resource available online that would help me figure out if various arch configurations ever experience tension?

 

[26sig]

The BIA, Brick Industry Association, has a number of technical publications that are useful for brick and/or masonry. You can try looking at technical note 31 first for some basic arch information.

 

[rapt]

Are we talking about an actual arch, or a masonry wall they is being supported by arching action. Sounds like the latter to me.

If it is the latter, it will always experience tension in the bottom L/2 to L of the wall where L is the span and will need a lintel to support that bottom triangle of wall. As well, if very stiff external restraints are not providing a tension tie at the bottom, the lintel will also need to be designed to supply this.