Seismic Analysis of Elevated Silo 1

[rlewistx]

I'm working on the analysis of an elevated sand storage silo. The silo is 65 feet high and 19 feet in diameter. The base of the round section and the top of the cone base is at elevation 29 feet. The bottom of the cone is at elevation 16 feet so truck can pass underneath. The frames are crossed braced only in the upper cone section.

I'm looking at section 15.7.10 of ASCE 7-05. I'm wondering how to calculate the natural period of the structure. I'm not designing the silo frame, just the foundation. The NEHRP section 14.4.7.9.5.2 state AWWA D100 has a method of determining the natural frequency for elevated tanks. I don't have a copy of AWWA D100. Can some who has a copy tell me if the standard would have sufficient information in it to help me determine the natural period for the structure from this limited information?

Are there other traditional methods for calculating the natural period of the silo structure?

Thanks for your help.

 

[COEngineeer]

I think to figure out the natural frequency or period you will need to do structural analysis on the structure to figure out the stiffness at each bay and the mass. I would just tell whoever design the frame to figure it out. I dont have my structural dynamic book with me, Ill look at it when Im home. It has been a while.

Never, but never question engineer's judgement

 

[jheidt2543]

I agree with COEngineer, the silo manufacturer had to figure this out for design of his tank framing, it should be no big deal for them to give you ALL the loads imparted to the foundation.

 

[JStephen]

AWWA D100 still requires some analysis of the deflection characteristics of the tank. The loads on the foundation should come from the silo designer.

 

[rlewistx]

The silo manufuacturer says he never has had to do a seismic design before. I guess he never sold it in seismic country. He forwarded the framing drawings to me and told me if I wanted it to do it myself. Unfortunately I have no control over the silo manufacturer. My client bought the silos and is tight with money so my complaining won't go far.

 

[JStephen]

Sounds like a sticky situation. There are some issues with liability and ethics here, although that's not the question at hand. Are you trying to design foundations for it, or just check the overall adequacy or what?

A couple of approaches come to mind. In the ASCE 7 equations, they are generally of the form V = "this but need not be greater than this". That is, there's an upper bound on the seismic loading regardless of the period. If the seismic accelerations are fairly low, you may be able to work with that upper bound and design your work without getting too overly-conservative.

If you are only designing foundations, the design loads on the foundation would not need to exceed the ultimate strength of the structure. If you can calculate the maximum shear and axial load the columns can generate, that may give you a means of design.

Since the lower part of the structure is not cross-braced, you could treat the entire upper structure as a rigid body, calculate deflection in the unbraced legs only, and derive a period without too much trouble. The actual period should be somewhat longer, and depending on the period, this may be a conservative estimate.