Tank Lateral Restaints

[someengineer]

Hello,

I was wondering if I could an opinion on this subject.

We are installing a plastic flat bottom tank in a building in a high seismic zone (BC Canada, west coast) and the question of required anchorage/restraint has come up. The tanks are such that both the overturning (empty and full) and sliding (empty and full) can be resisted by the self weight of the tanks alone.

There is a clause in the BC building code 2012 (4.1.8.18 8)) that stipulates that seismic forces cannot be resisted using friction loads and furthermore the structural commentaries specifically references the possibility of tanks "walking" under a seismic event.

My question is - would it be acceptable to provide lateral restraints at the bottom of the tank (i.e. angles bearing against the side of the tank) to prevent any lateral movement associated with sliding instead of anchors connected to the tank and foundation? I'm curious because the code is a little vague since it alludes to not using friction to resist sliding, yet it talks about walking which implies vertical movement. However the vertical movement is resisted by the self weight of the tanks which appears to be permissible by the code. Thoughts?

Thanks!

 

[jayrod12]

I think you still need to fasten it down and not just provide lateral passive restraint. The reason they say you can't use friction is if there is vertical acceleration the gravity load is temporarily reduced.

 

[JLNJ]

I would think "guide" angles as you mention would keep it from hopping away, but I don't know of a code reference.

 

[someengineer]

Thanks for your help, I think we will provide anchorage as opposed to lateral restraints to be on the safe side.

 

[dhengr]

Someengineer:
I think you need straps over the tanks to hold them down. Then, roll form some angles to the O.D. of the tank and 2-3' long and bolt the angles to the floor. You have to provide enough angles (lengths and angular location, in plan) so you are not applying the lateral loads to the bottom of the tank in such a concentrated way as to rupture/hurt the tank. And, maybe a rubber bearing pad btwn. the tank and the angles.

 

[someengineer]

dhengr, the tanks come with locations on top to attach cable tie downs which will be tied into anchor points in the foundation. It's pretty much is exactly what you are suggesting.

Thanks!

 

[humanengr]

Don't forget the dynamic coefficient of friction is "alot" less than the static coeff. That is probably an additional
reason for not allowing frictional resistance against sesimic loads.

 

[EngineeringEric]

I think Jayrod said this exact. Under potential seismic events their may not be any vertical self-weight... ie a dip of the building would eliminate friction for a second. numerous reductions in the force of gravity minus vertical acceleration (right now it is 1.0g, EQ may be 0.5g) coupled with lateral forces based on intertia of the mass may overcome reduced static friction and the thing will walk. Thats why you strap it down or provide flanges with tall enough vertical legs that the tank will not be able to hop out and stiff enough that they wont bend if the tank tries to slide out.