Example for Beam Flexure Design

Select the lightest W section for the simply supported beam shown below.  The beam length is 12ft.  The nominal loading is: w_d = 4.0 kip/ft (including beam weight) and w_l = 8.5 kip/ft.  Use grade 50 steel and assume lateral bracing is provided only at the ends of the section.

Solution:

Load Factors are: 1.2 for dead load and 1.6 for live load as per ASCE 7 specifications (these are also shown in the concrete portion of the provided manual).

W_total = 1.2*4.0 kip/ft + 1.6*8.5 kip/ft = 18.4 kip/ft

L = 12 ft

M_u = WL² / 8 = 18.4 kip/ft * (12 ft) ² / 8 = 331.2 kip*ft

φM_n = 331.2 kip*ft

Enter the chart with the moment needed and the unbraced length as shown below:

φM_n =331.2, L_b = 12'

Answer : W18x55