Fb the bending stress allowed

[bumpjones]

Looking at Appendix 5-67 in the AISC Manual of Steel Constuction formula (5):
Fb=12,000,000/(ld/Af) is less than or equal to 22,000psi
l is length, d is beam depth, Af is area of the compression flange. When I use this to evaluate truss loads availible for the installation of hoists, I end up with monster beams required for everything. Can any one point me in the right direction? Thanks.

 

[JAE]

bumpjones:
What edition of AISC are you looking at?

 

[bumpjones]

The sixth.

 

[JAE]

Wheee...that's a bit old. Current ASD is the 9th edition.

If you're working with trusses, and applying new loads to them (from your hoists) the Fb is primarily intended for bending stress, not a typical axial stress found in truss members. This, of course, is assuming you are attaching the new loads to the truss at the truss panel points.

Could you describe your situation a bit more? Normally, on an existing truss, we would avoid attaching between truss panel points, thereby avoiding bending in the truss members. That way, the truss members only take axial loads.

You would then use Fa allowable stresses instead of Fb.

 

[rakbaba]

to my opinion, the best way is to have bracings in the plane of the lower chords between the loaded truss and one of the adjacent trusses