Water Tower Analysis 1

For an existing 110' high elevated water tank located in seismic zone 4 would you recommend a static analysis per 97' UBC standards or a full blown site specific dynamic analysis. Analysis is being performed for a retrofit consideration. Per 97' UBC if the structures period is less than 0.7 sec then per code you are allowed to perform a static analysis, above 0.7 sec code recommends dynamic. We have heard from a few source that a dynamic analysis should definitely be performed on a inverted pendulum type structure like this. Please advise on your theories on how to accurately model this type of structure.

Thanks,
KSP Engineers

 

[Trussdoc]

I don't know much about water towers, but with the consequences of a possible collapse and the legal ramifications, I would definitely take the conservative path!

The thought of thousands of gallons of water sloshing around on top of a tower intuitively precludes a static analysis, and I would think a lawyer could easily convince a jury of that.

 

[VoyageofDiscovery]

Hi KSP Engineers,

My advice is to contact the original designer if possible.
Water tanks come in such a variety of design concepts, that local stresses are extremely important.
One example from my experience in a non-seismic location was that I initially assumed the steel tank to be a shell, when it was actually conceived as a membrane, essentially a bag of water. Experts in this field may be long gone as this area is very specialized and new construction of each type is somewhat limited.

Glad to see you are exercising extreme caution.

Regards

VOD

 

[JimParks]

I think if you go through the excercise of determining the period of the water tower you won't find it anywhere near (let alone below).7 seconds. (I'm having a hard time picturing a 110 foot water tower swaying back and forth in less than a second, even if it has a stiff bracing system, but I could be wrong.) So the question may be moot. I would agree with Trussdoc though that even if the period allowed a static analysis, it wouldn't seem very prudent.

Good luck with the project.

Jim

 

Use the seismic provisions in American Water Works Association standard D100-96 for design. They are based on UBC-94 not UBC-97; the differences probably won't be substantial. D100 is conservative because it does not allow sloshing for elevated tanks (ie. separating the water load into impulsive and convective components). Sloshing can reduce seismic forces substantially.

The period of most elevated tanks is almost always above 1-second. For a 110-ft tall water tank multi-leg tanks and waterspheroids (the golf ball on a tee look) I would expect the period to be above 1.5 second. Hydropillars and composite (concrete shaft) tanks have cylindrical shafts that are 50 to 70 percent of the tank diameter. With these structures the period is probably around 1-second or possibly a little less.

A lot of older elevated tanks have been built without regard to sesimic forces. Many perform adequately if connection details are robust. On multi-leg tanks the weak link is the tension only bracing system, especially the connections which generally are not capable of yielding the tie rod. The classic failure mode of these tanks in high winds or seismic is a spiral collapse after failure of a bracing element.

Contact Bob Wozniak at bow-tech@att.net. He is past chairman of the AWWA D100 committee, is retired, and does consulting work especially on rehab of tanks in California.