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Steel beam size

Discussion in 'Building' started by messimagician, 13 Aug 2017.

  1. messimagician

    messimagician

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    So after looking at the steel beam my SE as specified, I have noticed the depth between the flanges on steel beam is smaller than the actual joists on the 1st floor.

    The joists are 175mm and the beam which was specified is 178mm however the beam only as 146mm between the flanges so you can't sit the joists in between them

    I asked the SE to have it recessed in the ceiling so it's near flush. The SE specified using joist hangers but not actually said which to use. Is this a common thing having a beams flange depth smaller than the joists? I've tried searching but pretty much everything I've seen so far shows steel beams with the same depth as the joists whether on joists hangers or sat on the flange. I think I seen one where the joists were notched but I thought that was a bad idea seeing as we're talking over 30mm


    The steel beam is 2m with a span of 1.8m and will be supporting the inner leaf and external leaf on it. There is a 2.6m flat roof extension where the joists run parallel to 1st floor joists which the steel beam will also be supporting
     
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  3. tony1851

    tony1851

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    Are you using two steel beams? Otherwise, how do you support both the inner- and external leaves on a 102mm wide beam??

    (By the way, you can notch the joists if you want to sit them direct on the bottom flange; notching a 175 deep joist into a 178 deep UB will not be a problem). Alternatively, if you want to use joist hangers, you will need to bolt a piece of timber to the web.
     
  4. messimagician

    messimagician

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    The external leaf as a small galvanised beam sat on top of the extension joints and these joists then sit on the existing timber beam which is under the internal leaf. I'm not sure why it was done that way. I thought maybe it has something to do with air flow from the cavity and cross ventilation ect. If I decided on putting a second beam in it would restrict air flow I guess unless I put it below the joists.

    Ok that's great to know you can notch the top. If I were to have one side of the joists in the web do you then have to fasten timber to the under side of the steel beam to fix the plasterboard to?

    Or if I were to use hangers and have the ceiling flush do you need to notch out the hanger thickness at the bottom of the joists?

    Are there particular hangers I should use for this as my SE did not specify which ones to use. The ones I have seen are quite long that they wrap over the top of the beam and seeing as the hangers on one side will not only be supporting a roof but also external leaf as well. the thought of 40mm twist nails holding this up is crazy haha
     
  5. tony1851

    tony1851

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    You can notch the underside of the joists as well.
    (Still can't work out what's going on - you shouldn't have the skin of a wall supported off the joists.
    Are there any pics/details?). Scan0110.jpg
     
  6. messimagician

    messimagician

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    Eeek that's not good then. I've just done a drawing which hopefully helps. The extension is part of the kitchen and leads to the garage on the side of the house. The timber beam is on the inner leaf and supports both sets of joists and the steel beam on the outer leaf is above the joists of the extension. The exterior wall does have step cracking and was flagged up in the home buyers report

    When I did speak to SE about it he said it seemed OK with how it was done. It was something that builders use do but after running the calculations said it was not adequate

    If you can't do it like as in my drawing then maybe the only way would be to have two beams but have them under the joists just to make sure that it doesn't cause any issues with ventilation.
     

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  7. tony1851

    tony1851

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    I can't understand why the flat roof joists run onto the existing timber beam of the inner skin and under the steel beam?
    If ventilation was to be a problem, you should just have a warm flat roof (with the insulation on top of the deck). That completely avoids the ventilation problem.
    The correct way would be to have the underside of the new steel beam at the same level as the underside of the old timber beam, and run the new roof joists onto the bottom flange of the beam. That way you could achieve a flush ceiling.
    You seem to be making a complex and difficult design to solve a problem (ventilation) which could be solved in a far easier way.
     
  8. ^woody^

    ^woody^

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    You bolt a timber in the web, and then fix hangers to this and over the top of the beam.

    Loading the flange is how the cowboys do it.
     
  9. messimagician

    messimagician

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    It's a little difficult to show it all. The kitchen extension and garage flat roof runs the whole length of the side of the house, the fall of the flat roof also runs from front to back as well. The height of the flat roof prevents a warm roof as its too near a window on the landing. The only option was keep it a cold roof and have it re felted. I did talk to the roofers about a warm roof but they thought it was a waste seeing the 3/4 of the roof was covering the garage. Maybe it might of been beneficial if I was converting it


    I'm not sure you understand, this is not my design, this is how it's already been done, there is no new joists going in. If I understand what your saying then what your suggesting is having a suspended ceiling? That's not possible due to height 2.4m ceiling. I have no idea how long a go this work was carried out. It was obviously carried out well before warm roofs were thought of. The previous owners both passed away and it was being sold by solicitors. The house was built 1959

    I can only assume it was done this way because of ventilation and to allow electrical cables to pass through to the roof of the extension.

    I can only see two ways. recess the two steel beams and hope it doesn't cause an issue with ventilation or have both steel beams under the joists and box them in.

    If I go with having them under the joists then maybe I can get away with using smaller beams put in. It's not how I would have liked but at least I know it's done correctly

    I have just emailed the council again to double check for any reports on the property in case something was missed by the solicitor. I didn't say anything about an extension incase they put notice on it. If I did carry out any work to the beams and I did notify the council would they expect the whole extension to meet current standards?
    The SE said some thing about getting it done under retrospective work?
     
    Last edited: 13 Aug 2017
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  11. tony1851

    tony1851

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    No it isn't - done it loads of times. Why should it be a problem?
     
  12. ^woody^

    ^woody^

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    Because the web takes the load.
     
  13. Blagard

    Blagard

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    It should not be a problem if done correctly. NHBC Standards below.

    6.4 - S6
    Joists shall be properly trimmed into steelwork

    Joists trimmed into steelwork should be notched at both top and bottom and have 12mm/2mm projections respectively to allow for timber shrinkage.

    [​IMG]

    (b) bearings onto steelwork
    Strutting should be provided where solid timber joists bear on, or are notched into, steelwork to prevent rotation.

    [​IMG]
     
  14. tony1851

    tony1851

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    Yes, normally an incoming beam is bolted to the web via a welded end plate. In that way, no torsion is imposed on the
    supporting beam.
    But domestic floors are relatively lightly loaded, and the eccentricity caused by loading the bottom flange is almost always inconsequential. Here's one with the floor joists notched onto the bottom flange of a 152 x 152.(not too clear unfortunately) DSCF5171.JPG
     
  15. ^woody^

    ^woody^

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    Just because it can be done does not make it correct.

    The nhbc are one of the worst for lowering standards to suit the lowest denominator, rather than enforcing good practice and high standards
     
  16. ^woody^

    ^woody^

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    Above all though, it's so much easier and less of a faff.
     
  17. tony1851

    tony1851

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    Yes, yes, yes..... all the Codes say connect via the web blah, blah, blah............but in the real world resting timber floor joists on the flange works. As a matter of fact, the additional bending stress caused by the torsion can be calculated and allowed for, so no problem.
    Next.....
     
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