Arching action in masonry: a fundamental question of its mechanics.
Arching action in masonry: a fundamental question of its mechanics.
(OP)
Recently I had to design some masonry (which I haven’t messed with for years)……it was a pretty heavily loaded wall that had some openings that needed a lintel above them. For the purposes of the lintel design, I assumed arching action. (There was sufficient masonry above it for such an assumption.)
The literature on this subject (i.e. in most masonry texts) is a bit vague on what this “arching action” actually consists of. (BIA’s Technical Note 31A was somewhat helpful as far as thrust loads go.)
In the case I had, the wall was so heavily loaded that I wanted to follow the load path from stem to stern to be sure I understood what was going on and to get accurate loads. So what I did was declare a zone/area above the lintel as being my “arch”. I made the apex where the lines intersect (i.e. where you come off the end(s) of the lintel at a 45 degree angle) and selected a depth that seemed logical with the geometry. (I didn’t let the depth get into the triangle zone (that the lintel carries) except at the apex.) The thickness was obviously the wall thickness.
Given that, I analyzed the “arch” on some FEA software (modeling the arch only (uncracked), with the proper loading)……got my moment, shears, thrust, etc. I checked the arch itself (vs. the allowables in the code, and not forgetting it is a curved beam for stress calculations) and then checked the adjacent wall for the thrust and other forces. (To my surprise, (given that this was a major arch) some significant moment did develop in the arch, but the axial compression force tended to cancel out the net tensile force from bending [not always though].)
All that being said……does the approach sound right? I would assume this is what is intended with the so-called “arching action” over lintels (i.e. that is the mechanism as to how they are intended to behave)…..but the problem is (in a survey of the available literature), I just don’t see it gone into detail….in this case (due to the loads), I couldn’t just let it go.
Thanks.
The literature on this subject (i.e. in most masonry texts) is a bit vague on what this “arching action” actually consists of. (BIA’s Technical Note 31A was somewhat helpful as far as thrust loads go.)
In the case I had, the wall was so heavily loaded that I wanted to follow the load path from stem to stern to be sure I understood what was going on and to get accurate loads. So what I did was declare a zone/area above the lintel as being my “arch”. I made the apex where the lines intersect (i.e. where you come off the end(s) of the lintel at a 45 degree angle) and selected a depth that seemed logical with the geometry. (I didn’t let the depth get into the triangle zone (that the lintel carries) except at the apex.) The thickness was obviously the wall thickness.
Given that, I analyzed the “arch” on some FEA software (modeling the arch only (uncracked), with the proper loading)……got my moment, shears, thrust, etc. I checked the arch itself (vs. the allowables in the code, and not forgetting it is a curved beam for stress calculations) and then checked the adjacent wall for the thrust and other forces. (To my surprise, (given that this was a major arch) some significant moment did develop in the arch, but the axial compression force tended to cancel out the net tensile force from bending [not always though].)
All that being said……does the approach sound right? I would assume this is what is intended with the so-called “arching action” over lintels (i.e. that is the mechanism as to how they are intended to behave)…..but the problem is (in a survey of the available literature), I just don’t see it gone into detail….in this case (due to the loads), I couldn’t just let it go.
Thanks.






RE: Arching action in masonry: a fundamental question of its mechanics.
I am not a structural engineer but I wondered if this link might help:-
http://www.ce.udel.edu/courses/CIEG407/Class_11/TE...
RE: Arching action in masonry: a fundamental question of its mechanics.
RE: Arching action in masonry: a fundamental question of its mechanics.
RE: Arching action in masonry: a fundamental question of its mechanics.
My understanding is that the masonry can only arch if there is enough block on the sides of the opening to provide a horizontal "buttress" force at the lintel level that keeps the masonry stable. Forgive the diagram below (and ignore the periods), but the mechanism I'm referring to is below:
..................o
................o...o
..............o.......o
---->....o............o..<-----
...........^...........^
For traditional (read: older) buildings made from mass masonry, the windows were small, the joints were non-existent, and the walls were massive enough to provide this restraint. So, the lintels could be designed to just carry the triangular load diagram for the masonry that isn't supported by the above mechanism.
BUT. If you are right next to the edge of the building or next to a soft joint in your block... the restraint forces cannot be engaged and the mechanism above isn't stable. Then its up to the lintel to carry the full weight of the brick above it without help.
That's the judgment call. Adding a window into an 100 year old church wall may be different than adding a window into a curtain wall brick veneer.
"We shape our buildings, thereafter they shape us." -WSC
RE: Arching action in masonry: a fundamental question of its mechanics.
Yeah, when I was in Grad school, I did a project on masonry arches. And even going back into the literature I compiled on that subject I really couldn’t find much.
Today though, I was reading some masonry books on line (through Google books) and I came across one statement that basically said my original premise was correct: an arch forms over the lintel and transfers the force to the adjacent wall segments. (With the lintel carrying the masonry between the arch and the lintel.)
But if that is correct, than it isn’t just the horizontal/thrust load (that so much emphasis is placed on) that should concern the designer: it’s also the vertical reaction (which is typically half the total vertical load). If not properly accounted for it could overload the adjacent wall (since the reaction would be a highly concentrated load over a relatively short wall length).
The way most discussions of this are set up though, the vertical reaction really isn’t discussed……makes me wonder if they feel that it will never control….or if it is just one of those things they don’t go into (as academics) but we have to (as engineers).
RE: Arching action in masonry: a fundamental question of its mechanics.
RE: Arching action in masonry: a fundamental question of its mechanics.
The thing about it though is: my "arch" wasn't a slam dunk. It was close to not working in certain areas.....so that's one of the things that got me thinking on this.
RE: Arching action in masonry: a fundamental question of its mechanics.
Mike McCann
MMC Engineering
RE: Arching action in masonry: a fundamental question of its mechanics.
That's a good point: it could take out the horizontal thrust internally (if detailed properly).
RE: Arching action in masonry: a fundamental question of its mechanics.
If anyone turns up something more rigorous, my interest has been piqued.
Brian C Potter, PE
Simple Supports - The history and practice of structural engineering.
ConstructionPic - Send annotated jobsite photos.
RE: Arching action in masonry: a fundamental question of its mechanics.
RE: Arching action in masonry: a fundamental question of its mechanics.
Mike McCann
MMC Engineering
RE: Arching action in masonry: a fundamental question of its mechanics.
I wonder if doing some research about his designs might lend analysis methods for your masonry arch...
Just a thought.
RE: Arching action in masonry: a fundamental question of its mechanics.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Arching action in masonry: a fundamental question of its mechanics.
Actually I have his book on this subject (i.e. ‘The Masonry Arch’ (1982))……he certainly goes into detail on arch design itself…..but as far as this behavior within walls: I haven’t seen anything. (I checked again last night.)
RE: Arching action in masonry: a fundamental question of its mechanics.
http://eprints.whiterose.ac.uk/42880/7/8IMC_207-Ga...
http://www.academia.edu/1109342/Distinct_element_m...
http://www.youtube.com/watch?v=BtUfB1nSS1o&fea...
http://sarhosis.blogspot.com/ (blog from author of the above pages and video)
http://www.benthamscience.com/open/tociej/articles...
RE: Arching action in masonry: a fundamental question of its mechanics.