Handbook of Civil Engineering Calculations

the "Load Factor Design Selection Table for Beams" in Part 3 of the AISC LRFD
Manual

Calculation Procedure:

1. Determine if the beam is a compact section
   The W24 x 75 is a compact section. This can be verified by noting that in the Properties
   Tables in Part 1 of the AISC LRFD Manual both bf/2tfand hjtw for a W24 x 76 beam are
   less than the respective flange and web values of \p for Fy = 36 ksi (248 MPa).


2. Find the flexural design strength for minor-axis bending
   For minor- (or y-) axis bending, Mny = Mpy ZyFy regardless of unbraced length (Eq. [56]).
   The flexural design strength for minor-axis bending of a W24 x 75 is always equal to
   <M^ = bZyFy = 0.90 x 28.6 in
   3
   x 36 ksi = 927 kip-in = 77kip-ft (104 kNm).


3. Compute the flexural design strength for major-axis bending
   The flexural design strength for major-axis bending depends on Cb and Lb. For a simply
   supported member, the end moments M 1 = M 2 = O; Cb = 1.0.


4. Plot the results
   For O < Lb < (Lp = 8.0ft), QpMn = Q 1 JMp = 540 kip-ft (732 kNm).
   At Lb = Lr = 23.4 ft, QpMf1x = QbM,. = 343 kip-ft (465 kNm). Linear interpolation is re-
   quired for Lp<Lb< Lr, For Lb > Ln refer to the beam graphs in Part 3 of the AISC LRFD
   Manual
   Figure 366 shows the data plotted for this beam, after using data from the AISC table
   referred to above.

Related Calculations. This procedure is the work of Abraham J. Rokach,
MSCE, Associate Director of Education, American Institute of Steel Construction. SI val-
ues were prepared by the handbook editor.

DESIGNING WEB STIFFENERS

FOR WELDED BEAMS

The welded beam in Fig. 37a (selected from the table of Built-Up Wide-Flange Sections

in Part 3 of the AISC LRFD Manual) frames into the column in Fig. 37 b. Design web

stiffeners to double the shear strength of the web at the end panel.

Calculation Procedure:

1. Determine the nominal shear strength for a stiffened web
   At the end panels there is no tension field action. The nominal shear strength for a stiff-
   ened web is, using the AISC LRFD Manual equation, Vn = 0.644^C 11 ,. Assuming

^->234 /I C=JMOO_

tw ^F;

c

" (Mj^

Substituting, we obtain

T,K n^, r^ 44,000 A 26,400£

--

0

^'"G^-^--MF