AS 2304 Tank design & overturning

[sdz]

I'm designing a steel tank with no floor and a liner to contain water. Since the liner is ground supported any water in the tank does not contribute to overturning resistance.

However if there is a negative internal pressure acting on the roof that would resist overturning. Since there is a gap all around the roof edge there should be some -ve internal pressure but I'm inclined to think that the internal pressure should be taken from AS1170.2 Table 5.1(A, all walls equally permeable, Cpi = -0.3 or 0.0, in which case it would be taken as zero and does not help overturning.

Does that sound right to you?

 

[HTURKAK]

I understand, the liner would resist to overturning in case of a negative internal pressure.. is this true?.
I am familiar with Euro Codes... IMO, conservatively , you could ignore the effect of internal pressure due to wind .

Will you provide a sketch showing the tank dimensions, roof angle, the gap all around ?.

 

[sdz]

HTURKAK, approx 10m dia, 2.1m wall, 10 degree pitch on roof. It is a steel ring with no bottom. The liner provides a waterproof membrane and is supported by the ground and tank walls. Normally there should be very little tensile load in the liner. It would probably tear off the support around the top of the tank wall if there was any significant load.

AS 2304 - 4.5.2 Stability—Cylindrical water storage tanks
For the purpose of determining tank stability under wind actions, the tank shall be considered to be emptied of its contents.

In the past they have told clients to maintain 100mm minimum water level to resist overturning however my conclusion is that won't work. Initially since the liner is ground supported it does nothing. When the tank starts to lift it will lift the liner but water in that area will just flow away.

 

[BJI]

Use hold down bolts, just like any other tank where uplift requires it.

 

[HTURKAK]

Your conclusion is excellent.. The roof and shell weight SHALL resist to sliding and overturning due to wind loads ( also to seismic loads if applicable)..

If the weight of (roof + shell ) will not satisfy specified F.S. against OT and sliding, the use of anchorage is an option as Mr BJI (Mechanical) stated..

 

[Agent666]

I can't imagine any scenario where you wouldn't want to bolt the thing down. It would be unusual not to fix it down with respect to resisting any lateral loading I would have thought.

 

[sdz]

In a lot of cases these are used as farm tanks and they would prefer to not have anchor bolts if they can be avoided. But it looks like I have to tell them that's not going to work.

 

[RayGon]

The way is see this is what you have is a closed structural system that sits on the ground. Like a steel ball. The pressure inside the ball will not affect the stability of the ball. The pressure will not stop it from rolling as long as it is is round.

Stability and overturning is usually caused by outside forces not internal.