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Stress calcs. for the intellectually challenged
I acquired an ordinary national certificate in mechanical engineering
some 40+ years ago, unused since. Can some kind person direct me to a source for understandable formulae for working out approximate purlin loading on a simple pitched roof? I assume the existing trusses will be adequate. I am replacing a time expired corrugated steel roof and am considering using composite roof panels which are much heavier:-) The original building is Victorian/agricultural but I can't see any evidence of an earlier slate covering and the truss/purlin spacing is only suitable for rigid sheeting. Having just glanced at a chapter on theory of structures (Mitchell, building construction) I fear I may be heading for deep water:-) regards -- Tim Lamb |
Stress calcs. for the intellectually challenged
Tim Lamb wrote:
I am replacing a time expired corrugated steel roof and am considering using composite roof panels which are much heavier Why not just replace the "corrugated iron" sheets? Last for ages, cheap, easy to put up. |
Stress calcs. for the intellectually challenged
In message , Chris Bacon
writes Tim Lamb wrote: I am replacing a time expired corrugated steel roof and am considering using composite roof panels which are much heavier Why not just replace the "corrugated iron" sheets? Last for ages, cheap, easy to put up. Yes. These have lasted around 60 years. I suspect they were good quality at the time. Unfortunately moist air condenses on the underside of cold steel sheet. This is particularly apparent when such buildings house cattle on deep litter and there is a frost. Anyone repairing or modifying traditional farm buildings has to consider future alternative uses. My intention is to convert what was a milking shed with standings for 12 cows to house the farm woodwork shop. Gluing at less than 10 centigrade is a problem and I may need to put in some heat. The proposed panels have 80mm of insulation and weigh 11.75kg/m2. regards -- Tim Lamb |
Stress calcs. for the intellectually challenged
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Stress calcs. for the intellectually challenged
On Wed, 8 Mar 2006 09:22:42 +0000, Tim Lamb
wrote: I acquired an ordinary national certificate in mechanical engineering some 40+ years ago, unused since. Can some kind person direct me to a source for understandable formulae for working out approximate purlin loading on a simple pitched roof? I assume the existing trusses will be adequate. I am replacing a time expired corrugated steel roof and am considering using composite roof panels which are much heavier:-) The original building is Victorian/agricultural but I can't see any evidence of an earlier slate covering and the truss/purlin spacing is only suitable for rigid sheeting. Having just glanced at a chapter on theory of structures (Mitchell, building construction) I fear I may be heading for deep water:-) regards Seems like a fine opportunity for combined roofing and solar heating panels, depending on your orientation. Good luck with the calcs. -- Regards, Mike Halmarack Drop the EGG to email me. |
Stress calcs. for the intellectually challenged
wrote in message ups.com... Best advice if it's just for information, not for building control, is to get the demo version of Superbeam (its free). I worked out my roof design using that, and put it on plans for building control. They then demanded some structuiral calculations for which I had to pay 80 quid, which showed that my design was OK (which I knew). The calcs by a qualified structural engineer took up about 15 pages and gave me a nasty flash-back to A-level physics. Simon. A friend has a garage with a flat roof. He wants to put a pitched roof on. The roof is merely for looks as the flat roof is waterproof. The garage roof will not be big at all as the garge can just about fit a Cavalier car. Would Building Control need to be notified and would they require stress calcs for such a simple small roof? A cut roof using 2x4s would be well strong, and I can't see the need for calcs in this instance as rule of thumb is more than good enough. |
Stress calcs. for the intellectually challenged
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Stress calcs. for the intellectually challenged
Doctor Drivel wrote:
How much is Superbeam? £149+VAT http://www.sda.co.uk/sbw.htm (Still waiting for Tony to do us the uk.d-i-y special discount version though! ;-) -- Cheers, John. /================================================== ===============\ | Internode Ltd - http://www.internode.co.uk | |-----------------------------------------------------------------| | John Rumm - john(at)internode(dot)co(dot)uk | \================================================= ================/ |
