RAM | STAAD Forum - Natural Frequency of a portal frame
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Question

Natural Frequency of a portal frame

by
Bentley Communities Legacy Member
Expert
Created (Edited ) in RAM | STAAD Forum

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?

image

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