Eigenvalue 3 was found out of sequence and Negative Axial Force for Steel Column Secion in Etabs

[Leong Shi Ming]

 

Hi. I am a student and recently I started my first project on Etabs to model a  steel structure. I had faced some problems therefore I seek for your valuable advice. The problems are:

1) When I have done the analysis, there was a warning-“ Eigenvalue 3 was found out of sequence.” popped out.

2) Also when I started steel frame design, I found out that the axial force in the column is in negative value and some of them showing red colour.

May I know what exactly the problem is and how to resolve it? The Etabs file is attached below. I hope that someone can help me to clear my doubt. Thank you.

 

RUNNING ANALYSIS AS A SEPARATE PROCESS

 USING THE STANDARD SOLVER (PROVIDES COMPLETE INSTABILITY INFORMATION)

 NUMBER OF JOINTS                          =        1610

    WITH RESTRAINTS                        =         225

    WITH MASS                              =        1604

 NUMBER OF FRAME/TENDON ELEMENTS           =        1435

 NUMBER OF SHELL ELEMENTS                  =        1132

 NUMBER OF CONSTRAINTS/WELDS               =           3

 NUMBER OF LOAD PATTERNS                   =           7

 NUMBER OF ACCELERATION LOADS              =           9

 NUMBER OF LOAD CASES                      =           8

 NUMBER OF MASS SOURCES                    =           2

 ADDRESSABLE PHYSICAL MEMORY (RAM)         =       7.672 GB

 PARALLELIZATION OF ANALYSIS OPERATIONS:

 (Env. variable SAPFIRE_NUM_THREADS        =           0)

 NUMBER OF THREADS: STATE (AUTOMATIC)      =           2

 NUMBER OF THREADS: STIFFNESS (AUTOMATIC)  =           2

 NUMBER OF THREADS: EVENT (AUTOMATIC)      =           2

 NUMBER OF THREADS: MOVE (AUTOMATIC)       =           2

 NUMBER OF THREADS: RESPONSE (AUTOMATIC)   =           2

 NUMBER OF THREADS: SOLVE (AUTOMATIC)      =           2

 NUMBER OF THREADS: FORM (AUTOMATIC)       =           2

 E L E M E N T   F O R M A T I O N                                      12:32:51

 NUMBER OF COUPLED CONSTRAINT EQUATIONS    =           0

 L I N E A R   E Q U A T I O N   S O L U T I O N                        12:32:51

 FORMING STIFFNESS AT ZERO (UNSTRESSED) INITIAL CONDITIONS

 USING MASS SOURCE: MsSrc1

 TOTAL NUMBER OF EQUILIBRIUM EQUATIONS     =        8976

 APPROXIMATE "EFFECTIVE" BAND WIDTH        =         128

 NUMBER OF EQUATION STORAGE BLOCKS         =           1

 MAXIMUM BLOCK SIZE (8-BYTE TERMS)         =     1147837

 SIZE OF STIFFNESS FILE(S)                 =       8.792 MB

 NUMBER OF EQUATIONS TO SOLVE              =        8976

 ---------------------------------

 BASIC STABILITY CHECK FOR LINEAR LOAD CASES:

    NUMBER OF NEGATIVE STIFFNESS EIGENVALUES SHOULD BE ZERO FOR STABILITY.

    (NOTE: FURTHER CHECKS SHOULD BE CONSIDERED AS DEEMED NECESSARY,

     SUCH AS REVIEWING EIGEN MODES FOR MECHANISMS AND RIGID-BODY MOTION)

    NUMBER OF NEGATIVE EIGENVALUES         =           0,  OK.

 ---------------------------------

 L I N E A R   S T A T I C   C A S E S                                  12:32:52

 USING STIFFNESS AT ZERO (UNSTRESSED) INITIAL CONDITIONS

 TOTAL NUMBER OF CASES TO SOLVE            =           7

 NUMBER OF CASES TO SOLVE PER BLOCK        =           7

 LINEAR STATIC CASES TO BE SOLVED:

