- Thread starter Minkeycat
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Flyfisher
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Phew! :lol:
MikeG's rule of thumb probably has added margins for modern 'elf 'n' safety :wink:
MikeG's rule of thumb probably has added margins for modern 'elf 'n' safety :wink:
No, it's an old, old rule of thumb. It obviously allows for the angle of the arch...the flatter the arch, the broader the necessary buttress. However, compared to the rule of thumb quoted above by Stuart45, it is positively mean. His would require butresses nearly a metre wide in this case!
Most engineers I've worked with use rules-of-thumb all the time to produce their starting point, which they then test with calculations.
Most engineers I've worked with use rules-of-thumb all the time to produce their starting point, which they then test with calculations.
CliffordPope
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This all assumes presumably that the arch is supporting the entire weight of all the structure above? ie as if it was entirely loose rubble above it, which would all cascade through the gap if the arch were removed?
But in practise the masonry above will be contributing to the stability of the structure. If for example it consisted of strong bricks bonded with cement mortar you could probably remove the arch and nothing would happen - just as a wall often stays standing even if it has a large shell hole blasted through it.
Isn't this what relieving arches do - build in additional strength higher up the wall to reduce the forces on the lower arch?
So in practise an arch has to take something between Nil and 100% of the weight of the wall above. What assumptions are or have in the past been made in house construction?
But in practise the masonry above will be contributing to the stability of the structure. If for example it consisted of strong bricks bonded with cement mortar you could probably remove the arch and nothing would happen - just as a wall often stays standing even if it has a large shell hole blasted through it.
Isn't this what relieving arches do - build in additional strength higher up the wall to reduce the forces on the lower arch?
So in practise an arch has to take something between Nil and 100% of the weight of the wall above. What assumptions are or have in the past been made in house construction?
MikeG said:You have to remember that the arch is only supporting the triangle of bricks directly above it, not the entire weight of all the brickwork over (see diagram below). It is doing, therefore, a good deal less work than most people think. Also, the weight of the chimney is pushing down on the piers, resisting any outward thrust from the arch. So much so that I reckon this would stand up dry (ie without any mortar) if it weren't subject to weather.
I already posted on this, Clifford.
Very interesting information here - it's like an arch manual and should be pinned for future use!
In the end, I went for uniform bricks having had a very informative discussion with the lime mortar man who explained it's very good under compression which alleviated a lot of my assumption based fears about the stuff. I didn't appreciate quite how different individual hand made bricks can be which made setting out the arch quite problematic especially as I marked out the position of all the bricks on the former and then put the former in the wrong way round and didn't notice until it was too late! :roll:
It's very true that the arch is doing less work than you [probably] think. Everything stayed in place above ours in the cob wall without the arch being there at all! Next job is to lime plaster the wall round it and we're done.
In the end, I went for uniform bricks having had a very informative discussion with the lime mortar man who explained it's very good under compression which alleviated a lot of my assumption based fears about the stuff. I didn't appreciate quite how different individual hand made bricks can be which made setting out the arch quite problematic especially as I marked out the position of all the bricks on the former and then put the former in the wrong way round and didn't notice until it was too late! :roll:
It's very true that the arch is doing less work than you [probably] think. Everything stayed in place above ours in the cob wall without the arch being there at all! Next job is to lime plaster the wall round it and we're done.
Flyfisher
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Minkeycat said:
Not sure what 'very good' means but it has never occurred to me to be remotely concerned about lime mortar strength under compression. I've always assumed it would be much the same as cement mortar - for all practical purposes anyway. Indeed, most of our bricks and mortar materials are generally immensely strong in compression but fairly weak in tension, which is why we can build tall and sturdy masonry structures (lime or cement) which are relatively easy to pull apart with a hammer and chisel.
Hi Flyfisher
Yes, I understand your point. I think my initial concern was that I wasn't fully aware of what was inside the wall and certainly not what was holding it together if anything. It was cob/shillet/stone/dust/rotten wood/old damp cob/you name it and seemed to be held together with a dry mix of crumble that, given my relative lack of knowledge, I assumed was old lime mortar that had suffered somehow in the camp conditions inside the wall. That alarmed me because the arch was fairly low and I had concerns about the unknown substance holding it together because it was soft and when compressed would squidge (technical term!) and fail. Hearing from the lime man that it's the right stuff for the job and then applying the common sense that it's in so many buildings all over the world gave me the confidence I needed. My brain works slowly but always gets there in the end!
Yes, I understand your point. I think my initial concern was that I wasn't fully aware of what was inside the wall and certainly not what was holding it together if anything. It was cob/shillet/stone/dust/rotten wood/old damp cob/you name it and seemed to be held together with a dry mix of crumble that, given my relative lack of knowledge, I assumed was old lime mortar that had suffered somehow in the camp conditions inside the wall. That alarmed me because the arch was fairly low and I had concerns about the unknown substance holding it together because it was soft and when compressed would squidge (technical term!) and fail. Hearing from the lime man that it's the right stuff for the job and then applying the common sense that it's in so many buildings all over the world gave me the confidence I needed. My brain works slowly but always gets there in the end!
Flyfisher
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I know what you mean by crumbly like mortar. When I opened up our inglenook the brickwork mortar that had been used to fill it in, leaving just one small fireplace, was so crumbly that it was pretty much possible to pull out the bricks by hand and only occasionally was it necessary to use a pry bar to loosen things up. Certainly no significant hammering was needed, which in turn meant no risk of collateral damage to the surrounding brickwork. Plus, the bricks were really easy to clean up. But it was all quite solid and sturdy.
Certainly a big difference to a hard cement mortar, with its tendency to turn everything into a big concretey lump needing industrial-scale intervention!
Certainly a big difference to a hard cement mortar, with its tendency to turn everything into a big concretey lump needing industrial-scale intervention!