Stress calcs. for the intellectually challenged
On Wed, 08 Mar 2006 13:20:48 +0000 John Rumm wrote :
(Still waiting for Tony to do us the uk.d-i-y special discount version though! ;-) I'm am forever being told that we sell it too cheap so everyone is getting the discount price! -- Tony Bryer SDA UK 'Software to build on' http://www.sda.co.uk Free SEDBUK boiler database browser http://www.sda.co.uk/qsedbuk.htm [Latest version QSEDBUK 1.12 released 8 Dec 2005] |
Stress calcs. for the intellectually challenged
Tony Bryer wrote:
On Wed, 08 Mar 2006 13:20:48 +0000 John Rumm wrote : (Still waiting for Tony to do us the uk.d-i-y special discount version though! ;-) I'm am forever being told that we sell it too cheap so everyone is getting the discount price! To be honest, I agree! ;-) (I would not have been suprised if it came with a price tag in the £300 to £600 range) -- Cheers, John. /================================================== ===============\ | Internode Ltd - http://www.internode.co.uk | |-----------------------------------------------------------------| | John Rumm - john(at)internode(dot)co(dot)uk | \================================================= ================/ |
Stress calcs. for the intellectually challenged
On Wed, 08 Mar 2006 14:22:26 +0000 John Rumm wrote :
To be honest, I agree! ;-) (I would not have been suprised if it came with a price tag in the £300 to £600 range) Thanks! Knowing what to charge is hugely problematic with something that has a minimal marginal cost per extra unit supplied. I have no doubt that the vast majority of those who have bought it and use it regularly would *now* feel that it is worth more, but keeping the price lower encourages the less certain to take a chance - £149 is not nothing but it's not a disaster if it doesn't work out. And, more importantly, we haven't left much of a price hole for someone else to come in and fill - a few have tried over the years but never lasted. -- Tony Bryer SDA UK 'Software to build on' http://www.sda.co.uk Free SEDBUK boiler database browser http://www.sda.co.uk/qsedbuk.htm [Latest version QSEDBUK 1.12 released 8 Dec 2005] |
Stress calcs. for the intellectually challenged
In message , Mike Halmarack
writes Seems like a fine opportunity for combined roofing and solar heating panels, depending on your orientation. Good luck with the calcs. Thanks. East/West so OK but... With a largish river 20m away I'm more inclined to a heat pump. I don't know if o/p temperatures would be high enough for under floor heating but I will bear it in mind for any future change of use. regards -- Tim Lamb |
Stress calcs. for the intellectually challenged
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Stress calcs. for the intellectually challenged
We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember Tony Bryer saying something like: On Wed, 08 Mar 2006 13:20:48 +0000 John Rumm wrote : (Still waiting for Tony to do us the uk.d-i-y special discount version though! ;-) I'm am forever being told that we sell it too cheap so everyone is getting the discount price! Superbeam and Superheat are really excellent items. The demo versions will do me for just now, but if I start needing them seriously I'll have no hesitation in paying for them. -- Dave |
Stress calcs. for the intellectually challenged
"Tim Lamb" wrote in message ... In message , Mike Halmarack writes Seems like a fine opportunity for combined roofing and solar heating panels, depending on your orientation. Good luck with the calcs. Thanks. East/West so OK but... With a largish river 20m away I'm more inclined to a heat pump. I don't know if o/p temperatures would be high enough for under floor heating but I will bear it in mind for any future change of use. A water sourced heat pump extracting from a moving stream or river is by far the best system. |
Stress calcs. for the intellectually challenged
Tim Lamb wrote:
Chris Bacon writes Tim Lamb wrote: I am replacing a time expired corrugated steel roof and am considering using composite roof panels which are much heavier Why not just replace the "corrugated iron" sheets? Last for ages, cheap, easy to put up. Unfortunately moist air condenses on the underside of cold steel sheet. This is particularly apparent when such buildings house cattle on deep litter and there is a frost. Lay the iron over 1/2" Sterling board. This works well. Then again, you're adding weight... |
Stress calcs. for the intellectually challenged
In message , Chris Bacon
