How to Handle Dynamic Loading on Structural Steel Platform

[KB4444]

I have an independent platform made of steel that is going to be used to support two machines. The machines will not move on the platform but the internal components and motor can cause a dynamic load, which was provided by the machine suppliers. The dynamic load is point loads that are transferred evenly through each of the machine legs, which are connected to the platform.

Can a simple static analysis of the point loads be done from the dynamic loads provided? Is there an extra load factor required for building structures, for example I know bridge design applies an extra 1.3 factor on top of the 1.5 or 1.7 LL factor?

 

[canwesteng]

Does the dynamic load come with a frequency? If it's being driving by an electric motor, would be best modelled as cyclic load with sin/cos function. Do you have any criteria from the equipment manufacturer as to what acceptable deflection is?

 

[KB4444]

The only information that is provided along with the dynamic load being applied is that the bowl inside the machine rotates at 4200rpm. There is no deflection limits requirement.

 

[JoshPlumSE]

4200 rpm = 70 Hz, right?

So, you're going to want to check your structure to see if you have any natural frequencies around 70Hz. If there are no natural frequencies within about 20% of 70 Hz, then you're probably good. Though, you probably also want to make sure you don't have any at 35 Hz either.

If you do have natural frequencies that are close then you have a couple of options:
a) Come up with an amplifier to determine an "equivalent static force" based on the ratio between the frequency of the equipment and the frequency of the structure.
b) Run a time history analysis of the equipment running at 4200RPM to see how it affects the structure.

 

[JStephen]

The few times I've run into this kind of thing, the manufacturers gave an "equivalent static load" for design and also a maximum deflection, the point with the deflection being to make it stiff enough to limit vibration issues. You might check with the manufacturer to see if they have any limitations there, and also to see what kind of load factors need to be applied or have already been applied to the specified loads.

 

[KB4444]

Thanks for both answers!

JoshPlumSE - We are also planning to use isolators which I believe will help reduce the chance of having matching frequencies.

JStephen - This situation we have would be similar to what you mentioned. The dynamic load is provided as a point load which would be their "equivalent static load". It would make a lot of sense for a maximum deflection to be provided but that is not included.

Would you need to account for fatigue with the dynamic load (static point load) or is that usually included when converting to a static equivalent load?

 

[canwesteng]

The range of the dynamic load should not be stressing the steel enough that fatigue comes into play, otherwise the machine will probably shake itself into pieces. You do have to worry about vibrations loosening bolts. At 70 Hz, you can't high tune the structure, so giving a required deflection wouldn't really make sense.