Universal beam calculation

Discussion in 'Builders' Talk' started by NAF, Jan 18, 2006.

  1. NAF

    NAF Member

    Hi, I'm replacing a section (2.2m) of first floor internal suporting wall with a universal beam (2.5m).
    The beam will have to support floor joists, p/b, flooring only (no walls etc.)
    Rooms above whose floor joists will be supported by the universal beam (ub):
    1. have an area of 13.6 m2
    2. dead load (0.75 kN/m2) + live load (1.5 kN/m2) = 2.25 kN/m2
    3. load is 13.6 x 2.25 = 30.5 kN
    4. beam only suports half the load (one end of joists supported by wall so load is 0.5 x 30.5 = 15.25 kN

    I've looked on the Corus site at

    http://www.corusconstruction.com/en/design_and_innovation/structural_design/the_blue_book/section_properties/british_standardsections/joists/

    and there are lots of specs for different sized universal beams.
    Looking at a 152(h) x 89(w) x 16(kg/m) beam the spec is:
    1. buckling resistance for a 2.5m ub = 19.8 kNm
    2. Compression resistance for a 2.5 m ub: Pcx = 529 kN, Pcy = 222 kN
    etc.

    Please can someone tell me what are the factors I need to look at to meet my loading requirements.

    Thank you,

    Neil
  2. NAF

    NAF Member

  3. Lightning McQueen

    Lightning McQueen New Member

    You've omitted something very important for steelwork calculation - partial safety factors.
    In your case these are 1.4 for dead load and 1.6 for live load.

    ULS loading = 1.4 x 0.75 + 1.6 x 1.5 = 3.45kN/m2 so load on beam is 23.5 kN.

    0.75kN/m2 may be a little high for a domestic suspended floor but it's your floor I suppose so you know exactly what there is.

    Anyway, 2.2m clear opening with 150mm bearing each end - take span of steelwork as bearing to the centre of bearings so span = 2.35m.

    For span and load as given above, bending moment is 6.9 kNm. Therefore you need a beam with a buckling capacity greater than than 6.9 kNm for your span.

    Experience tells me that your 152 x 89 x 16 UB should be fine over this span. Write up your calc, plop it onto the desk of your BCO for checking and you should be on your way.
  4. NAF

    NAF Member

    Thank you Skil6513.

    Please can you tell me how the bending moment is calculated - is that a Superbeam output?

    Thank you
  5. Lightning McQueen

    Lightning McQueen New Member

    No problem, NAF.

    You don't need superbeam or any other fancy software for this problem - it can be considered a simply supported beam. The loading from the floor can also be considered a uniformly distributed load i.e. could be expressed in kN per metre run.

    For simply supported beam carrying UDL your bending moment is wL/8 where w = total load on beam and L = span of beam.

    For completeness and BCO submission you should also considered shear forces and deflection of the beam.

    For your ss beam shear force is half the total load, say 12 kN. Shear capacity of 152 x 89 16 UB is 113.2 kN, so no problem there.

    Deflection is usually worked out considering the unfactored live load only, i.e. 1.5 kN/m2, and for a ss beam carrying UDL deflection = (5/384)*(wL^3)/(EI).

    w = load on beam (10.2 kN in your case)
    L = span of beam
    E = Young's modulus for steel (205 kN/mm2)
    I = second moment of area of beam (834 cm^4 for 152x89x16 UB)

    In your case this gives 1.0 mm deflection which is span/2350 therefore deflection is not a problem.
  6. ceg999

    ceg999 Member

    Hi, apologies for gatecrashing this topic, but do you know if there is an equivalent, to the Corus site, for timber calculations?...thanks
  7. Lightning McQueen

    Lightning McQueen New Member

    Try the TRADA website.

    http://www.trada.co.uk/

    I think they have span tables for basic timber elements such as joists/rafters etc.
  8. ceg999

    ceg999 Member

    Hi Skil, you got me all excited there for a moment and I got straight onto the site ... unfortunately TRADA ask you to sign up to full membership before letting you look at their tables (about £130). I only want to test some ideas so thats a bit dear. but thanks.
  9. Lightning McQueen

    Lightning McQueen New Member

    OK, try this link.

    http://www.odpm.gov.uk/index.asp?id=1162863

    It's to the website of the ODPM. At the bottom of the page you can download a pdf copy of the 1992 building regs. This version of the building regs has been superceded now but have a look at Appendix A where you will find span tables for timber in the old strength classes of SC3 and SC4. It may be helpful in giving you an "idea".
  10. ceg999

    ceg999 Member

    Hi Skil, thanks again...I've already got the span tables but what I was interested in was the timber equivalent of the Corus tables showing stuff like elasticity modulus and stress/load factors. I have the maths but need the properties data to test out some A frame designs. regards
  11. Lightning McQueen

    Lightning McQueen New Member

    ceq999,

    The elastic modulus for timber is a variable quantity, however for stress graded timbers the minimum values are 5800 N/mm2 for grade C16 up to 7200 N/mm2 for grade C24. These are the figures quoted in the BS.

    Section properties are easy for timber because it's all rectangular cross sections.

    Area = b * d (you probably knew that)

    Section modulus, z = (1/6) * b * d^2

    Second area moment, I = (1/12) * b * d^3

    b = breadth of section and d = depth of section.

    That is all you should need to know.

    Good luck,

    Skil.
  12. ceg999

    ceg999 Member

    Skil, your my hero....thanks
  13. Lightning McQueen

    Lightning McQueen New Member

    It's nice to be somebody's hero, I suppose.

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