10 Member AB is part of a steelframed building and is govern

#10

Member AB is part of a steel-framed building, and is governed by gravity loads that create the bending moment diagram shown, where . The top flange of the beam is laterally braced throughout, and the bottom flange is braced at points A and B only. The beam is to be designed to resist the design moment, Mu?=?|M1|. The appropriate values of the unbraced length, Lb, and bending coefficient, Cb, are

(A)

Lb?=?L1

Cb> 1.0

(B)

Lb?=?L1 + L2 + L3

Cb>1.0

(C)

Lb?=?L2

Cb?=? 1.0

(D)

Lb?=?L1

Cb?=? 1.0

Solution

C is the most appropriate option as compared to others.As the point of contraflexure for a fixed beam lies at point slightly away from the support.Point of contraflexure is, where the bending moment changes its sign.

So L2 is the unsupported length available.