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[manikanta73]

I am performing modal analysis on Staad, Sap and Etabs. The modal mass excited from the three softwares is different but the natural frequency is same. Anyone please tell why there is a difference in mass excited. Below are the parameters which I have considered.

Plan area 4m x 4m
4 columns, G+1 building
Column Size: 400mm x 400mm
Beam Size: 300mm x 350mm
Load 100kN applied at all joints.
Diaphargm is applied to all floors.
E= 25x10⁶ kN/m²
Poisons ratio= 0.17
----------------

In the modal analysis we can specify the number of modes to be considered say 5. It gives the modal mass and frequencies for the first five modes. Can we get the modal mass and frequencies only for the 5th mode (first four modes are not required).

[JVCSNL]

Dear Member,

With regard to different software there will certainly be some difference in how the modal analysis is done.  It depends on the algorithm used to solve the eigen value problem.

As far as Nat. freq and overall mass participation in each mode is nearly same, the results shall be acceptable.

Concerning to your second question, you need to understand the natural frequencies are obtained and sorted in ascending order.  This is required because we need to know maximum time period first.  

In fact, many algorithms provide the natural freq. in random order and later they are sorted as the lowest frequency of utmost importance to engineer.  If you see SAP, which uses subspace iteration method, analysis screen will show you that the method attempts to obtain the lowest frequency first.  

Suggesting to read the basics of structural dynamics to address these doubts.  

Regards,

Jignesh V Chokshi

[anshugoel]

Can you put up the results from each of the program on website (may first few modes) ?

[manikanta73]

Load
(kN)     Mode     Modal Mass Participation Factor         Time Period (Sec)
          Staad                        Sap           Etabs           Staad     Sap     Etabs
          Mx          My        Mx     My         Mx        My               
100     
1     0.430     0.432     0     0.862     0.493     0.369     0.378     0.378     0.378
    
2     0.432     0.430     0.862     0     0.369     0.493     0.378     0.378     0.378

3       0               0             0        0        0          0  0.325     0.325     0.325
    
4     0.130     0.006     0.049     0.087     0.064     0.071     0.100     0.100     0.100
    
5     0.006     0.130     0.087     0.049     0.071     0.064     0.100     0.100     0.100

Density is taken as zero and mass source is applied in sap and etabs

[manikanta73]

Mr JVCSNL

In all the softwares i am using eigen vector method, but the mass excited is different.

If the softwares having different algorithms to do eigen analysis then how they are giving same frequencies and different modal mass excited.

[anshugoel]

[anshugoel]

Can you please put up the CPU time for all three software, for the same problem set ?

I am betting SAP is the fastest, followed by STAAD and Etabs is last ?

[logeshcivil]

Dear sefians
Here I have attached floor plan as well as the ETABS model. How do I increase the sum of first three modal mass participation ratio to 65%?
I have increased the modal numbers to arrive at participation ratio of 90%.
I have provided stiffness modifier as per IS 16700.

Thank you