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Calculating beam loads/spans


oldgit72
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Does anyone know a web site where it is possible (without needing to be a mathematician) to simply calculate what size of beam is required to span a given distance/support a given load? Having googled my requirements, I can only seem to find American sites that speak a language I do not understand.

 

Thanks

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Thanks TP but no good as most of the US sites seem only to quote beams with 2-4 inch widths and I am looking to calculate for oak in metric sizes of  15cm x15cm.  My barn which I plan to renovate next year is 15m (14m internally) wide and I want to know if 15cm oak beams @60cm centres will span 5 meters in order that I can divide the ground floor/3 with 2 block built internal walls running front to back and supporting the beams. Hope this makes sense.
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If its any help we and other friend's houses have oak beams at 1 metre centres which seems to be the norm round here,  The ones I have measured are spanning 5 metres and are approximately 16cm x 16 cm, so that is 6" square, some are 14  cm wide by 16 cm deep.  You could ask at a timber yard what the builders use. If theybare being "supported" I would think 15 cm square would be more than adequate.
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Are you going to have a floor loading above these joists? - I work on 1.5KN/m2 (around 150kg), will there be an other load bearing partitions etc? Will you be screwing ceiling plasterboards directly to the joists, in between them or using suspentes?

Finally is there any particular reason why you have chosen a square section to resist a bending load?

All the joist tables if you can find them are for rectangular section structural softwood restrained with noggins or joist straps and are calculated on a deflection basis so that any ceiling screwed directly beneath will not crack under the applied load, if you intend to use a suspended ceiling then or plasterboard between the joists then it will be largely immune.

All of my info on material strengths etc is in the UK, only last week i was faced with a similar problem, I needed to put a load bearing ceiling in my grenier that could support my weight plus storage etc, due to height restrictions I had to have the minimum depth of timber with the plasterboard screwed directly underneath it, moreover the load had to be taken by chevrons that are already inadequate for the tile loadings, in short a can of worms.

What I did, and what you might want to consider, is to set up a test rig on the floor with 3 of the chosen joists (in my case demi-chevrons) sat on blocks at the required centres, I then screwed a floor panel to it and jumped on it to see what the deflection looked and felt like. In my case I now have 60mm by 38mm "joists" (demi-chevrons) at 40cm crs spanning 3.3 metres but with the central span supported by tige filetées at 1m crs, no way could it support a 1.5kn floor loading but in my case I reckon it will be OK.

I have some existing joists that are definitely overspanned, the floor bounces and the ceilings beneat have cracked, they have 175 by 63mm bastaings at 40cm crs spanning just under 5 metres, this might give you something to mull over.

If you can answer the above questions then I would be able to do some deflection calcs for you next week when I am in the UK. Its a bit to late now to do my own calcs [:D]

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Not much use for 150 by 150 oak beams unfortunately.

Sorry to ask once more Dave & Olive but what is with the "Hi Ok" ?

I dont think in ye good old days anyone actually calculated the load bearing capacity of oak beams but relied on experience and/or empirical methods like I suggested, if they had done the calcs they would have very quickly sected a more suitable section of timber like 150 by 75mm.

Doubling the width of the timber raises its its load bearing capacity by a factor of 2 but doubling the depth of section raises the capacity by the square of the depth.

So for an example if a 1m length of 75 by 150 can safely carry 500kg then doubling the width to 150 by 150 will increase it to 1000kg but doubling the depth (same volume of wood) will increase it to 2000kg.

That is one of my gripes with the very limited sections of wood available in France, those based around chevrons (80 by 60) are about as much use structurally as a chocolate teapot, oh to be able to get my hands on some 4" by 2" [:-))]

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[quote]

Are you going to have a floor loading above these joists? - I work on 1.5KN/m2 (around 150kg), will there be an other load bearing partitions etc? Will you be screwing ceiling plasterboards directly to the joists, in between them or using suspentes?

