Effects of Vertical Acceleration on Liquid-Containing Tank (ACI 350.3-06)

[Erudawn]

ACI 350.3-06 Sec R4.1.4 states that the effective fluid pressure shall be increased or decreased due to the effects of vertical acceleration, but it doesn't clearly specify under which vertical seismic direction it should be increased or decreased.

For instance, if I am considering a vertical earthquake force (EQv) in which the seismic inertia of the concrete structure is upwards, I imagine that the entire concrete structure moves upwards along with the base of the tank, while the contained liquid remains stationary, thereby increasing the downward fluid pressure acting on the base.

Does this mean that the effects of vertical acceleration on the contained liquid are always opposite to its effect on the concrete structure? (i.e., when the vertical effect of seismic on the concrete structure is upwards, its effect on the contained liquid is downwards, and vice versa).

 

[KootK]

That's how I read it. I imagine that the main concern is usually for the effect that this has on the horizontal fluid presexerted on vertical surfaces.

 

[JStephen]

Normally, in the steel tank codes, vertical acceleration is treated as an impulsive acceleration, and the acceleration of the tank and the contents would be considered the same and in the same direction. In one direction, you get increased pressures and increased soil bearing, in the other direction, reduced uplift resistance.

It is possible to come up with natural frequences for both the contents vibrating vertically and for the container subject to vertical acceleration. Assuming those natural frequencies are different, vibrations of each are not necessarily in phase, so you could have accelerations in opposite directions. But that is not the design case normally considered.

 

[HTURKAK]

Not only the downward pressure but horizontal water pressure will also increase or decrease . The horiontal pressure for the vertical seismic effect pvy = üv*γL (HL – y) .Pls look to the definitions.


Your approach is OK .

Pls look 5.3.2—Combining dynamic forces for rectangular tanks
...The hydrodynamic force at any given height y from the base shall be determined by (5-1)

Py = SQQRT ((Piy + Pwy)**2 + Pcy**2+ (pvyB)**2 )

Just write (Guidelines for seismic design of liquid storage tanks ) search the web . One of the outcomes the following doc. which has worked examples .



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