static and dynamic loads

[jay156]

I'm sorry if this is a stupid question, but this type of thing has never come up in my work until now.

A client is putting up a new building, and on the third floor they are going to put two mixers. The mixer drawing gives me two loads, A static load of 20,000 lbs, and a dynamic load of 173,000 lbs., which sounds absurdly huge. It gives no period or frequency, no amplitude or anything I'd expect for "dynamic loading" which admittedly, I've only had introductory classes on way back in college.

My question is, how the heck do I design this building? Am I supposed to just treat the dynamic load as though it's a simple vertical load and size the beams and columns to be outrageously huge to support it? If we add an inertia base, could I design the building based just on the static load?

Thanks.

 

[TLHS]

Call the vendor and ask them what the numbers mean. It sounds like it's a static equivalent design load or something. It could also be the worst case operating load, which is likely some sort of jamming or emergency shutdown load. You're really not going to know unless you talk to the person who wrote it.

 

[JStephen]

For mixers, we normally get loads or moments which are static equivalent loads, with estimated dynamic loads increased by various factors to account for vibration or fatigue or deflection or whatever. And on occasion, maximum deflections as well.

In some cases, those loads may not need to be carried through the structure. For example, if the dynamic load is intended to make the beams supporting it stiff enough to avoid vibration, you wouldn't necessarily need to include the full amount in your foundation loading. And if stiffness is the concern, you wouldn't necessarily need to combine that with wind or seismic loads.

 

[GregLocock]

It could easily be the out of balance force from a mixer if all the dough gets on one side of the paddle, 9g is a fair bit but nothing like what we get on engines. If it is out of balance watch for resonances as the thing spins up and down, and the direction of the force will also vary. It may be cheaper to install a seismic mass on isolators rather than trying to react the forces throughout the building, and it will certainly be a better working environment.

Cheers

Greg Locock


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[concretemasonry]

I agree with Greg that is best to try to have an isolation system rather than carry the loads into the structure. With something like a mixer that may have number of possible mixtures during typical cycle. The start-up and loading of the mixer can be unpredictable and cause some strange loads. Even in a normal operation cycle the harmonics can do strange things, depending on the natural frequency of the structure.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[Ron]

I also agree with Greg....I've worked on a lot of "permanent park" amusement rides housed in buildings, exerting dynamic loads....isolate them!! Some transmission will occur regardless, but the effort should be to minimize (mitigate) that transmission back to the structural frame....remember, the building might not always be used in the same way.