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Thread: Roof Design Update TG9_18

  1. #1
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    Default Roof Design Update TG9_18

    Anyone had the chance to review the new Roof design guidance published on the NASC website?

    Seems to me like they are making the design a lot more onerous (time £££) with new load cases, zoning and wind approach?

    Is this in force now with the previous rescinded - all seems to have been done very quietly? I quite liked the old code - nice and simple so why change it - although I think it is probably aligned to Eurocodes now?

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    All that they have done is to lay out the things that you should take into account when you design temporary roofs if you decide to or have to follow the current codes of practice (which have been around now since 1995 in pretty much the same format). The real load cases are a lot more complicated than even they have suggested because design wind speeds from different directions are different. If you use the Euro or British wind codes for roofs, you only get tabulated information on wind zones pressure coefficients for orthogonal wind directions but you should consider winds from more than just 4 directions, giving different load intensities and different zones. Introduce variable internal pressures to account for the wind getting through all sorts of holes in side sheeting from different directions and your head will really start hurting.
    If you had to design every temporary roof to comply with EN1991-1-4, the industry would grind to a halt because there is no computer program out there that can do it properly and the cost of the number of hours necessary to prove the simplest roof is way over the cost of the hire.
    It is another example of a representative body taking a complicated situation and multiplying the complication by a large factor instead of simplifying it by the use of experience. The academics behind Eurocodes are the original source of this problem because they don't live in the real world where things have to get done. Instead, I think that they treat the development of codes as an 'interesting discussion'.
    It may have been done quietly but on their web site it says currently being updated so perhaps they have not finished it yet?

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  4. #3
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    I don't think it is any more onerous than the previous version, just had to be updated to fall in line with BS EN 16508-2015 (with some NASC changes to suit the UK). I don't agree that you can't quickly design temporary roofs to BS EN 1991-1-4; the code allows only orthogonal directions to be used along with no directional factors giving a conservative result. You can choose to refine using directional, seasonal, size etc. factors if you wish. You can also apply the pressure and force coefficients as net values to give a quicker simpler and more conservative result for some roof and wall combinations. For the very simplest monopitch roofs it is easy to create a calculation template that suits almost all scenarios. I personally don't use TG9 as i find EN 1991 covers a more comprehensive range of building geometries more suit to the types of roof i typically design. You can download TG9:18 from the website if you are a member, they've probably just not received the printed copies from the printer yet.

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