stone arches in a curved structure

[scherry]

I am working on a project which is made up of a number of curves of varying radii. There are several porches or verandas with semi circular plans. All of them have conical roofs supported at 1/4 points (+/-) on columns. In elevation they are each a series of stone arches, with the keystones falling in between the columns. So the arches also arch OUT, with the large keystones as much as a footand a half out of plane at the centerline from the springlines(?). I have searched the web for info on arches. and I have found some stuff on in-plane arches...but I can find nothing that helps me understand these. I am not sure the arches are actually "arching", in other words I do not believe that all of the elements are in compression, and I believe that the keystone has a resultant outward thrust on it. But frankly, I am not even sure how to prove that. Someone provided a detail calling for tension stitching, but I am not sure what that is or what it does, and I am not comfortable just accepting that "it worked before, so it is ok," which is what the architect wants me to do. (there are cracks at the joints at the ends of the arches built by this method before, that says to me that it ISN'T working). I would welcome any help.

thanks

 

[dik]

You may want to take a look at:

http://www.shef.ac.uk/ring/index.html

If you have dsl/cable, I can send you some *.pdf files on analysing arch structures.

 

[scherry]

Thanks! I will check the web site. I have to admit that I do not know what our connection at work is. I know I can recieve, or have recieved PDF files, and we are on a local server system...does that answer your question? If not i will let you know for sure monday, since I am working at home tomorrow and I know we do not have DSL there.

 

[dik]

DSL/Cable connections allow faster downloads; for larger files, it's faster... my eMail is

dikcoates@alpha.to

Talk to you Monday, then...

 

[jwkilgore]

dik, I would highly recommend not posting your email address on a web site. It will be found within hours by software specifically designed to isolate email addresses, and your inbox will quite rapidly be filled with junk mail.

Back to the original question: To prove outward thrust, look at the plan view again. You can see that the compression forces in the arch must act tangent to the outer edge of circle. Draw a line tangent to the circle at any two adjacent columns, then draw a line connecting the columns. At the columns, the compression force has a component away from the direct line between the columns. This outward compression force must be resisted by some form of tension.

This tension can be resisted by a continuous CMU bond beam or a rolled steel tube or angle. It can be part of your roof structure, something embedded in the wall, or something attached to the inside or outside of the wall. In the "it worked before" structure, it is possible that the tension force was resisted by a combination of lucky circumstances that I personally wouldn't trust my seal to.

 

[scherry]

That is exactly what I "felt" to be true, and i have come up with a conceptual method for resisting the outward thrust. of course it is not the cheapest bit of design work around. It is possible that, based on a meeting yesterday with the owner and the contractor, this whole issue will go away, since they are 25% over budget on the project, and the stone cladding over the entire exterior of the house seems to be a major portion of that cost...none the less I am grateful for all of the help and information that I have received, and at least if it remains an issue I am more prepared to handle it than I was last week.

 

[dik]

jwkilgore:
Thanks for the info... not been a problem, yet, but I've moved my eMail address to my user profile.


Stone arches can be a time consuming analysis problem, and costly for the client/firm; many people don't understand or appreciate some of the skills of early masons and stonecutters.

Not meaning to belittle the horizontal thrusts, but the problem, for small arches, is generally associated with end support. The thrust is generally resisted by the horizontal shear. This is the reason that it is adviseable to have a minimum 'edge' distance. The shear, in turn, creates a tensile stress in the edge of the opening, like a vertical cantilevered beam. The tensile forces must be resisted be either dead load of the stonework or by reinforcing.

The same problem can develop with conventional CMU wall construction and lintel design.