Lintel Calculations - Newtons and things.

[JoshRogan]

(I apologise in advance, far too long winded - the last two sentances are probably all you need to read but I know how you guys like your info!)

Just running through some caculations and I would be grateful if anyone could confirm my 'workings'. Obviously I will check with lintel company etc for finalising but I like to try and work things out for myself to get a better understanding.

I've been using the following for calculating the mass of an area
www.planningportal.gov.uk/uploads/br/BR_PDF_ADE_2003.pdf
Approved Document E, Annex A3.1 (table A.2 Page 73

My garage opening is 3.0m wide and blockwork is made up of 440x215x100 DCBs laid FLAT. My blocks weigh in at 18kg each. The overal length of this side of the wall is 4.8m and the gable wall will extend no higher than 2.5m above the lintel to an apex roof and will be of blocks laid on EDGE, single skin. For the purpose of this exercise i'm leaving out any affect of the roof on the lintel and wall surface finish. I have chosen a lintel length of 3.6m, a bit on the generous side perhaps.

So, from my method.
1) The triangle AREA of the gable wall above the lintel will be 9.0m2 [0.5 (3.6 * 2.5 ) * 2]
2) Mass of 1m2, with blocks laid on edge being 18kg, will be 190 Kg/m2 (table A.2)
3) Therefore mass of blocks above lintel will be 9x190=1710kg

So I think I have that bit sorted but where I am stuck is that when you look at any Lintel spec they give a unit of kN. Now I imagine the k is simply a 1,000 units so where the spec may say 20kN it in fact is 20,000N?

Also, then, is my result of 1710kg directly comparable to 20,000N??

Many thanks

c.
PS One thing I thought of, will I need a pier rising up to the apex?

 

[jeds]

To get to Newtons you simply multiply by a factor for gravity - which is 9.81. In fact engineers don't bother with 9.81 but tend to use 10 instead. So 1710kg is 17100Newtons. Or, divide by 1000 to get 17.1kN.

You should also multiply your load calculations by a factor for safety. For a house this would normally be 1.4 for dead loads and 1.6 for live loads. For storage buildings you can use 1.2 for both.

As a double check you should also calculate for deflection of the beam. (K*WL²/EI) The maximim allowable is about 3mm per metre. If a beam is good for deflection it is normally OK for loading as well.

Just one other thing. When calculating a triangular gable you should normally allow for the fact that that the load is greater at the centre of span (at the maximum point of bending) but diminishes towards the edge. This feature produces and odd looking curved shape to the bending moment diagram which is slightly more tricky to calculate. My advice is to calculate the gable as a square but leave off the top two courses - which are relatively insignificant and can be safely ignored.

 

[aconnor2]

send a sketch to IG Lintels they'll design it for you

 

[JoshRogan]

Jeds - Many thanks for that concise answer, I thought my calcs were out, forgot about the conversion to Newtons.

Would I apply the same principle when working out lintels on the opposite side of the garage, where I have a door and a window that require lintels. I.E. would I just take the rectangle above each span individualy to work out the calcs for each span (or, of course, use one big lintel across both if they are close enough)?

Aconner2 - thanks, IG are on my list I think (thank god most of them have PDFs brochures of all their products!). They all seem to have technical departments that are willing to specify, which is good. Must be a lot of competition, did not realise there were quite so many lintel makers.

Cheers.

C