joist span calculation formula?

Discussion in 'Building Construction' started by John, May 25, 2005.

  1. John

    John Guest

    I wonder if anyone can point me in the direction of a formula for
    calculating the maximum span of a joist?

    The 'knowns' are these (I'm in the UK, by the way, so everything is in SI
    units):

    spacing (mm)
    load per unit length (kN/m)
    fibre stress of timber (N/mm2)
    breadth of joists (mm)
    depth of joists (mm)

    The 'unknown' is what maximum span would result for a given set of
    parameters - I assume it would be possible to derive it from the data
    available...

    For example:
    joists centres: 353mm
    load per unit length: 0.9kN/m
    fibre stress of timber: 7.5N/mm2
    breadth of joist: 47mm
    depth of joist: 200mm
    span of joist: ?

    The reason for this is that I'm (trying to) draw up a set of tables for
    preliminary joist sizing (UK span tables only provide metric spacing - I'm
    doing a set with imperial spacing for use with alterations of old
    buildings).

    Hope somebody can help with this one!
     
    John, May 25, 2005
    #1
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  2. John

    Guest

    Canadian Wood Council publishes tables for various species, spacing, &
    spans.

    TB
     
    , May 26, 2005
    #2
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  3. John

    RicodJour Guest

    wrote:
    > Canadian Wood Council publishes tables for various species, spacing, &
    > spans.
    >
    > TB


    And they also have an online span tool - SpanCalc, or something like
    that. Very useful.

    R
     
    RicodJour, May 26, 2005
    #3
  4. John

    Bob Morrison Guest

    In a previous post John says...
    > I wonder if anyone can point me in the direction of a formula for
    > calculating the maximum span of a joist?
    >

    John:

    There are three parts to your question: bending strength,shear strength
    and deflection limitations.

    For floor joists with normal floor loading we can generally ignore the
    shear calculation, so that leaves flexure and deflection.

    For flexure, the bending formula is fb = M/Sx or

    fb = (wl^2)/8Sx where Sx = (bd^2)/6
    fb = (6wl^2)/(8bd^2)

    L = sqrt[(8bd^2)*fb/(6w)] make sure your units are compatible


    For deflection of floors:

    L/360 = (5wL^4)/(384EI) where I= (bd^3)/12

    I will leave you to work out the math.

    Your "maximum" span is the smaller of the two L's

    --
    Bob Morrison, PE, SE
    R L Morrison Engineering Co
    Structural & Civil Engineering
    Poulsbo WA
     
    Bob Morrison, May 26, 2005
    #4
  5. John

    Roarmeister Guest

    On 25 May 2005 16:02:43 -0700, wrote:

    >Canadian Wood Council publishes tables for various species, spacing, &
    >spans.


    You might have given him the actual link if you are going to suggest it.
    http://www.cwc.ca/design/tools/calcs/SpanCalc_2002/

    But this may or may not help him if the UK doesn't use the same dimension
    standards as North America *OR* since they probably have different wood
    species than what is grown in NA.

    The original poster should check to see if there is an equivalent site in
    the UK.
     
    Roarmeister, May 26, 2005
    #5
  6. John

    Guest

    Thanks for the link.
    I've been using the printed tables.
    TB
     
    , May 26, 2005
    #6
  7. John

    Roarmeister Guest

    On 25 May 2005 20:12:09 -0700, wrote:

    >Thanks for the link.
    >I've been using the printed tables.
    >TB


    If you back up the link you can see even more goodies...
    http://www.cwc.ca/design/tools/

    wood design (structural)
    code check
    moisture shrinkage calc
    designing for tall walls
    span calculator
    board foot calculator

    have fun....
     
    Roarmeister, May 27, 2005
    #7
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