 

Above the beams will be partitions but not load bearing ie plasterboard fixed to 3x2 (equivaent metric)  timber or possibly metal frames. I am also intending (finances permitting) to fix 3cm thickness oak t+g flooring. Other than that, the load bearing will be normal 1st floor furniture, bathroom fittings,water tank and people. From your numbers, are you saying that 1sqm of beam surface can support 150kg? or have I misunderstood? If this is right then by my calcs, 250 linear metres of 15cm beams would support 5625kg (250x0.15x150). But surely this depends on the depth as well as the surface or were you referring to m3 not m2. Please clarify.

Finally is there any particular reason why you have chosen a square section to resist a bending load? Aesthetics and tradition around here (charente)

All the joist tables if you can find them are for rectangular section structural softwood restrained with noggins or joist straps and are calculated on a deflection basis so that any ceiling screwed directly beneath will not crack under the applied load, if you intend to use a suspended ceiling then or plasterboard between the joists then it will be largely immune. The plan is to fix soundproofing insulation and plasterboard between the joists probaby fixed to batons fixed to the sides of the beams..

All of my info on material strengths etc is in the UK, only last week i was faced with a similar problem, I needed to put a load bearing ceiling in my grenier that could support my weight plus storage etc, due to height restrictions I had to have the minimum depth of timber with the plasterboard screwed directly underneath it, moreover the load had to be taken by chevrons that are already inadequate for the tile loadings, in short a can of worms.

What I did, and what you might want to consider, is to set up a test rig on the floor with 3 of the chosen joists (in my case demi-chevrons) sat on blocks at the required centres, I then screwed a floor panel to it and jumped on it to see what the deflection looked and felt like. In my case I now have 60mm by 38mm "joists" (demi-chevrons) at 40cm crs spanning 3.3 metres but with the central span supported by tige filetées at 1m crs, no way could it support a 1.5kn floor loading but in my case I reckon it will be OK.

I have some existing joists that are definitely overspanned, the floor bounces and the ceilings beneat have cracked, they have 175 by 63mm bastaings at 40cm crs spanning just under 5 metres, this might give you something to mull over. Presumably softwood not oak?

If you can answer the above questions then I would be able to do some deflection calcs for you next week when I am in the UK. Its a bit to late now to do my own calcs [:D]

[/quote]

 

The other issue I have (once I have determined Beam sizes) is what size block walls I will need to support the beams. The Barn is 14 metres wide by 11 meters deep and I am planning 2x block walls running front to back (11 metres) in order to divide the footprint into 3 equal sections providing central support for the beams. Hope this makes sense.

 

Heres a floorplan if it helps:

 

[IMG]http://www.freewebs.com/16vert/photos/Big%20Barn%20plans/paint%20barn%20gf%20only-2.bmp[/IMG]

 

Thanks

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Oldgit.

The 1.5KN/m2 is the accepted uniformly distributed domestic floor loading, i.e. the average weight carried by each square metre. 

To select a suitable beam section you calculate the the weight of the floor and ceiling/insulation materials per M2 you add to this the 1.5KN/m2, this figure is multiplied by the floor area and then divided by the number of beams supporting the load, finally the self weight of the beam is added, this gives the load to be carried by each beam.

If using standard softwood joists then you would select a suitable one from the tables, for other materials like oak, unless someone can find some published tables, the proper calculations begin.

You are correct, the floor that I referred to is supported by softwood bastaings.

I am jealous of the amount of space that you have to work with, you would get 4 of the appartments/studios that I am building in that space!

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Ted Benson's book "Building the Timber Frame House" contains tables and formulae for calculating beam sizes. I think I've left the book in France but have set up a spreadsheet with the formulae for calculating beam deflexion.

Plugging in your figures indicates a deflexion of around 1/200 of the beam length for a 150x150 oak beam supporting 600kg - I'm assuming total load of 200kg/sq m. This indicates your beams are not up to the job as Benson suggests maximum deflexion of 1/360 which would require beams 180 (vertical) x150 (horiz).

I'm not a structural engineer so can only reflect information from the book but I've used the tables and formulae for beams and posts I'm installing and seem to get the right result in terms of rigidity.

There was a thread on here a while ago in which someone was asking about the floor stiffness required to lay ceramic tiles on a wood floor - that included a link to a site with a set of tables - if you can find that you will get more info.

In the meantime I'll have another go at tracking down the book

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  • 4 weeks later...

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