Rsj size

Discussion in 'Builders' Talk' started by Suddy, Jun 14, 2019.

  1. Suddy

    Suddy New Member

    Hi there I’m a self employed joiner with small workshop 50ft x 25ft there is 3x wooden beams 9”x4” 25ft long supporting the tin roof, these beams have probably been up for 40 years, now my question is what’s the smallest rsj I can replace it with, building regs doesn’t really matter as it’s my personal workshop only me goes in there. Surely a similar size rsj would work? Maybe 8”x4” I only want to replace as the old ones are rotted, the tin roof consists of 5”x2” timber with tin sheeting on top, not a lot of weight really.
    Just tried to upload a photo but says file is too large
    Any info I will take on board please
     
  2. Severntrent

    Severntrent Active Member

    Google structural steel Blue Book. From this publication you will be able to determine suitable RSJ sizes depending what loading you feel is appropriate. Whilst the actual weight of the roof might not be great and you are not bothered about building regs you may want to consider the extra loading due to snow/ access requirements
     
  3. kiaora

    kiaora Well-Known Member

    Hi
    It may cost less to ask a Structual engineer, and not over do the job ?
    Say it’s not for building control, and your risk.

    Maybe timber with a flech plate ! Easier to install ?


    Good luck

    Regards
    Peter
     
  4. Suddy

    Suddy New Member

    Yes just don’t want the timbers eventualy snapping I know it may never happen but always better being safe then sorry, I did ask a local steel yard and they said I would need something around 18” because of the distance it covers, he said that was to building regs I know, just trying to work out what I’d get away with now in rsj, surely a 8” one would carry more than the equivalent one in wood? I think a structural engineer would be a good shout mate. Just don’t want to put something up and it really bending under is own weigh because then I wouldn’t be really achieving anything.

    Steve
     
  5. Jimbo

    Jimbo Well-Known Member

    I recently had some steels designed, cost was £150 per steel (for the design).
     
  6. Severntrent

    Severntrent Active Member

    Trouble is a 8 X 4 web shape steel is not quite the same as a solid piece of 9 X 4 wood so a logical direct comparison is not possible. even with a solid steel of 8 X 4 it would have a self weight considerably more than a wood beam. I guess the easiest way forward is to determine what your current wood beam can take and then select a steel to take the same bending moment and maximum deflection. A structural engineer should be able to do that quite easily whilst ignoring any building regs associated with a properly designed steel.
     
  7. stevie22

    stevie22 Active Member

    If you have a design to Regs deflection will determine the beam size in wood or timber especially on longer spans. Steel will bend a long long way before it fails and even then it won't break only. A 203x100 steel will be stronger than a 9x4 timber but will tend to buckle at higher loads. If it were mine I'd go for a 203x133 UB which is stronger and less liable to buckle.
     
  8. Mr Rusty

    Mr Rusty Well-Known Member

    What is the roof construction? gables & purlins? or trussed? If it's a trussed type construction remember the horizontals also stop the roof spreading (trying to flatten out). Pictures would help. There are online calculators which can help calculate beam sizes as long as you can work out the loads.
     
  9. Suddy

    Suddy New Member

    Hi there the roof is just a flat roof with a slight slope on it, with timbers 6”x2” resting on the stated 9x4 timbers, I’ve just posted a picture. Thanks Steve
     

    Attached Files:

  10. Mr Rusty

    Mr Rusty Well-Known Member

    Try this https://www.steelbeamcalculator.co.uk/?new it's a fairly straightforward application and there are worked examples. You can run the demo for free, but you'll need to buy a month's subs to run an 8m span. Run it as a uniform distributed load and flat roof (even though it has a very slight slope - not enough to make a difference)
     
  11. stevie22

    stevie22 Active Member

    That's a wing and a prayer roof!
     
  12. Mr Rusty

    Mr Rusty Well-Known Member

    I thought that, but not being an expert I thought I would keep quiet - doesn't seem to have much substance for a large area...........
     
  13. Severntrent

    Severntrent Active Member

    But its stood the test of time and even takes a ladder loading. A good snowfall will test it
     
  14. stevie22

    stevie22 Active Member

    A bad one will test it more!
     
  15. Severntrent

    Severntrent Active Member

    Depends if you're a skier
     
  16. Suddy

    Suddy New Member

    Haha they been in that position for over 60 years haha it first had the old asbestos sheets on it, then went to tin and then I replaced the tin with new tin 5 years ago
     
  17. Severntrent

    Severntrent Active Member

    Working backwards to see what bending moment the wooden beam is capable of taking and then selecting a steel beam than can take this bending moment gives a 152 x 89 UB. The deflection at midspan would be 25mm. The calculation has been done in very simplistic terms and ultimately is based on a total load acting on the beam of 4kN, when you consider that building regs have something of around 0.75kn/m2 factored into their roof loading for wind/snow loads, then based on that, each metre run of the wooden beams would be required to take about 3.8kn thus giving a total load of 29 kN. You can understand where your local steel yard specing an 18" beam is coming from.
     
  18. Suddy

    Suddy New Member

    Haha thanks but that was all very foreign to me
     
  19. Severntrent

    Severntrent Active Member

    Except for the 18"??:)

    In simple terms a 152 x 89 UB is as strong as your wooden beams so a 200 x 100 UB (8" x 4") will be even stronger
     
    stevie22 likes this.
  20. Suddy

    Suddy New Member

    Thank you for your help I’ll prob go for a 8x5 to be on safe side matey
     

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