Maximum Unsupported Beam Length Lu 2

[BigBakwas77]

Does anyone have any information about how the values of Maximum Unsupported Beam Length (Lu) is derived in the Beam Selection Table of CISC Blue Book (Handbook of Steel Construction)? I have seen some threads on Eng-Tips.com which has some formulas to calculate Lu based on AISC Imperial values which I could not replicate the answers in Metric to match the values given in the Blue Book.

 

[robyengIT]

for example, if it can helps, in Italy we used tables similar to the attached ones. Pages 1-2 calculation according to max allowable stress (1200 or 1400 kg/cm2); pages 3-4 similar calculations but enforcing limit to max deflection (L/300 or L/500). Formulas used are shown.
Full manual available if needed

 

[CANPRO]

BigBakwas77, check out CSA S16 (not sure what year you're working with) cl. 13.6. The code is in the first portion of the steel handbook - the tables are there for convenience, but you should be able to replicate all of the values using the code provisions.

 

[BigBakwas77]

@CANPRO
Thank you for your reply. I have used Cl. 13.6 of CSA 16-09 while creating a spreadsheet for Steel Beam design. What I am looking for specifically at this stage is some sort of formula/s to determine Lu for a selected section which should match the numbers provided in the Blue Book. The following thread has such formulas to work out Lu which I am unable replicate in Metric units.

https://www.eng-tips.com/viewthread.cfm?qid=194345

 

[jayrod12]

Well you should be able to back calculate the Mr in the table to determine the appropriate Lu, just modify your spreadsheet accordingly.

I guess my question would be why are you trying to reverse engineer the table?

If you're talking about the Lu noted in the 5th column, I believe that is back calculated for the appropriate My or Mp depending on the bending class of section.

 

[CANPRO]

The Lu provided in the book is the unsupported length at which stability governs the beam strength and you can no longer reach the full potential of the section (phi*Sx*Fy). If you want to recreate that value from the book, set your equation for the unbraced strength of the beam equal to phi*Sx*Fy and solve for L.