I have recently been working to prove STAADs ability to provide natural frequencies for a range of frames and structures by comparing STAADs calculated natural frequencies to hand calculations. Unfortunately I have been unable to match any natural frequency for any frame I have modelled, however I have been able to match the natural frequencies for simply supported beams, cantilevers and fixed beams.
The current structure I am modelling is a simple portal frame as attached and I am attempting to match the natural frequency calculated in the textbook "Formulas for natural frequency and mode shape - R.D. Blevins" Which calculated the natural frequency for the frame to be 7.04Hz, yet STAAD calculates a frequency of 22.7Hz
I have attached the problem, the manually calculated result and the STAAD code.
Out of plane motion is prevented.
The frame is to made of piping with 1cm outside diameter and 0.8cm inside diameter.
Why is STAAD not giving me the same natural frequency of 7.04Hz?
STAAD Code
STAAD SPACE
START JOB INFORMATION
ENGINEER DATE 26-Aug-20
END JOB INFORMATION
INPUT WIDTH 79
UNIT METER KN
JOINT COORDINATES
1 0 0 0; 2 1.24 0 0; 3 0 1.24 0; 4 1.24 1.24 0; 5 0 0.137778 0; 6 0 0.275556 0;
7 0 0.413333 0; 8 0 0.551111 0; 9 0 0.688889 0; 10 0 0.826667 0;
11 0 0.964444 0; 12 0 1.10222 0; 13 0.137778 1.24 0; 14 0.275556 1.24 0;
15 0.413333 1.24 0; 16 0.551111 1.24 0; 17 0.688889 1.24 0; 18 0.826667 1.24 0;
19 0.964444 1.24 0; 20 1.10222 1.24 0; 21 1.24 1.10222 0; 22 1.24 0.964444 0;
23 1.24 0.826667 0; 24 1.24 0.688889 0; 25 1.24 0.551111 0; 26 1.24 0.413333 0;
27 1.24 0.275556 0; 28 1.24 0.137778 0;
MEMBER INCIDENCES
1 1 5; 2 3 13; 3 4 21; 4 5 6; 5 6 7; 6 7 8; 7 8 9; 8 9 10; 9 10 11; 10 11 12;
11 12 3; 12 13 14; 13 14 15; 14 15 16; 15 16 17; 16 17 18; 17 18 19; 18 19 20;
19 20 4; 20 21 22; 21 22 23; 22 23 24; 23 24 25; 24 25 26; 25 26 27; 26 27 28;
27 28 2;
MEMBER PROPERTY BRITISH
1 TO 27 TABLE ST PIPE OD 0.01 ID 0.008
DEFINE MATERIAL START
ISOTROPIC ALUMINUM_EX
E 6.89e+007
POISSON 0.33
DENSITY 27.1663
ALPHA 2.3e-005
DAMP 0.03
G 2.59202e+007
ISOTROPIC STEEL
E 2.05e+008
POISSON 0.3
DENSITY 76.8195
ALPHA 1.2e-005
DAMP 0.03
TYPE STEEL
STRENGTH FY 253200 FU 407800 RY 1.5 RT 1.2
ISOTROPIC ALUMINUM
E 6.89476e+007
POISSON 0.33
DENSITY 26.6018
ALPHA 2.3e-005
DAMP 0.03
END DEFINE MATERIAL
CONSTANTS
MATERIAL ALUMINUM_EX MEMB 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 10 10 11 11 -
12 12 13 13 14 14 15 15 16 16 17 17 18 18 19 19 20 20 21 21 22 22 23 23 24 -
24 25 25 26 26 27 27
SUPPORTS
1 2 PINNED
LOAD 1 LOADTYPE None TITLE LOAD CASE 1
SELFWEIGHT Y -1
MODAL CALCULATION REQUESTED
PERFORM ANALYSIS PRINT ALL
FINISH