AWWA D110 Prestressed Concrete Tank Comparison 1

[Gumpmaster]

Has anyone seen a good comparison between the different types of AWWA D110 tanks (Type 1, Type 2 and Type 3)? I'm specifically looking for comparisons on the long term performance of those tanks.

We typically specify Type 1 tanks. The Type 2 tanks are much cheaper, but I have to believe that they have more problems with corrosion on the long term.

Thanks.

 

[JedClampett]

Well, you could ask the suppliers.
But seriously, we're used or had experience with all the types. I can't say one is better than the other.
Type 1 is supplied by DYK in our area. Good supplier, good tanks, although they can get a bit independent on the foundation design.
The Type 2 I'm familiar with is supplied by Crom, mostly in the southeast. Once again, very good reputation, to the point of all tanks in Florida being called Crom Tanks, (like Kleenex). They service tanks, even very old ones. We had a digester that was over 50 years old that they inspected and repaired (not for free, but still). You would think corrosion is an issue, but they seem to have the trick in avoiding it.
I'm not as familiar with the Type 3. I think they're supplied by Natgun, but I'm not sure. And they've merged with DYK, so that complicates matters.
As far as specifying them, prestressed tankage is regional. If you're in Florida, DYK stays away and Crom bids. In California, it's pretty much DYK and no one else. Type 2 is pretty inexpensive, but I don't think the others are drastically more. Therefore, you're better off finding out who's been providing prestressed tanks in the areas, specifying that type and just riding along.

 

[HarveyTanksYou]

Good Questions Gumpmaster,

And also, good information from JedClampett. As someone who has designed hundreds of wire-wrapped prestressed concrete tanks (both Type II and Type III), a former owner of a prestressed concrete tank firm in the southeastern US and a current owner of a nationwide prestressed concrete tank firm, this one is right down my alley.

In the interest of full disclosure, I'm a proponent of any wire-wrapped tank with a steel shell diaphragm (Type II, III and IV). The steel shell diaphragm provides a positive waterstop in the wall which guarantees watertight performance assuming construction is performed properly. Yes, Type I does contain vertical prestressing which should help to produce a more watertight tank structure but there is no positive waterstop like the steel shell diaphragm. I've seen it work fantastically thousands of times over my career.

To start off with, despite what most industry suppliers will tell you, structurally, all these tanks are the same. They are all designed using the same equations for plates and shells by Timoshenko, equations for design of seismic developed by GW Housner in the 1950’s and typical reinforced concrete design. Yes, the diaphragm location is different and requires slightly different design methodology as it is typically used for temperature and shrinkage steel on either the inside or outside face depending on Type but as any structural engineer knows, that is only semantics.

The real difference is in means and methods of construction and thus diaphragm location. Type II tanks are constructed of shotcrete and the diaphragm is located on the inside face of the wall with a minimum of 1" of cover. Shotcrete is a cement rich grout mix comprised of fine aggregate, water and cement. The cement content is very high in the mix (1:3 against the diaphragm) thus producing excellent corrosion resistance and long term durability. Any problem with a Type II tank comes from construction quality (just like all other structures). When shotcrete is applied in accordance with the guidelines of ACI 506 by an experienced nozzelman the result is excellent encasement of the diaphragm and long term durability. When it is not, hollows can develop between the shotcrete and diaphragm with the result being corrosion issues and poor durability. That said, when good quality control measures are followed, the result will always be an excellent tank structure. I have personally performed destructive inspections many Type II tanks that have been in service for decades and have found shiny steel diaphragm on the tank when good quality shotcrete has been applied. I have also seen the opposite in very, very limited cases. The problem can be exacerbated in cold climates where ice lensing occurs and has served to give Type II a bad reputation in some instances where the real blame was due to poor quality.

