Eigenfrequency/dynamic load

[LMW.dk]

Hey guys. I have a construction of steel structure ( beams columns etc) and a light construction. I have to find the exact eigenfrequency of my construction. The self weight is = 0,8 kN/m2 and a service load of 5 kN/m2 ( according to eurocode)

However that gives me high value of eigenfrequency hz=4 hz. Which is high i have to have about 8-10 hz.

Is it possible to reduce the dynamic load for the service load?

Im doing the calculation in fem design.

 

[ThomasH]

Hi
If you need the exact eigenfrequency I believe that you have to start with the correct mass and you probably won't find that in the Eurocode. You need the mass associated with the the structures dynamic behavior, not the design load that you find in the Eurocode. They can be the same but I doubt that .

And what is the requirement 8-10 Hz based on? What is the dynamic load that are you designing your structure for?

Thomas

 

[HTURKAK]

You are expected to calculate the seismic mass acc. to EC-8 Clause ( 3.2.4 Combinations of the seismic action with other actions )

(G + ψEi Q)

If Q= 5 kN/m2 , this variable action probably for workshop ..

and seismic weight could be wi= 0,8 + 0,3* 5 = 2,3 kN/m2


For seismic weight, we shall consider the dead weight of structure plus only the portion of the mass or weight tied to the structure . Hence, live loads such as loose furniture, loose equipment, human occupants, overhead crane live loads etc. not included.

 

[ThomasH]

@HTURKAK

Why do you think this is a seismic calculation? What am I missing ?

Thomas

 

[HTURKAK]

The O.P. is looking for the eigenfrequencies or natural frequencies of a steel bldg. acc. to EC. and his question (Is it possible to reduce the dynamic load for the service load?)

When he used the seismic wt (D+Q ) 5,8 kN/m2 he found eigenfrequency =4 hz ..

Better if the OP clarifies his question if it is not seismic ..

 

[LMW.dk]

Hmm i dont know if u use fem design. But my model is attached. Can you tell me what i can do in order to reduce the eigenfrequencies?

 

[centondollar]

Start off by thinking about what affects the natural angular frequencies and natural frequencies. Definitions below are valid for a 1 DoF system, but which also provide a qualitative framework for larger systems:

omega_n = sqrt(k/m) -----> f_n = (1/2*pi) * sqrt(k/m) where k = stiffness and m = mass

What happens when you increase mass and keep stiffness constant? What happens when you increase stiffness and keep mass constant? Think about it. This is engineering.

 

[ThomasH]

@LMW.dk

What is the purpose of the analysis? Vibration machinery, comfort for people walking on the floor, seismic?

In my experience, the governing criteria for a dynamic analysis is seldom only a frequency.

Thomas

 

[LMW.dk]

The purpose of the analysis sis the comfort for people walking on the floor.

 

[ThomasH]

And where have you found a criteria for comfort based on natural frequency? They are usually based on acceleration.

I think you need to start with the correct mass for the structure. Once you have that you can calculate the frequency and then the response.

Edit: What method are you using for the analysis?
A warning based om experience. Steel is lightweight so you can achieve a high frequency when reducing the mass. However, the reduced mass also means that the floor may be easy to vibrate. So you also need a frequency dependent load model.

Thomas.

 

[HTURKAK]

I just see the OP's clarification that , the post for the floor vibration.

- The following doc. ( SCI 354 ) suggests for frequency calculation ( self weight+ services+ 10% of imposed load+ ceiling).

- EN 1990 UK national annex suggests 30% of imposed loads when considering deformation at serviceability limit state .

- I attached ( SCI 354 ) and suggest you to look worked examples of this doc.