15-Bending Coefficient (Steel Structural Design & Prof. Shehab Mourad)

1. 1 Prepared by Prof. Shehab Mourad – Department of Civil Eng. - KSU
Where, M max : is the largest moment in an unbraced segment
MA, MB & MC: are the moments at the 1/4, 1/2 & 3/4 point in the
segment, respectively.
Bending Coefficient (Cb)
Cb is a moment coefficient to account for the effect of different
moment gradients on lateral - torsional buckling
Cb = 1.0 for a uniform moment diagram
Which represent the worst case
of compression flange buckling,
resulting in the least beam moment resistance
In general, Cb = 12.5 M max
2.5 M max + 3 M A + 4 MB + 3 MC
+
M M
MA
MB = Mmax
MC
Lb = L
1/4 Lb 1/4 Lb 1/4 Lb 1/4 Lb
Compression flang is laterally
supported at ends only , Cb = 1.14
Lb = L/2 Lb = L/2
1/4 Lb 1/4 Lb 1/4 Lb 1/4 Lb
MA
Mmax
MB
MC
Compression flang is laterally
supported at mid span , Cb = 1.30

2. 2 Prepared by Prof. Shehab Mourad – Department of Civil Eng. - KSU
Some typical values of Cb for different type of loading and different points of
laterally supported compression flange

3. 3 Prepared by Prof. Shehab Mourad – Department of Civil Eng. - KSU
Beam shear strength
Shear resistance factor ( f v) = 0.90
LRFD strength requirement : f v Vn ≥ Vu
Area of web, Aw = d tw