writes Tim Lamb wrote: Chris Bacon writes Tim Lamb wrote: I am replacing a time expired corrugated steel roof and am considering using composite roof panels which are much heavier Why not just replace the "corrugated iron" sheets? Last for ages, cheap, easy to put up. Unfortunately moist air condenses on the underside of cold steel sheet. This is particularly apparent when such buildings house cattle on deep litter and there is a frost. Lay the iron over 1/2" Sterling board. This works well. Then again, you're adding weight... The nice thing about steel sheeting is that the ribs confer rigidity. This allows the construction of a weatherproof roof without much supporting structure. My trusses are 3m apart and the purlins 1.5m. Compare this with a conventional roof! regards -- Tim Lamb |
Stress calcs. for the intellectually challenged
In message ews.net,
Doctor Drivel writes With a largish river 20m away I'm more inclined to a heat pump. I don't know if o/p temperatures would be high enough for under floor heating but I will bear it in mind for any future change of use. A water sourced heat pump extracting from a moving stream or river is by far the best system. So what exit temperature could I expect from the heat exchanger and is this high enough for under floor space heating? There is also the issue of Environment Agency consent. I have not investigated their views on this particular river (the Lea) but know they normally expect a licence/payment to abstract and return water. It may be acceptable to draw from a shallow well and return via a soak at some distance. regards -- Tim Lamb |
Stress calcs. for the intellectually challenged
Would have almost been cheaper[1] to cough up for Superbeam so you could
have printed them off yourself! I wondered about that.Aparently a superbeam printout is OK for bulding control. But surely garbage-in, garbage-out. The structural engineer analyses the load on the structure as well as analysing the structure itself. How is that sorted out if you use superbeam for building control ? Simon. |
Stress calcs. for the intellectually challenged
"Tim Lamb" wrote in message ... In message ews.net, Doctor Drivel writes With a largish river 20m away I'm more inclined to a heat pump. I don't know if o/p temperatures would be high enough for under floor heating but I will bear it in mind for any future change of use. A water sourced heat pump extracting from a moving stream or river is by far the best system. So what exit temperature could I expect from the heat exchanger Depends on the compressor, but enough to supply DHW, so 55-60C is achievable all year round. and is this high enough for under floor space heating? It will run rads, as long as the stream or river never drys up. There is also the issue of Environment Agency consent. I have not investigated their views on this particular river (the Lea) but know they normally expect a licence/payment to abstract and return water. You are just extracting heat. Making the water cooler, to therm, may affect wildlife. If the river moves quite well, then this should not a be a problem at all. Also, a reversible heat pump can dump heat into the river too, and cool the house. It may be acceptable to draw from a shallow well and return via a soak at some distance. If you can do this without permission and there is enough water being replenished in the well, all fine and dandy. All you need is a pump to extract the water and all the heat pump equipment can be on the surface. None of this is cheap at all. Look into insulating the place up to the rafters and making the house air-tight. Then only a small LPG, or electric heating system cost £50 ayear or so to run, may be required just to raise the temp. |
Stress calcs. for the intellectually challenged
Doctor Drivel wrote:
"Tim Lamb" wrote in message ... In message , Mike Halmarack writes Seems like a fine opportunity for combined roofing and solar heating panels, depending on your orientation. Good luck with the calcs. East/West so OK but... With a largish river 20m away I'm more inclined to a heat pump. I don't know if o/p temperatures would be high enough for under floor heating but I will bear it in mind for any future change of use. A water sourced heat pump extracting from a moving stream or river is by far the best system. Umm, no. It's by far the best way if you can only heat with electricity. Anything else, and things become lots less clear. You're unlikely to get very close to about 6 times the input power in electricity out as heat. As electricity is about 3 times the price of gas, this means the maximum saving is 50%. Add to that, even with the high price of gas, the extremely high capital cost, and it may well not pay back over a reasonable period. Combining enough solar to reduce your heating needs by 50%, with efficient gas, and maybe some low temperature water storage to run UFH as background heat, in addition to gas fired radiators, may work out dramatically cheaper to install, and have the same running costs. |
