Adding TJI Joist Table to Ram SS

I am planning to add a Trus Joist TJI constant shear joist table to Ram SS.  I have done similar tables for other joists.  To get the deflection close, I modify the moment of inertia by the modular ratio of the Modulus of Elasticity of the joist to steel.  I am a little lost trying to use this approach for TJI joists.

For example, a 14" TJI-210 has an EI = 462 x 10^6 in^2-lbs.  The deflection formula is D = 22.5wL^4/EI + 2.67wL^2/(d x 10^5).  I think it would be safe to ignore the 2nd term in that equation and the answer would still be close.  The second term changes slightly for wider flanges.

w = uniform load (plf), L = span (ft), d = depth of joist (in), D = deflection (in), EI (tabulated)

Can someone show me how to derive a good value for a modified Moment of Inertia I for the Ram SS joist table?

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  • In our joist tables we expect the user to define a table of allowable live loads per joist length. The intent is that the allowable live load LLi in our nomenclature is the live load that produce deflection = Length/360.

    For each joist a section is created with the following format:
    Label Depth W
    Length1 TL1 LL1
    Length2 TL2 LL2
    Length3 TL3 LL3

    Lengthn TLn LLn
    where:
    Label is the name of the joist (13 characters or less),
    Depth is the depth (in. or mm),
    W is the weight per unit length (lb/ft or kg/m),
    Lengthi is the length for which the uniform load capacity is TLi and LLi (ft or m),
    TLi is the total uniform load capacity (lb/ft or kN/m),
    LLi is the uniform live load capacity which causes a deflection of L/360, not to exceed TLi (lb/ft or kN/m).

    So if you substitute that into your equation above, and then solve for w I think you'll have it.

    In case you are interested, we added some support for Virtual Joist Girders in 14.06.01.

     



Reply
  • In our joist tables we expect the user to define a table of allowable live loads per joist length. The intent is that the allowable live load LLi in our nomenclature is the live load that produce deflection = Length/360.

    For each joist a section is created with the following format:
    Label Depth W
    Length1 TL1 LL1
    Length2 TL2 LL2
    Length3 TL3 LL3

    Lengthn TLn LLn
    where:
    Label is the name of the joist (13 characters or less),
    Depth is the depth (in. or mm),
    W is the weight per unit length (lb/ft or kg/m),
    Lengthi is the length for which the uniform load capacity is TLi and LLi (ft or m),
    TLi is the total uniform load capacity (lb/ft or kN/m),
    LLi is the uniform live load capacity which causes a deflection of L/360, not to exceed TLi (lb/ft or kN/m).

    So if you substitute that into your equation above, and then solve for w I think you'll have it.

    In case you are interested, we added some support for Virtual Joist Girders in 14.06.01.

     



Children
  • Seth, Thanks for the suggestion. I had not considered making a Standard Joist Table. I have used the Constant Shear Table format for my other tables. The Constant Shear Joist tables are more versatile in handling different types of loads. The Standard Joist tables don't do well with anything but a uniform load.

    If I have to turn a joist into a header beam to support one or more joists to create a stair opening or duct shaft opening, then I get concentrated loads on the header and the header puts a concentrated load on the joist it frames into. That's one of the reasons why I choose the Constant Shear tables.

    Now back to my original question, I need to derive the value for a modified Moment of Inertia I for the TJI joists. Can you help?

    Program Enhancement Suggestion: I would like to have the ability to use more than 2 Constant Shear Joist tables in a single model. I would like to use tables for 2 species of wood, Microllam LVL, Parallam PSL, Trus Joist TJI and maybe Glulam or some other joist/beam type. That brings my total up to 6 Constant Shear Joist tables. Then the marketing guys can say Ram SS can handle wood framing!!!
  • I missed your tag about constant shear initially. The E that the program uses is 29,000 ksi, so you just need to take your calculated EI value and divide by 29,000 to get the appropriate I value to use in the tables. Keep in mind, that when the program is designing constant shear joists it factors the deflection up times 1.15 per the SJI standard before checking against deflection limits. If your material does not require such an amplification, then that factor should be added to your effective I value.

    First time I've ever had anyone ask for most than 2 constant shear tables. Using a combined table might be worth investigating, but it would put a little more work on user selection since the program might pick a different species than you had in mind.

    If you are not using Ram Elements, we do wood design there, BTW.



    Answer Verified By: SVGregory 

  • Thanks, Seth. After I worked through the deflection equation and got the units straight, it was as simple as dividing EI by 29. Also, I multiplied the I for the lumber by 1.15 to correct it for the SJI factor used by the program. The TJI joists should use the SJI factor to account for the deflection in the second term of the equation.

    Has Allen Adams or anyone else at Bentley considered expanding the number Constant Shear joist tables available to a single model?