Seismic Overturning Moment on Silo

[Deadblow]

Hello,
I calculated the seismic response coefficient, Cs = 0.0534. I know that I get my base shear by taking Cs x (the weight). How do I calculate the overturning moment? The silo is 39'-9" tall and 10'-10" in diameter.

Thanks!

 

[BAretired]

Would it be 20.Cs.W?

BA

 

[JStephen]

Assume the force acts at the center of gravity of silo and contents.

 

[Deadblow]

Ok, that's what I figured but thank you for your confirmation!

 

[BadgerPE]

Is this a liquid storage tank? Have wave/sloshing effects been considered?

 

[Deadblow]

It is a liquid storage tank and no I did not account for any wave/sloshing effects. May I consider the weight of the liquid as part of the resisting moment or only the dead load of the silo itself? Are you able to point me toward a good reference? Also, I'm not quite sure how to analyze the wind load on the empty silo. Any references and/or codes?

Thanks

 

[BadgerPE]

I haven't ever run the numbers myself, but all silos that have been submitted to me (FND designer) have been designed to API 650 for seismic forces.

 

[vhengr]

While earthquakes the sloshing effect of liquid on the side wall creates additional horizontal forces which need to be considered.

Basically idea is; Impulsive mass of liquid acts on the lower part of the tank and convective mass of liquid on the upper part. During seismic event; the mass of fluid in the lower part of tank behave like rigidly connected with tank and moves with wall and imparts seismic inertia forces called impulsive mass. Slushing fluid in the upper part; termed as convective mass which oscillate with their own natural frequency introduced hydrodynamic pressure on a wall.

All over; the response of a tank structure in EQ action and how to account for the sloshing effect in structre design is an intricate problem.

You may refer the ACI 350-3 for designing the tank for hydrodynamic effect. Step by step method for calculating the hydrodynamic forces on tanks and time period has been provided.

also you can find detailed guidance on this subject is available at website National Information Centre of Earthquake Engineering at IIT Kanpur; INDIA.

http://www.nicee.org/IITK-GSDMA_Codes.php

May find helpful to you.


Best Regards_ VH

 

[JStephen]

If it's for liquid storage, it's a tank, not a silo.
If it's a flat-bottom tank on grade, then you can use API-650, AWWA D100 or similar standards- is the tank itself designed to any standard?
Generally, for tanks of any size, the consideration of sloshing reduces the seismic loading.
Also refer to ASCE 7. One of the approaches used there is to treat the tank contents as a rigid mass, ignoring sloshing. If seismic loading is not high, that will work okay.

 

[Deadblow]

Ok, thanks for all the help everyone. I analyzed the empty tank for wind load using the part of chapter 6 of ASCE 7 for chimneys, tanks, etc. Then I analyzed the full tank for seismic according to chapter 15 of ASCE 7. Seismic controls the base shear and wind controls the overturning. The tank itself is designed by others, I am just responsible for the anchorage and the foundation. I didn't know that ASCE specifically addressed "Ground-Supported Storage Tanks for Liquids" so I apologize for my ignorance.

 

[structSU10]

With the couple tank foundations I have done, I was able to get the wind overturning and seismic overturning from the tank design engineer, as well as the appropriate dead loads, and could create my own load combinations to design the foundation. It usually wasn't too painful of a process, and I didn't have to develop any loading myself.