Lateral Stability

 

Beam Stability Factor, C_L

reduced moment capacity if lateral torsional buckling can occur  

cold-formed steel trusses

C_L = 1 if fully braced  (roof or floor sheathing on compression side of beam, l_u = 0)

 

Unbraced Length, l_u

distance between points of lateral support on compressive edge

Lateral Support of Wood Beams in Residential Structures

span → moments, stresses, deflections
unbraced length → stability

 

Slenderness

depth-to-thickness  (large for steel, small for wood)

1. rules of thumb in NDS section 4.4.1

for example, if d/b = 6...

...bridging...

...or full-depth solid blocking spaced at 8' max. interval, ...

...full lateral support on compression edge, ...

...and supported at bearing points against rotation

2. slenderness ratio

R_B = sqrt (l_ed / b²)

see NDS table 3.3.3 for effective unbraced length, l_e  (similar to column buckling)

0 ≤ R_B ≤ 50

see textbook for graph of R_B, C_L equations, Euler buckling stress, and which E values to use