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?
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 WLength1 TL1 LL1Length2 TL2 LL2Length3 TL3 LL3…Lengthn TLn LLnwhere: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.
Answer Verified By: SVGregory