Piled Flat Slab Dynamic Loads

[LivingTheBeam]

Hi All,

I am designing a piled flat slab that will support a tank farm. These tanks will be subjected to an agitation process which will produce a cyclic horizontal load on the circular tanks. I was wondering if anyone has any experience or advice for situations like this. My main areas of concern are:

1) does the flat slab need to be designed/checked for dynamic loads?

2) as this horizontal load from the agitation process will induce varying/cyclic type loads on the legs of the tanks, should this horizontal load be combined with wind loads when checking push/pull loads on the supports for checking punching shear etc? (the tanks are external)

3) has anybody got good design example for or procedure for analysing flat slabs for high point loads?


Many thanks for any replies

 

[SlideRuleEra]

Three cases:

A. If the agitator is supported by tank walls, neither the tank legs nor the slab will be subjected to torque from the swirling liquid.

B. If the agitator is supported by a structure that bears on the slab (not the tank walls), the tanks legs will be subjected to torque from the swirling liquid, but the slab will not.

C. If the agitator is supported by a structure that has independent support (i.e. neither the tank walls nor the slab support the agitator), both the tank legs and the slab will be subjected to torque from the swirling liquid.

In cases "B" and "C", the torque that the tank legs and/or slab are subjected to will be less than the torque that the motor driving the agitator(s) produces. Usually, agitator motors are of fairly modest power. For a tank with significant supporting legs on a piled slab... IMHO, the agitator torque and it's effect on the slab will be trivial. Affect on the tank legs depends on the leg detail... but is probably trivial, too.

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[LivingTheBeam]

Thanks for your response SlideRuleEra,

I have not designed anything like this previously and have little experience with tanks and agitators.

In case "A" above, how does the agitator being fixed to the tank walls not produce any torque on the legs/slab. Will the swirling liquid still not exert this force on the tank which will be transferred to the leg supports and then into the slab?

For example in the attached image, would this be considered to be an example of case A?

Thanks

 

[SAIL3]

Case A....assume no liquid in tank, then tank walls, support stl and slab would experience torque from agitator motor...if liquid is present, then the torque of motor is transferred to the liquid and eventually reacted by tank walls and thereby cancelled out....

 

[SAIL3]

on further thought, there is still a nagging question...globally, if one applies an external torque, one would expect a global reaction to this torque..soo, I am not sure, at the moment, what the correct ans is....

 

[SAIL3]

more mud in the water...if assume the tank is on rollers and free to rotate....
no liquid...I would expect no rotation of tank..
liquid present...I would expect the tank to rotate....

 

[SlideRuleEra]

LivingTheBeam - The image you posted is a good example of Case "A". The reason there is no torque on the tank legs or slab is Newton's Third Law of Motion: For every action there is an equal and opposite reaction.

The motor/gearbox is fixed (probably bolted) to the the rigid tank. When the agitator is operating the liquid swirls in a certain direction, say, clockwise (looking down from above). The motor/gearbox mount has to resist the swirling force to keep the motor/gearbox from spinning counter-clockwise. The two forces (clockwise and counter-clockwise) must be exactly equal and are completely contained by the tank walls. Neither the tank legs nor the slab are subjected to any torque.

Consider an experiment, fill the tank with a viscous liquid, say, molasses. Unbolt the motor/gearbox for the tank and turn it on. Chances are the agitator will stay almost completely stationary... but the motor/gearbox will spin counter-clockwise on top of the tank.

For your project, find out the power of the motor powering the agitator... it won't be much, depending on tank size, maybe a few horsepower. The motor's rotational speed will be geared down to reduce the speed of the agitator. Typical agitators rotate at low speed, just enough to keep solids in suspension or liquids mixed.

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