How to calculate a ridge beam size

i've had some requests about how to do a ridge beam and i want to start by explaining how to do a timber ridge beam uh because this is probably the easiest option that you've got for having a ridge beam for a a short span so for example if you've got house here and you're building a extension so single storey extension and then you're having a ridge beam up here going to the gable end and then you've got your your roof here so we're going to just draw this for you um essentially what you've got is you've got rafters sitting on a wall plate here and a wall plate there and then you've got the ridge beam supporting it in the middle um and then obviously the the roof covering over the top looking at it on the side view we would have a gable end here we'd have a ridge beam sitting on that we might have the original property here with an opening on some beams like so so if this is a cavity wall that's what this is here outer leaf and inner leaf so essentially what we've got is this beam here and what that beam's doing if we look on this section here so this particular drawing is a section through here looking in that direction if we were to draw this properly we'd say that we'd label it up say a a and then we would write here section a a and of course we wouldn't have that bit there so that's basically what you've got you've got a span from wall to wall let's say three meters and we've got a span of the ridge beam from here to here of let's say three meters as well now what i teach in terms of loading is to keep things simple so if we do the loadings here i would say for the roof you should take two kilonewtons per meter squared and that gives you an allowance of about 1.25 dead load and 0.75 live load so that's where this 2 comes from and for timber for british standards you don't need to factor it so it stays at two for steel you multiply it by 1.5 so for steel you would use three so the first question is how much load is being attracted to this ridge beam and the answer is if we look at things on plan so we've got the wall of the original house here and now we've got our new extension to the house here three meters by three meters and the ridge beam is going through the middle so the roof is spanning from wall plate to ridge and from this wall plate to the ridge so the effective area that we're having to support on the ridge quite simply is half of that and half of this so half of that span there and half of that span there so it's basically 3 meters divided by 2 is 1.5 meters so we have 1.5 meters of contribution of that roof so this area here is what is effectively sitting on that ridge beam so we're going to take 2 kilonewtons per meter squared and we'll say the loading w is equal to two kilonewtons per meter squared multiplied by 1.5 meters so that's equal to three and the units are kilonewtons per meter so that's what we call the udl or unifor formerly distributed load along this ridge beam so we've now got the loading on that beam the next thing we need to know is the moment which is the if we think about the cross section of our beam the top section here is going to go in compression and the bottom section is going to go into tension and these are equal and opposite so if we take a view along here in that direction this is in compression this is in tension and they're equal and opposite and so what we've what we call this around a midpoint is a moment because you can see you can see this circular nature of that around the point so that's called a moment so the bending moment in a beam there's a very easy formula to get hold of what a bending moment in a beam is and you've probably come across it before it's wl squared over eight and that's for udl which is what we've got so three multiplied by the length l 3 squared over 8. so let's do this on a calculator 3 times 3 squared divided by 8 equals 3.37 so we'll say 3.4 3.4 and the units are kilo newton meters so that's our moment now once we have the moments in our beam we can select a beam size the formula we need here is zed required because what i've got here for you to download if you after you've watched the video just don't download this from the download link that i've provided it's a little pdf document timber calculations the easy way and what we have in here is for various sizes of steel so depths and breadths we have what's called zed which is the section modulus now that is the geometric property that we that we need to to have in a timber beam in order for it to be strong enough so just bear with me as we go through this very easy calculation so the z value that we require so z require the z value we require is the moment divided by the allowable stress now we know what our moment is already because we derived it a minute ago it was 3.4 kilo newton meters now what we just need to do is multiply that by 10 to the sixth so what that means is multiply it by a million because we want the units to be newton millimeters because the section that we are going to get is going to be in millimeters breadth and depth and we're going to divide that by the allowable strength of the material so once you've got your download select the strength class of timber you've got so the most common in the uk are c16 and c24 so we'll go in this case with c24 and the allowable strength for bending parallel to the grain here is 7.5 so 7.5 so all we're doing is we're dividing one number by the other so let's have a look 3.4 so on this particular calculator shift and there's a little one button here that says 10 to the x so press that one and six so 3.4 times 10 to the six divided by seven point five four five three and then we can see there's another three figures before the decimal place so the notation for that is times times 10 to the 3. now if you notice on the sheet everything in this column is times 10 to the three so what that means is when you get your answer where you've got three figures after the sorry before the decimal place just ignore those and read those three here so four five three four five three times ten to the three millimeters to the cubed so all we need to do is read down here where we've got something larger than 453 so we could have a 200 by 75 beam or we could have two two five by sixty three actually what i'm going to do is i'm gonna go with two lots of uh of 175 by 50. 175 by 50. two times 175 by 50. and that is the size of the ridge beam that you will need so download for yourself the timber calculations the easy way tables which will give you everything you need to know in table form for doing this calculation get yourself back to the bottom of the video get yourself a piece of paper and design a ridge beam for yourself give it a go first with the same dimensions that i was using three meters beam span and three meters from wall to wall and then have a go at something larger or smaller than that if you want to leave me any questions in the comment section below the video and please do i'll try and get to those uh what once a week or so the other thing i would say is that once you've specified your beam and made it strong enough as we've done now you also just need to check whether the deflection in that beam is going to be too great for your ceiling and that's a criteria just to stop the ceiling uh cracking and the finish is cracking on that ceiling if you've if you've got it plastered now that's covered in another video and i'll leave the link for that in the comments below

Channel: Robin de Jongh

Views: 1,741

Keywords: beam calculation, building math, calculate a timber beam, how to calculate a ridge beam size, how to size a ridge beam, ridge beam, ridge beam calculation, ridge board, robin de jongh, roof ridge beam size, structural calculations, timber beam calculation, timber beam design, timber ridge beam, what size is a ridge beam, wood ridge beam

Id: CyEsXFINGVM

Length: 13min 55sec (835 seconds)

Published: Fri Oct 08 2021