steel spans Best answer on the web

Posted in: darrelrussell.com edit
07 Jan 2009
  • Hello!
    I am thinking of building a balcony off the back of my house. The
    total dimensions will be 6' x 30' with the longest unsupported span
    being 15'. I would like to use 3"x6"x3/16" rectangular steel tube
    laid out on one of the following schedules A) 24" centers(best) B) 18" centers(better) or C) 16" centers(good). Could you tell me what the deflection would be?


  • Sorry to bother you again, but I just looked at some info that changes what I just said in the previous request. The figure I gave you of 100 psf dead load was for balconies in public buildings. I see that values used in residential construction are in the 50 to 60 psf range.


  • I was looking back at this question and I am having second thoughts. I solved the problem as if the 3x6 beams are cantilevered out from the house. Now I notice the 15' dimension you mention. Are you using columns. Do the beams run parallel with the house wall. If so the numbers I gave you are wrong. I will be glad to recalculate. Just let me know.


  • Okay blisstout, here we go. The formula comes from my "AISC Manual of Steel Construction" for a cantilever beam:
    Deflection = wl^4 / 8EI

    Where:
    w = (live load + dead load) x spacing x (1/144)
    NOTE: 1/144 is a factor to keep the units correct.

    l = length of beam in inches = 72

    E = constant for steel = 30,000,000 psi

    I = moment of inertia for 3x6 tubing = 14.0 in^4 (from AISC)

    -------------------------------------------------------------------------------
    A. For 24 inch centers
    w = 65 x 24 x (1/144) = 10.83 # per inch.
    Deflection = 10.83 x 26,873,856 / 8 x 30,000,000 x 14.0 = 0.087 inches

    -------------------------------------------------------------------------------
    B. For 18 inch centers
    w = 65 x 18 x (1/144) = 8.125 # per inch
    Deflection = 8.125 x 26,873,856 / 8 x 30,000,000 x 14.0 = 0.065 inches

    -------------------------------------------------------------------------------C. For 16 inch centers w = 65 x 18 x (1/144) = 7.22 # per inch
    Deflection = 7.22 x 26,873,856 / 8 x 30,000,000 x 14.0 = 0.058 inches

    If there is something that you don't understand, please ask for a clarification.

    Redhoss


  • Thanks for getting back to me. The dead load will be less then 5psf(Trex brand decking) and I think that Residential Building Code for Seattle requires that it be built to hold 60psf of live load but I'm not sure...


  • I can do this for you, but first we need to consider loadings. Normally live load for this type of construction is considered to be 100 psf because of the potential for having many people on the balcony. Depending on the material you plan to use for the deck (floor) you would expect for a dead load of something in the range of 5 to 10 psf. Of course the deflection expected will vary greatly depending on what live/dead loads you want to use. The actual defection calcs are not difficult once we decide the loading. Can you help me.