V. AISC LRFD - Double Angle Compression Capacity

To determine the design compressive strength of a long-leg-back-to-back double angle made of American L4X3.5X1/2 angles with 3/4 in. separation.

Reference

Hand calculation using the following reference:

AISC Load Factor Resistance Design, 3rd Edition, Example 4.7, page 4-14.

Problem

Member is 8 ft long. Ends are pinned. Unbraced length should be assumed as the distance between ends.

F_y = 36 ksi

Comparison

The reported STAAD.Pro value is based on the term PNC in the STAAD.Pro output.

表 1. Comparison of results
Result Type		Theory	STAAD.Pro	Difference
Design strength (kips)		101	101.22	none

STAAD Input

The file C:\Users\Public\Public Documents\STAAD.Pro 2023\Samples \Verification Models\09 Steel Design\US\AISC\AISC LRFD - Double Angle Compression Capacity.STD is typically installed with the program.

STAAD SPACE AXIAL COMPRESSION CAPACITY PER AISC LRFD 3RD ED
START JOB INFORMATION
ENGINEER DATE 23-Sep-18
END JOB INFORMATION
*
*  EXAMPLE PROBLEM 4.7, PAGE 4-14, AISC LRFD 3RD ED.
*  CAPACITY (PNC) SHOULD BE ABOUT 101 KIPS
*
UNIT FEET KIP
JOINT COORDINATES
1 0 0 0; 2 0 8 0;
MEMBER INCIDENCES
1 1 2;
UNIT INCHES KIP
MEMBER PROPERTY AMERICAN
1 TABLE LD L40355 SP 0.75
DEFINE MATERIAL START
ISOTROPIC MATERIAL1
E 29000
POISSON 0.3
END DEFINE MATERIAL
CONSTANTS
MATERIAL MATERIAL1 ALL
SUPPORTS
1 PINNED
2 FIXED BUT FY MX MZ
LOAD 1
JOINT LOAD
2 FY -100
PERFORM ANALYSIS
PARAMETER 1
CODE LRFD
FYLD 36 ALL
TRACK 1 ALL
CHECK CODE ALL
FINISH

STAAD Output

                         STAAD.Pro CODE CHECKING - (LRFD 3RD EDITION)   v1.0
                         ***********************
 ALL UNITS ARE - KIP  INCH (UNLESS OTHERWISE Noted)
 MEMBER     TABLE       RESULT/   CRITICAL COND/     RATIO/     LOADING/
                          FX            MY             MZ       LOCATION
 =======================================================================
      1  LD   L40355                   (AISC SECTIONS)
                           PASS     LRFD-H1-1A-C       0.988         1
                      100.00 C          0.00           0.00        0.00
 +---------------------------------------------------------------------+
 | DESIGN STRENGTHS FOR MEMBER     1       UNITS - KIP IN              |
 | PNC=  101.22  PNT=  145.80  MNZ=   81.97  MNY=  107.03  VN=   48.68 | 
 +---------------------------------------------------------------------+