Steel Portal Frame Frequency 2

[JKW05]

I have attached a sketch and calculations for a simple frame which will have a very small impact load applied downward at the mid-span of the beam. The required minimum natural frequency of the structure measured along the vertical axis at the mid-span of the beam is 200 Hz. From the attached calculations, it appears that column deformation alone makes this unattainable. (For simplification, I have not included the applied load in the calculations (200 lbs.) as that would not have an impact on my conclusion). Am I missing something here? Any assistance would be appreciated. Thanks.

 

[Agent666]

Ah, yeah sorry. It should be delta!

It's in metric, so 0.18*100 (10 approximates g=9.81m/s^2 taking it outside the sqrt.

Sorry also missed that explanation in your 4th post. Maybe I'm reading it wrong, but aren't they saying the 'thing' you use to test should have the 200 hz frequency, not the structure being tested?

 

[JKW05]

The frame is used to test safety harnesses.

The standard specifies the minimum natural frequency of the drop test structure. Don't know that I can buy into it meaning anything other than the structure....

Thanks for your input.

JKW

 

[bones206]

I made a little RISA model for comparison. I restricted the nodes in the X and Z directions.

 

[JKW05]

Thanks Bones!

Mode 1 = 118 HZ is certainly close to the 111 Hz from my original post.

JKW