Unbraced length input 2

[johndeng]

Forgive me, another silly question.

What do you put for beam unbraced length, a very simple example, a 20' roof beam with metal deck on top. What do you put for(Lb pos, Lb neg, L33, L22, and LTorsion)?
Lb pos, Lb neg, is the unbraced length for top and bottom flange bending
L33, L22, is for the unbraced compression length in each direction
LTorsion is for the unbraced torsion length

 

[andrews2]

I'm going to assume that L33 correlates to buckling about the strong axis and L22 correlates to buckling about the weak axis. If I have that backwards, be sure to flip.

The roof deck will prevent lateral translation of the top flange and compression buckling about the weak axis. Strong axis buckling, bottom flange translation, and torsion are not being resisted by anything so I would use the following:

Lb+ = 2'
Lb- = 20'
L33 = 20'
L22 = 2'
LTor = 20'

The 2' dimension is just an arbitrary number I typically use, but basically it should just be a small number roughly equal to the spacing of the deck flutes.

 

[KootK]

I'm going to assume that we're talking about a simple span beam here. No cantilevers or continuity over supports. I'll also assume that you don't have any bracing elements in play other than the deck.

Your braced lengths will depend upon which direction the strong axis of the deck is run. If flutes run parallel to the beam, then one usually uses the full length of the beam between supports for all of the unbraced lengths (the deck does nothing). If the flutes run perpendicular to the beam, then it's common to treat the beam as follows:

Positive bending LTB bracing = 0-ish. Maybe 24" oc to reflect deck fastener spacing, 5' to reflect joist spacing, or whatever.
Negative bending LTB bracing = span between supports.
Strong axis axial bracing = span between supports.
Weak axis axial bracing = span between supports. This is really a conservative shortcut to address torsional buckling about the top flange conveniently. Technically, the beam is braced continuously against weak axis axial load buckling.

 

[SethGuthrie]

Looks like Ram Elements, check the program help (F1 key), or this:

https://communities.bentley.com/pro...180/steel-unbraced-lengths-in-ram-elements-tn