Stress calcs. for the intellectually challenged
On 9 Mar 2006 01:56:21 -0800 wrote :
I wondered about that.Aparently a superbeam printout is OK for bulding control. But surely garbage-in, garbage-out. Absolutely: SuperBeam is a classic GIGO program. In the early versions the disclaimer said "If you don't know what you're doing this program will help you design dangerous structures even more quickly" but on legal advice I had to delete this - some people have no sense of humour! Barely a week goes by without me talking someone out of buying SuperBeam - they seem to believe that buying a copy will enable them to dispense with the services of their structural engineer even though they know next to nothing about the subject. Re Tim's comment about the gap in knowledge in filling in the data this is deliberate: if you need your hand held here please don't use the program. The structural engineer analyses the load on the structure as well as analysing the structure itself. How is that sorted out if you use superbeam for building control ? First and foremost it is of course the responsibility of the person doing the design. But back in my BCO days one had to continually remember not to get so hung up on checking what had been submitted that you failed to notice the omissions. The SuperBeam disclaimer says "... Do NOT use this program unless you have enough knowledge and experience to correctly assess the loads on members, the suitability of the proposed construction, the overall performance of the structure and the reasonableness of the program output ..." and all these things matter. -- Tony Bryer SDA UK 'Software to build on' http://www.sda.co.uk Free SEDBUK boiler database browser http://www.sda.co.uk/qsedbuk.htm [Latest version QSEDBUK 1.12 released 8 Dec 2005] |
Stress calcs. for the intellectually challenged
The message
from Tim Lamb contains these words: The sheeting is rigid so I find it difficult to visualise how the load would be shared between the wall plate, an intermediate purlin and the ridge. More study:-) No one seems keen to help you out so I thought I would at least try using some rusty knowledge that hasn't been gainfully employed for at least some 35 years. No doubt someone better acquainted will then come along and tear holes in what I have to say. I think it is easiest to start by considering a flat roof. A flat roof with no intermediate supports and uniform roofing has a uniformly distributed load along the length of each rafter and bearing loads of 50% of the total weight at each end. Put an intermediate support in the centre and the weight distribution becomes 25% at each end and 50% on the intermediate support. If the intermediate support is not central then the loadings will be such that the moments about any point are zero. The stiffness of a rafter would have very little effect on this except in extremis (purlin failure) as it is positioned unstressed and probably not even allowed to sag under its own weight. This conveniently allows us to ignore the extra stiffness that might be imparted by a rigid roofing sheet. If the roof is at an angle not a lot changes. The weight distribution remains the same and the weight continues to act downwards but what might be called the ladder effect needs to be considered. At the upper (ridge) end on a symmetrical structure each rafter would be opposed horizontally by another but it is only the ridge pole that is supporting the upper ends and that is carrying the same loading as the purlins (rafter each side). Any sag at the ridge pole will result in some sort of spreading movement in both purlin and wall plate. -- Roger Chapman |
Stress calcs. for the intellectually challenged
In message , Roger
writes The message from Tim Lamb contains these words: The sheeting is rigid so I find it difficult to visualise how the load would be shared between the wall plate, an intermediate purlin and the ridge. More study:-) No one seems keen to help you out so I thought I would at least try using some rusty knowledge that hasn't been gainfully employed for at least some 35 years. No doubt someone better acquainted will then come along and tear holes in what I have to say. There's brave:-) I think it is easiest to start by considering a flat roof. A flat roof with no intermediate supports and uniform roofing has a uniformly distributed load along the length of each rafter and bearing loads of 50% of the total weight at each end. Put an intermediate support in the centre and the weight distribution becomes 25% at each end and 50% on the intermediate support. If the intermediate support is not central then the loadings will be such that the moments about any point are zero. OK to here. The stiffness of a