# V. AISC LRFD - Wide Flange Flexural Strength 1

To determine the design flexural strength of an ASTM A992 W18X40 bent about its strong axis on a 35-ft span with a uniformly distributed load.

## Reference

Hand calculation using the following reference:

AISC Load Factor Resistance Design, 3rd Edition, Example 5.1, Case (a), page 5-12.

## Details

The beam is braced such that Lb = 2 ft. The deflection at mid-span due to a uniformly distributed service load of 1 kip/ft is also to be calculated.

 Fy = 50 ksi

 Length = 35 ft

## Validation

The beam is continuously braced, so by observation Lb < Lp.

The design flexural strength is:

ϕbMn = ϕb[FyZx] = 0.9×[50 ksi × 78.4 in3] = 3,528 k·in

The maximum deflection (at midpsan) due to a uniform load of 1 kip/ft is:

## Results

Table 1. Comparison of results
Design flexural strength, ϕbMn (in·kips) 3,528

3,528

none
Deflection (in) 1.90 1.8918 negligible

The file C:\Users\Public\Public Documents\STAAD.Pro CONNECT Edition\Samples\ Verification Models\09 Steel Design\US\AISC\AISC LRFD - Wide Flange Flexural Strength 1.STD is typically installed with the program.

Note: The UNT parameter is set to 1.0 in. in order to approximate a fully-braced condition.
STAAD SPACE BENDING CAPACITY PER AISC LRFD 3RD ED
START JOB INFORMATION
ENGINEER DATE 23-Sep-18
END JOB INFORMATION
*
*  EXAMPLE PROBLEM 5.1, CASE (A), PAGE 5-12, AISC LRFD 3RD ED.
*  CAPACITY (MNZ) SHOULD BE ABOUT 294 KIP-FT
*
UNIT FEET KIP
JOINT COORDINATES
1 0 0 0; 2 35 0 0;
MEMBER INCIDENCES
1 1 2;
MEMBER PROPERTY AMERICAN
1 TABLE ST W18X40
UNIT FEET KIP
DEFINE MATERIAL START
ISOTROPIC MATERIAL1
E 4.176e+06
POISSON 0.3
END DEFINE MATERIAL
UNIT FEET KIP
CONSTANTS
MATERIAL MATERIAL1 ALL
SUPPORTS
1 PINNED
2 FIXED BUT MZ
1 UNI GY -1
1 1.8
PERFORM ANALYSIS
SECTION 0.5 ALL
PRINT SECTION DISPL
UNIT FEET KIP
PARAMETER 1
CODE LRFD
MAIN 1 ALL
FYLD 7200 ALL
UNT 2 ALL
TRACK 1 ALL
CHECK CODE ALL
FINISH


  SECTION  DISPL
BENDING CAPACITY PER AISC LRFD 3RD ED                    -- PAGE NO.    3
MEMBER SECTION DISPLACEMENTS
----------------------------
UNITS ARE - INCH
MEMB  LOAD     GLOBAL X,Y,Z DISPL FROM START TO END JOINTS AT 1/12TH PTS
1     1       0.0000    0.0000    0.0000    0.0000   -0.4973    0.0000
0.0000   -0.9571    0.0000    0.0000   -1.3475    0.0000
0.0000   -1.6441    0.0000    0.0000   -1.8290    0.0000
0.0000   -1.8918    0.0000    0.0000   -1.8290    0.0000
0.0000   -1.6441    0.0000    0.0000   -1.3475    0.0000
0.0000   -0.9571    0.0000    0.0000   -0.4973    0.0000
0.0000    0.0000    0.0000
MAX LOCAL  DISP =    1.89185   AT     210.00  LOAD    1   L/DISP=    222
************ END OF SECT DISPL RESULTS ***********
38. UNIT FEET KIP
39. PARAMETER 1
40. CODE LRFD
41. MAIN 1 ALL
42. FYLD 7200 ALL
43. UNT 2 ALL
44. TRACK 1 ALL
45. CHECK CODE ALL
STEEL DESIGN
BENDING CAPACITY PER AISC LRFD 3RD ED                    -- PAGE NO.    4
STAAD.Pro CODE CHECKING - (LRFD 3RD EDITION)   v1.0
***********************
ALL UNITS ARE - KIP  FEET (UNLESS OTHERWISE Noted)