Type III tanks are constructed by casting the corewall in a tilt-up panel. You should know that is primarily done for ease and economy of construction as these tanks were first constructed in areas with short construction seasons and high labor rates. This construction technique has the added benefit of locating the steel shell diaphragm behind a minimum of 4” of cast concrete. You might say, problem solved; however, these tank Types have other details that require excellent attention to quality for proper construction and particular for watertightness. With good quality construction techniques, these are also great structures and should provide many years of durable service. Again, I have also personally seen the opposite in limited cases where construction quality was poor.

I could go on and on, but, in short, either tank Type will work great for most purposes if you have experienced personnel designing and constructing the product. Quality control is the key on these projects, yes, just like all other field constructed products. Historically, industry suppliers have long tried to fool good engineers into specifying one Type of tank over the other solely in the interest of reducing competition. Unfortunately, up until recently, no one has come along to challenge this notion. For example, why would a Type II tank be an excellent choice in Sunny Florida and not in Sunny Texas or Sunny California? Conversely, why could a Type III tank not be constructed in any of those three locations as well with excellent durability? Hum... let me try to think of a good reason.

Be wary of other tricks of the industry as well such as unachievable experience requirements to “qualify” for construction.



K. Ryan Harvey, PE
Vice President - PCT Division
Wire-Wrapped Prestressed Concrete Tanks
Caldwell Tanks, Inc.

 

[Gumpmaster]

Thanks Harvey,

Good information. I've specified a good many Type 1 tanks, but not the others.

FYI, part of our reluctance to specify type 2 & 3 tanks in high seismic areas is the relatively small amount of reinforcing between the walls and the foundation. I'd be interested if you have any experience with type 2 & 3 tanks that have gone through earthquakes and how they performed.

I appreciate the point of view from someone in the industry.

Thanks Also JedClampett.

Does anyone else have experience in this area, positive or negative?

 

[JedClampett]

There was a prestressed tank failure (

http://articles.latimes.com/1998/sep/25/local/me-26262/2)

in California about 15 years ago. I don't think it was a seismic issue, more of a corrosion issue.

 

[HarveyTanksYou]

Gumpmaster,

Another good question. I have heard this a few times from people on the west coast. Type I, II, III and IV tanks are all designed using the same seismic load equations that are either provided in ACI 350.3 or AWWA D110, the later of which is being revised and will be out in a few months to change the methodology to an acceleration based design as opposed to the outdated zonal based design. Again, these design methods are based on research and development by GW Housner which was performed for the nuclear industry back in the 1950's.

So, now to the resistance side. All of these tanks are also detailed the same way to provide resistance in a seismic event. The floor wall joint of any of these tanks are classified as "partially restrained base" joints, meaning, the wall rests on a elastomeric bearing pad and is free to move in the radial direction. The "partial restraint" is what provides the resistance during a seismic event and that restraint is provided through the use of 7-wire stranded cables that run from the footing up into the wall. These cables are wrapped in an elastomeric neoprene where they pass from the footing into the wall of the tank and that neoprene allows the wall to move radially. It is the number of cables provided that provides the proper amount of resistance. In high seismic regions a substantial number of cables will be used for the restraint. That said, again, all of these tanks (I, II, III and IV) are detailed exactly the same in this area with all design requirements being provided in Code or Standards documents.

As for tanks which have gone through seismic events, I understand that there are several Type II and III tanks built in the early days of the industry on the west coast that have lived through seismic events. That said, since the design and detailing is exactly the same for all of these types of tanks, I wouldn't quite understand the need to specify one of the other just because one Type had not been through a seismic event. With all respect, I find that to be another tricky selling point that some in the industry use to eliminate competition and I could certainly refer you to other very respected structural engineers who would agree with that.

As for the tanks that failed in Westminster that JedClampett points out, those particular tanks were of the "Pritzker" design with a high concentration of tendons at the roof and base of the structure with a wall "spanning" between the two areas. A few of these tanks were built in the 1950's. An interesting concept that unfortunately lacked the structural redundancy of the D110 design.

Many thanks for allowing me to answer the questions and for the interest in learning the true story on the tank types.



K. Ryan Harvey, PE
Vice President - PCT Division
Wire-Wrapped Prestressed Concrete Tanks
Caldwell Tanks, Inc.