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how do I calculate loading?


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I'm about to begin doing the floor of a 1st floor granery. It has 200mm x 100mm joists on 450mm centres running to and from wall sockets (not notches) at each end. From end to end the room is 8.5m but the joists are supported halfway along on a stone bearer wall (not quite in the middle; 4m, then 4.5m).

The covering boarding is shot to Hell, but the joists are in perfect condition (no shakes, cranks, knots or worm-holes).

I am planning to strip off the boarding, replace it with 21mm chipboard, covered with high density foam insulation sheets with a serpent of 16mm pipes for the underfloor heating on top, embedded in 50mm of concrete, then something on the top but probably not as heavy as floor tiles.

Question 1 :

How can I calculate how much per metre this club sandwich will weigh?

Question 2 :

How do I find out if my joists, as detailed above, will support this weight?



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Hi Paul,

Firstly I assume your joists are 450mm centres and not 45mm as your post suggests. If infact they are 45mm centres just go ahead as there will be no problem.

By adding chipboard you are in theory creating a T joist which will add some structural strength to your floor.

Since the overall condition of you joists can not really be determined accurately . I would suggest a small experiment subjecting each joist to a point load test which will give you an indication that they will be strong enough to support your floor. The overall load on the floor will not however be classified as a point load as it will be evenly distributed across the surface area of the floor.

This can be done quite simply by calculating the total weight of concrete, chipboard flooring and the water weight within the heating pipes in total for each of the two areas and then dividing it by the amount of joists you have in each case. Then subject each joist to a point load equal to that in the centre of the joist. Note any deflection or structural damage to the joist and move onto the next one.

Weights for 50mm concrete are 122.2336kg per square metre and chipboard (22mm) is 18kg per square metre.

Hope this helps Paul.

I expect I will be ripped apart by the experts waiting on the wings who have not answered your post already as not the way they would do this.


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As well as the "dead-load" (weight of floor, finishes etc.) you want to consider "live-load" (weight of people). Would your floor cope with a full room of party goers all dancing / jumping about in tune?

Apologies for the mixture of imperial units, but trust me the following daft looking formula is a good rule of thumb for joist sizes - it's not a proper calculation but it just gives you an idea of what you may want.

Here goes: take the span of the beams in feet, halve the number and call it inches. Then add one and round up the inches to the next whole number and that is the depth (in inches ) required of your 2 inch (50mm) wide joists.

Example: Span is 11 feet, half is 5.5, call this inches and add one - so this gives 5.5 + 1 = 6.5, say 7 inches. Therefore for joists at 450mm centres you need 50mm wide joists of depth of 177.8, say 175mm.

Get out clause:

No guarantee is given or implied. No offer is being made. This is not advice to be taken. I know nothing. Do not rely on this information. Use a qualified structural /civil engineer to provied calculations.

Sounds daft - but "educated" builders in London have been using this for years!!!   



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       just  a reply to the last post " london builders have been using this for years " no wonder .

             ok the right way I was tought in college was sorry it`s imperial but you will get the jift .it`s twice the width in yards plus 1.. so for a 12ft span you will need 4x2+1= 9x2 .this is the standard size for floor joists in the uk @ 16" centres the french tend to use 200X63 or 8x2and 1/2 in old money and wider centres in fact there is more wood in the french way but the floors bounce a bit .

               just for info and no malice intended


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A quick check on the joist sizes gives a bending stress of approximately 5.3 N/mm sq and a deflection of 12.8mm,for the 4.5m span.This is acceptable for a general grade structural softwood.This includes a standard domestic live load.

Sorry for not replying earlier,just returned from france.

Regards Tony MIStructE

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[quote]A quick check on the joist sizes gives a bending stress of approximately 5.3 N/mm sq and a deflection of 12.8mm,for the 4.5m span.This is acceptable for a general grade structural softwood.This includ...[/quote]


 thanks for taking the time to look it up, now could I trouble you further to translate it please? (my technical college was bombed, you know!)

What does this mean in terms that an idiot like me can understand? Can you give me an idea of how much weight I can safely put on the joists? 


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Paul,The approx capacity of a 200x100 joist spanning 4.5 metres (general grade softwood) is 680kg total load.

The dead load is approx 325kg which leaves you with a live load capacity of 355kg (approx four and a half adults per joist !)This is slightly more than domestic design live loading at 150 kg per sq metre.

Regards Tony

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