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Comparing miles per hour between AWWA and ASCE 7 will not give you a direct apples to apples comparison.  The comparison needs to be on wind pressures and resulting overturning moments.  The 100 mph criteria was developed decades past based on controlling deflections and providing a rule of thumb aaproach.

In AWWA D100-96, we added (for the first time) provisions for escalation which is especially important for this tall tank. 225 ft (I assume to top of roof ) is a tall elevated tank.  The typical tank is about 100 ft the bottom capacity line.  It is likely the next version we hope to improve the wind design rules and bring them into agreement with ASCE 7 / IBC 2000 loads and teh avaliable technology.  We are using ASCE 7 directly for the composite elevated tank standard being developed.

The AWWA stds are recommended minimums.  Tanks in coastal areas are typically designed for a wind speed grater than 100 mph AWWA.  We never intended that D100 address all local issues and I often emphasize the localization of the requirements for snow, wind and other loads when I give seminars on tank design.

However,  tanks designed to AWWA D100 100 mph stds have had an excellent performance history.  We can point to many examples where tanks have weather wind speeds far in excess of the 100 mph in hurricanes and tornadoes.  In part, this is the conservatism in the strucutral design requirements ( for example a factor of safety of 3 for direct + wind on bearing pressure).  We are discussing permitting a 1/3 increase in the next edition of D100 for wind load / foundation design.

Perhaps a greater influence is the design criteria for the wind check to be applied to an empty tank, which does not often ocurr in practice.  The extra ballast from the water improves the overturning factor of safety.  In fact, we tell owners to fill up there tanks if a hurricane is approaching. 

Quite often flying debris is the cause of an elevated tank failure. Debris can fail the rods, struts, or cause dents/buckles in the shell.  Yet these structures are quite ductile.  We have sen tanks as the only structure left standing in the path of a tornado. I have seen a buckle from debris in the shell of of a single pedestal tank approx 5 ft square and 8-10 inches deep.

At this height, a single pedestal, small capacity tank should also be checked for low speed wind resonance which may actually govern the anchorage and foundation overturning.

I suggest you check the loads per both, evaluate the factors of safety given the soil and type of foundation, the type or styl of tank, the likely failure scenarios, operating parameters for the tanks ( probability of the tank being empty other than at maintenance or repaint), and then decide if the issue needs to be pushed farther.

Hope this helps.

Stephen W. Meier, PE, SE
Vice President, Engineering & Technology
Tank Industry Consultants, Inc.
684 W Boughton Rd, Suite 101
Bolingbrook, IL 60440
630-226-0745 Voice
630-226-0802 Fax
317-394-0854 Pager