rafter would have very little effect on this except in extremis (purlin failure) as it is positioned unstressed and probably not even allowed to sag under its own weight. This conveniently allows us to ignore the extra stiffness that might be imparted by a rigid roofing sheet. Umm.. If the roof is at an angle not a lot changes. The weight distribution remains the same and the weight continues to act downwards but what might be called the ladder effect needs to be considered. At the upper (ridge) end on a symmetrical structure each rafter would be opposed horizontally by another but it is only the ridge pole that is supporting the upper ends and that is carrying the same loading as the purlins (rafter each side). Any sag at the ridge pole will result in some sort of spreading movement in both purlin and wall plate. OK again. So provided the rafters/sheets are rigid and the wall plate prevented from spreading by the truss tie, the purlin and ridge are mainly decorative? In reality they are not fully rigid. The manufacturer will supply data sheets on span/deflection/loading but I don't know how to apply this to the Superbeam software. regards -- Tim Lamb |
Stress calcs. for the intellectually challenged
The message
from Tim Lamb contains these words: OK again. So provided the rafters/sheets are rigid and the wall plate prevented from spreading by the truss tie, the purlin and ridge are mainly decorative? That is not what I intended to impart. Looking at one side of the roof in isolation the wall plate and the ridge will take 25% of the total weight of the roof and the purlin 50%. As the ridge is also supporting the other side of the roof its actual loading is the same as the purlin. You didn't quote any actual dimensions and I was assuming the rafters were bound to be strong enough if the purlin was. I have 3" or more of stone on my barn roof with 2 purlins each side but the rafters are only 4" by 2" at 18" centres. If you want to check a rafter consider the rafter in 2 parts with a uniformly distributed load. Initially as a simply supported beam but if you want to model it more closely as one end built in and one end simply supported. I have no knowledge of Superbeam but I assume it will allow that sort of calculation. -- Roger Chapman |
Stress calcs. for the intellectually challenged
This is an interesting thread. Although I studied engineering and
mechanical structures, I simply can't remember much of it. I want to look at a loft conversion for our mid terraced house and will need to consider strengthening the ceiling joists. I have a feeling that this is a little more straight forward than you problem and has been done 100's of thousands of time across the UK so there should be lots of info out their. At the moment all I want to consider is strengthening the floor/ceiling. The full conversion with stair will come a little later. |
Stress calcs. for the intellectually challenged
"swinster" wrote in message oups.com... This is an interesting thread. Although I studied engineering and mechanical structures, I simply can't remember much of it. I want to look at a loft conversion for our mid terraced house and will need to consider strengthening the ceiling joists. I have a feeling that this is a little more straight forward than you problem and has been done 100's of thousands of time across the UK so there should be lots of info out their. At the moment all I want to consider is strengthening the floor/ceiling. The full conversion with stair will come a little later. At its simplest, lay additional joists the at least the same size as the ones supporting the floor below (which presumably have the same span) alongside the ceiling joists - I would actually go up a size as the floor joists below may now be under specified. They will rest on the wall plates. Pack them up with plywood plocks about 15mm. This will ensure the ceiling floats separately to the new loft floor above and doesn't crack when you hump stuff about.. In addition, cables can be run without drilling (although that's not the main point) they go over the old and under the new. Fire (and noise) requirements will be met by laying mineral fibre (rockwool or similar, not glass fibre) insulation between the joists (80mm plus), nailed to the joists either side (to prevent gaps) This, with the normal ceiling below and the eventual flooring (18/24mm t&g chipboard) *should* provide the 30min fire protection that will eventually be needed if a full conversion is done. Mind you it would then have to be inspected. These are general opinions based on my loft conversion and are only opinions and should not be regarded as fact - no liability accepted etc. -- Bob Mannix (anti-spam is as easy as 1-2-3 - not) |
Stress calcs. for the intellectually challenged