 CASE: DEAD

 CASE: LIVE

 CASE: WINDX

 CASE: WINDY

 CASE: MANSORY

 CASE: FINISHES

 CASE: ~LLRF

 E I G E N   M O D A L   A N A L Y S I S                                12:32:52

 CASE: MODAL1

 USING STIFFNESS AT ZERO (UNSTRESSED) INITIAL CONDITIONS

 USING MASS SOURCE: MsSrc1

 NUMBER OF STIFFNESS DEGREES OF FREEDOM    =        8976

 NUMBER OF MASS DEGREES OF FREEDOM         =        2782

 MAXIMUM NUMBER OF EIGEN MODES SOUGHT      =          12

 MINIMUM NUMBER OF EIGEN MODES SOUGHT      =           1

 NUMBER OF RESIDUAL-MASS MODES SOUGHT      =           0

 NUMBER OF SUBSPACE VECTORS USED           =          24

 RELATIVE CONVERGENCE TOLERANCE            =    1.00E-09

 FREQUENCY SHIFT  (CENTER) (CYC/TIME)      =     .000000

 FREQUENCY CUTOFF (RADIUS) (CYC/TIME)      =  -INFINITY-

 ALLOW AUTOMATIC FREQUENCY SHIFTING        =         YES

 Original stiffness at shift : EV= 0.0000000E+00, f=     .000000, T=  -INFINITY-

 Number of eigenvalues below shift =      0

 Forming stiffness, new shift: EV= 1.0000000E-03, f=    0.005033, T=  198.691765

 Number of eigenvalues below shift =    251

 Forming stiffness, new shift: EV= 5.0000000E-04, f=    0.003559, T=  280.992589

 Number of eigenvalues below shift =    146

 Forming stiffness, new shift: EV= 2.5000000E-04, f=    0.002516, T=  397.383531

 Number of eigenvalues below shift =     83

 Forming stiffness, new shift: EV= 1.2500000E-04, f=    0.001779, T=  561.985178

 Number of eigenvalues below shift =     47

 Found mode      1 of     12:  EV= 1.2519803E-04, f=    0.001781, T=  561.540539

 Found mode      2 of     12:  EV= 1.2519803E-04, f=    0.001781, T=  561.540539

 Forming stiffness, new shift: EV= 6.2500000E-05, f=    0.001258, T=  794.767061

 Number of eigenvalues below shift =     27

 Forming stiffness, new shift: EV= 3.1250000E-05, f=    0.000890, T=    1123.970

 Number of eigenvalues below shift =     14

 Forming stiffness, new shift: EV= 1.5625000E-05, f=    0.000629, T=    1589.534

 Number of eigenvalues below shift =      2

 Forming stiffness, new shift: EV= 7.8125000E-06, f=    0.000445, T=    2247.941

 Number of eigenvalues below shift =      0

 Found mode      3 of     12:  EV= 1.2916113E-05, f=    0.000572, T=    1748.292

 

 * * * W A R N I N G * * *

 EIGENVALUE           3 WAS FOUND OUT OF SEQUENCE

 Found mode      4 of     12:  EV= 1.2916113E-05, f=    0.000572, T=    1748.292

 Found mode      5 of     12:  EV= 2.3206534E-05, f=    0.000767, T=    1304.292

 Found mode      6 of     12:  EV= 2.3206534E-05, f=    0.000767, T=    1304.292

 Found mode      7 of     12:  EV= 2.3217637E-05, f=    0.000767, T=    1303.980

 Found mode      8 of     12:  EV= 2.3217637E-05, f=    0.000767, T=    1303.980

 Found mode      9 of     12:  EV= 2.3992956E-05, f=    0.000780, T=    1282.738

 Found mode     10 of     12:  EV= 2.3992956E-05, f=    0.000780, T=    1282.738

 Found mode     11 of     12:  EV= 2.9689614E-05, f=    0.000867, T=    1153.128

 Found mode     12 of     12:  EV= 2.9689614E-05, f=    0.000867, T=    1153.128

 NUMBER OF EIGEN MODES FOUND               =          12

 NUMBER OF ITERATIONS PERFORMED            =          48

 NUMBER OF STIFFNESS SHIFTS                =           8

 

 

 

 

Etabs problem file.EDB Column report.pdf

Edited February 19, 2022 by Leong Shi Ming

[UmarMakhzumi]

On 2/18/2022 at 11:26 PM, Leong Shi Ming said:

1) When I have done the analysis, there was a warning-“ Eigenvalue 3 was found out of sequence.” popped out

Leong,

Here is the description of this error:

Quote

Eigenvalue was found out of sequence

The purpose of this page is to explain the warning message "Eigenvalue was found out of sequence".

Modes are found in order of increasing distance of frequency from the shift. If lower frequency is found after a larger frequency, the program display the "out of sequence" warning. Please note that this is a warning, not an error, and may occur if the structure is sensitive to instabilities.

A structure that is unstable when unloaded will have some modes with zero frequency. These modes may correspond to rigid- body motion of an inadequately supported structure, or to mechanisms that may be present within the structure. It is not possible to compute the static response of such a structure. However, by using a small negative shift, the lowest- frequency vibration modes of the structure, including the zero- frequency in stability modes, can be found.

Link: https://wiki.csiamerica.com/display/kb/Eigenvalue+was+found+out+of+sequence

Please check your model for instability although the above does say that it is a warning not an error but that is my suggestion.

On 2/18/2022 at 11:26 PM, Leong Shi Ming said:

2) Also when I started steel frame design, I found out that the axial force in the column is in negative value and some of them showing red colour.

Negative force in column means that you column is in tension. This is common condition. For lateral loads, one side columns are normally in tension and other side in compression.

Thanks.

[Leong Shi Ming]

On 2/27/2022 at 10:12 AM, UmarMakhzumi said:

Leong,

Here is the description of this error:

Please check your model for instability although the above does say that it is a warning not an error but that is my suggestion.

Negative force in column means that you column is in tension. This is common condition. For lateral loads, one side columns are normally in tension and other side in compression.

Thanks.

Hi, Mr. Umar. Thanks for your reply to clear my doubt. May I know is there any quick way to locate the part causing the eigenvalue instability problem? 

[UmarMakhzumi]

On 2/28/2022 at 12:56 AM, Leong Shi Ming said:

Hi, Mr. Umar. Thanks for your reply to clear my doubt. May I know is there any quick way to locate the part causing the eigenvalue instability problem? 

Hi Leong Shi Ming,

The easiest way is to look at your "Joint Displacements" and hone down on joints where displacements is very very big.

Also, checking displaced / deformation shape helps.

Hope that helps.

Thanks.