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aluminum 6160 t6 s beam: finding fbx and fby
i am trying to do a combined stress analysis on an aluminum s6 beam tf=~5/16 bf=~3 3/8 the aluminum is 6160 t6; fy=40ksi. the beam has an unbraced length and span of 14'. i am unfamilliar with how to find the allowable stress of aluminum in the major and minor axis when the unbraced length exceeds lc & lu which i assume this span does. if anyone could give me either the equations or the raw numbers i would greatly appreciate it. (if their are multiple equations the most conservative would be fine.)


you need the aluminum association design manual and time to become familiar with it.
that aside, an aluminum association 6" i-beam with a 4" flange thickness of .35 and web thickness of .21 has section moduli if 8.50 and 1.87 and radii of gyration of 2.53 and 0.97 about the strong and weak axis respectfully.
assuming you mean 6061-t6 alloy, the maximum extreme fiber, net section tension is 19ksi when the beam is bent about it's strong axis and 28ksi when the beam is bent about it's weak axis.
for compression you should check (at a minimum) exreme fiber gross section compression, flange buckling and web shear. for gross section, the l/r is 66 for the strong axis which would calculate an allowable stress of 15.7ksi.
for weak axis bending you would be checking the flange as a flat plate with one edge supported, the b/t would be 28.6 which would calculate an allowable stress of 1.7ksi.
for web shear, the h/t is 17.1 shich would calculate an allowable shear stress is 12ksi.
before you use any of this, you should check my numbers, assumptions, the support condition and the load condition. if there is risk involved, you should get a pe familiar with aluminum design to review all calculations.
if there are bolt holes, the calculations need to be revised.
if there is any welding, the allowable stresses go way down in the vicinity of the welds.
good luck and be careful!!

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