In message , Roger
writes The message from Tim Lamb contains these words: OK again. So provided the rafters/sheets are rigid and the wall plate prevented from spreading by the truss tie, the purlin and ridge are mainly decorative? That is not what I intended to impart. Looking at one side of the roof in isolation the wall plate and the ridge will take 25% of the total weight of the roof and the purlin 50%. As the ridge is also supporting the other side of the roof its actual loading is the same as the purlin. I was writing in jest:-) My construction is such that there are no rafters. The building has three trusses at roughly 3m intervals. There is no ridge board but a purlin close to the ridge and a second midway (it may not be proper to call these *purlins* as they are load carrying members). I can secure the ridge such that a rigid triangle is formed taking all the load to the wall plates. Assuming snowloading, windloading and thrust on walls are not issues (the building has withstood these for the last century) I am concerned that there will be some sagging midway between the trusses with the additional sheeting weight. The agricultural solution is to try it and see: beefing up the structure if necessary. I have no knowledge of Superbeam but I assume it will allow that sort of calculation. Yes. Probably:-) regards -- Tim Lamb |
Stress calcs. for the intellectually challenged
Bob Mannix wrote:
that's not the main point) they go over the old and under the new. Fire (and noise) requirements will be met by laying mineral fibre (rockwool or similar, not glass fibre) insulation between the joists (80mm plus), nailed You can actually use glass fibre - it has the same grade A fire rating. (our BCO wanted it laid on chicken wire which was fixed to the joists) -- Cheers, John. /================================================== ===============\ | Internode Ltd - http://www.internode.co.uk | |-----------------------------------------------------------------| | John Rumm - john(at)internode(dot)co(dot)uk | \================================================= ================/ |
Stress calcs. for the intellectually challenged
The message
from Tim Lamb contains these words: OK again. So provided the rafters/sheets are rigid and the wall plate prevented from spreading by the truss tie, the purlin and ridge are mainly decorative? That is not what I intended to impart. Looking at one side of the roof in isolation the wall plate and the ridge will take 25% of the total weight of the roof and the purlin 50%. As the ridge is also supporting the other side of the roof its actual loading is the same as the purlin. I was writing in jest:-) I am afraid that passed me by. My construction is such that there are no rafters. The building has three trusses at roughly 3m intervals. There is no ridge board but a purlin close to the ridge and a second midway (it may not be proper to call these *purlins* as they are load carrying members). I can secure the ridge such that a rigid triangle is formed taking all the load to the wall plates. Assuming snowloading, windloading and thrust on walls are not issues (the building has withstood these for the last century) I am concerned that there will be some sagging midway between the trusses with the additional sheeting weight. I am not 100% convinced I understand your explanation but my current assumption is that you have a barn some 40 feet long and at every intermediate 10 feet there is a triangular roof truss which supports 4 horizontal timbers (2 at approximate mid points, 2 near the ridge) to which the original roof sheeting was fixed. If that is the case and your only worry is the new sheeting sagging there is a very simple solution. Ask the supplier what the maximum allowable clear span is. If the roof is shallow enough to be walked on it might also be worth establishing whether that would require a shorter clear span. AIUI you were also worried about the strength of the horizontal supports but not of the supporting trusses. In the above set-up the horizontal members should be relatively lightweight* but the rafter (for want of a better word) sections of the truss would probably have to be more substantial (carrying more load than the purlin), considerably more substantial if there is little or no cross bracing. The agricultural solution is to try it and see: beefing up the structure if necessary. Doesn't work too well if failure precedes beefing up. :-) *I have a gut feeling that 6" x 2" (floor joist for 10 feet span) might be overkill, 4" x 2" would need checking out but to do that you would need to know what to add for weather and safety constraints and there I can't help. -- Roger Chapman |
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