Page 158 - Handbook of Civil Engineering Calculations, Second Edition

STRUCTURAL STEEL DESIGN 1.141

Use 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 76 is a compact section. This can be verified by noting that in the Properties
Tables in Part 1 of the AISC LRFD Manual, both b f /2t f and h c /t w for a W24 76 beam
are less than the respective flange and web values of p for F y 36 ksi (248 MPa).
2. Find the flexural design strength for minor-axis bending
For minor- (or y-) axis bending, M ny M py Z y F y regardless of unbraced length (Eq. [56]).
The flexural design strength for minor-axis bending of a W24 76 is always equal to
3
b M ny b Z y F y 0.90 28.6 in 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 C b and L b . For a simply
supported member, the end moments M 1 M 2 0; C b 1.0.
4. Plot the results
For 0 < L b < (L p 8.0ft), p M n b M p 540 kip-ft (732 kNm).
At L b L r 23.4 ft, p M nx b M r 343 kip-ft (465 kNm). Linear interpolation is
required for L p < L b < L r . For L b > L r , refer to the beam graphs in Part 3 of the AISC
LRFD Manual.
Figure 36b 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, As-
sociate Director of Education, American Institute of Steel Construction. SI values 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. 37b. 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, V n 0.64A w F y C w . Assuming

h k 44,000
> 234 , C v

2
t w (h/t w ) F y
F y
Substituting, we obtain
44,000 26,400 k
V n 0.6A w F y A w
2 2
(h/t w ) F y (h/t w )