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Correct weight for 130mm celotex in kN/m2 at 0.25?

Discussion in 'Floors, Stairs and Lofts' started by Bowlzi, 28 Apr 2017.

  1. Bowlzi

    Bowlzi

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    Hi,
    I'm looking at some calculations for a beam. Is it correct that celotex is listed as 0.25kn/m2 ?

    Only unsure as it is listing plasterboard and skim at only 0.12kn/m2 and 25mm flooring at 0.13kn/m2

    Surely it's not heavier than these two things?

    Thanks
     
  2. tony1851

    tony1851

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    Nothing like that - someone's put the decimal point in the wrong place.
    But if you are doing calculations for a beam, why are you being so precise with your loads - it can't be that critical, surely?
     
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  3. John D v2.0

    John D v2.0

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    Yeah should be 0.025kn.
     
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  4. Bowlzi

    Bowlzi

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    Thanks for your replies.
    I am looking at the calculations given to me for a 6m ridge beam that has come out a 203x203, 46kg and it seems huge to me for a ridge beam. It is a straight forward flat rood loft dormer of aprox 5m and velux type windows in pitched side.
    I can't see why something so heavy as this is required to support at the ridge when the dormer is built up from the outside wall and it will be just a felt roof on top.
    Let me know if there is any better way of explaining with drawings or pictures.
    Thanks
     
  5. tony1851

    tony1851

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    Far too big. On a 6m span flat-roof dormer with conventional pitched roof to the front, either a 203 x 133 x 25 or a 203 x 133 x 30.
    Have you been at the on-line calculators again? :LOL:
     
    Last edited: 29 Apr 2017
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  6. Bowlzi

    Bowlzi

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    Yes, that is what I have thought. The calculations are from a paid an engineer. It was originally a 5.1m 203x133 with a support post at either end (one in stair wall) I did not want the post and wanted it to span into gable wall as was discussed originally with them.
    This is the new full length beam in the calculations. 1 beam with, no join, supported at ends only.
    There is no beam under Ashlar wall now, just the new 47x175 floor joists resting on exterior and spine at aprox 3.2m
    It also lists every beam in pitched roof to be made 2x 47x125 bolted together at 300centres. Currently has singles of 50x100 as you would expect.

    It just sounds overkill and a lot more work hence my query on weights as I wanted to go back and ask if there has been an error.
    Scaffolding is up already...
    Structurally I can't see why they have done this?

    Thanks
     
  7. tony1851

    tony1851

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    Because they're incorrect!.

    Why not post the calcs here (minus your address) so we can have a look!
     
    Last edited: 2 May 2017
  8. Bowlzi

    Bowlzi

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  9. tony1851

    tony1851

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    The loadings seem reasonable - if a touch on the high side, but that's just my opinion, though the insulation is of course too high.

    What is very strange is the data for beam 6 (ridge beam). Unfortunately they have given no figures to show how they arrived at this size, but let's see how this could have been done:

    The reactions (ie the load at each end) suggest a total load of 43 kN on the beam. Even if this was 'unfactored' (ie without the safety factors added) this load would be no more than approx. 43 x 1.5, say 64.5 kN. This would give a bending moment of 49kNm. Even if the beam was regarded as completely unrestrained by the roof structure, that section of beam would have a capacity of more than 70kNm; there are plenty of smaller sections which would easily cope with that loading and span.

    The size of the bearing plate at each end - 400 x 100 - is also odd for a low load such as 21.5kN, unless your walls are made of merangue.

    To specify a Universal Column section for a domestic loft conversion ridge is just bizarre. The problem here is that you are paying twice for this - once for the incorrect/sloppy calcs, and then again over the odds for such a heavy beam, when a cheaper one would suffice.
     
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  10. Bowlzi

    Bowlzi

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    Thanks.
    Surely the beam a is where one end will be and other on a fabricated steel post? Bricks are white engineering bricks.
    I'll try and upload other data pages.
     
  11. Bowlzi

    Bowlzi

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  12. tony1851

    tony1851

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    Beam A is presumably supporting the ridge beam over a window? If so, the 127 x 75 is OK.
    The other end of the ridge against the gable is to be supported on a steel post? If there is a loadbearing wall directly beneath the post, then fine. If not, it is easier to support the ridge off a 1m length of 100x100 box section with welded lugs on it bolted to the wall.

    But for the ridge beam, the figures on the sheet are of course OK, but the spreadsheet does not 'tell' the SE what beam to use - it merely clarifies whether or not the beam chosen by the SE will satisfy all the parameters, which it will.
    But you could check a massive 1m-deep girder on the spreadsheet and it will still say it is OK; it won't tell you that it's 'too big'.
    The skill is in choosing the smallest practicable section, and as above there are a number of lighter, cheaper and more practical sections.

    If you have a lighter beam, you might avoid having to use a crane, or alternatively you could have the beam in 2 sections to be bolted together, though there are fab. costs with that.
    Maybe have another word with your SE?
     
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  13. Bowlzi

    Bowlzi

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    Yes there is load bearing wall below and a steel post to the chimney side of this beam.
    It has all been very stressful. all I have ever asked for was a single beam (to avoid fab costs) because it can come in through the new window opening.

    Such a shame as this should have been quite simple but for the SE no doing what was asked for in the first place.

    What is your opinion of the 94x122 (2x47)joists being bolted together at 300 centres. This seems unnecessary too when you see what is there currently?

    Your replies are much appreciated.

    Thanks
     
  14. Ian H

    Ian H

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    If he were to join 2 beams for the ridge would that increase the section size? I mean if a 1 piece beam was a 203mm would a 2 piece have to be a 254mm for example?

    I lifted a 6mtr 254mm beam into a bedroom ceiling and hate to imagine lifting one up to a roof ridge.
     
  15. tony1851

    tony1851

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    No, it would be the same section-size all along the beam. The splice is designed to transmit the stress across the break in the beam and usually has plates bolted to the top and bottom flanges, and one bolted to the web. The snag is that the plate and bolts stick down below the underside of the beam. You also have to use special 'friction-grip' bolts - ordinary bolts won't do.
    Google 'pics of steel beam splices' for examples.
     
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