Wood water tankseismic analysis

[TFL]

I was wondering if someone could offer me some guidance. I have been struggling to find a reference that talks about the analysis of wood stave tanks. I am looking at an existing redwood tank to try to offer an opinion as to it’s seismic resistance . There seams to be a common trend out here(CA) of eliminating them – partially due to other factors such as water quality and maintenance. I have even found some non-technical references that talks about the “inherent seismic instability of redwood tanks.” I have not found any technical references to back up that statement. I was hoping someone's experience could help.

The Tanks are similar to:

http://www.vintagelog.com/images/redtank.jpg

Any help would be greatly appreciated.

 

[msquared48]

If my memory serves me right, the slats are positioned vertcally with the horizintal cables used to take the lateral thrust from the water pressure in tension, varying in size and spacing over the height of the tank. I believe that the slats are not attached under any specific structural design loading condition to specifically transfer a certain amount of shear between the slats. Therein, I believe, lies the weakness. In fact, as with a wood stave barrel, the swelling of the wood at the joints is relied upon to keep the fluid in the structure. When the structure is filled initially, there is leaking that decreases with time as the wood swells to seal the joints.

In an earthquake, with the water sloshing, say 4 to 6 feet from side to side as an example, and the lateral fluid pressures varying accordingly, the slats have little resistance mechanism other than interstitial friction to develop any diapragm forces to be used as a shearwall to resist the seismic forces. Failure would be in a layover mode. There is also the aspect of exciting the natural frequency of the fluid in the structure, causing further forces on the tank.

I believe that I have some design literature on these structures somewhere in my archives. I will try to find it.

Mike McCann
MMC Engineering

 

[CarlB]

You may find a contact at this site who could provide some assistance:

http://www.timbertanks.com/

They state: "Deliberate and inherent design features have been incorporated to give very high earthquake resistance to TimberTanks. This has been proved in practice during a number of severe earthquakes in New Zealand."

 

[msquared48]

Sorry, but I could not locate it.

Mike McCann
MMC Engineering