natural frequency portal frame

[aouiche]

Hey every one

I have s simple steel frame structure the analysis of this structure give 15 natural frequency results
2.067609179
5.763759994
13.08651498
13.39066267
22.05520949
31.60227434
35.89970274
39.23440003
50.30038346
61.72612467
73.51958827
88.75778292
112.1136364
119.6581416
134.0316193
next the structure should be submit to lateral force 150*sin(f*t).the author indicate that natural frequency is 12.69 Why I have 15 valus, thanks for help

 

[GregLocock]

Usually the lowest non zero global mode is the most important, so in your case f=2.067 hz.

However, in the shear excitement of messing about with things you don't understand, you have forgotten to convert to rad/s, which is the units for f in the force equation.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[aouiche]

Thanks,but we have the value of 13 which appear twice why i think it is the solution of my question

 

[GregLocock]

Are your results in Hz or rad/s?

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[aouiche]

Sorry it's in rad/s