Tips for Specifying Composite Elevated Water Storage Tanks
Elevated water storage has been used for over 100 years to ensure sufficient supply and operational flexibility for water systems.
Elevated water storage has been used for over 100 years to ensure sufficient supply and operational flexibility for water systems. These structures traditionally have been constructed with a welded steel tank container supported by steel columns or a steel pedestal. A variation uses a steel reinforced concrete pedestal to support the all-welded steel tank - this relatively new tank is referred to as a composite elevated tank (CET).
Form construction using 6 foot forms
The CET was first constructed in Canada in the late 1970s and the U.S. in the late 1980s. In 2005, the CET accounted for nearly 15% of all elevated tanks bid in the U.S. and over 60% of elevated tanks equal to or greater than 1 million gallons. Currently there are over 500 CETs in the U.S., none of which have been designed or constructed to an accepted industry standard.
Fifteen percent of all elevated tanks bid in the US in 2005 were composite elevated tanks.
The design, fabrication and construction for all-steel tanks is governed by the American Water Works Association Standard ANSI/AWWA D100, first published in 1935. This standard has experienced major revisions over the past 70 years and incorporates the accumulated knowledge and experience of purchasers and manufacturers. There currently is no such standard available for composite elevated tanks.
Form construction using 4 foot forms.
Acknowledging the need for a standard, in 1992 the American Water Works Association formed a committee (Standards Committee 170) to develop a standard for the CET. This committee consists of purchasers, consulting engineers and manufacturers of CETs. Though the standard is not complete, many of the sections within the standard have been approved, including the section pertaining to the concrete support structure. For specific information pertaining to the current status of the standard, contact Paul Olson at American Water Works Association via e-mail at email@example.com.
In 1998, the American Concrete Institute issued ACI 371R-98 “Guide for the Analysis, Design, and Construction of Concrete-Pedestal Water Towers.” This document has been used as a source guide for the AWWA Standard and can be used as a current tool in developing project specifications. Until the AWWA Standard is issued, owners and engineers must rely on this guide and manufacturers’ in-house reference documents to develop project specifications.
In relying on an independent tank manufacturer’s specification, owners and engineers can unknowingly limit qualified competition from bidding the work, costing their ratepayers significantly.
Three of the primary CET manufacturers in the U.S. are Caldwell Tanks, CB&I and Landmark. Each of these companies has developed their own proprietary design for the CET concrete pedestal, the pedestal-to-tank interface, and the steel tank container. Specifications should be specific enough to ensure quality construction and a safe structure, yet broad enough to allow all companies to competitively bid the project using their proprietary design and construction techniques.
Two specific components of project specifications that tend to limit competition and increase price pertain to specific methods of construction (i.e. concrete pedestal form height) and contractor qualifications.
Pedestal Form Height
The current draft from the AWWA CET Standards Committee and ACI 371R-98 does not limit the concrete pedestal form height. Specifications should never state a required form material or minimum or maximum form height, as this unnecessarily limits competitive bidding and has proven to significantly increase the costs for CETs.
Each of the three CET suppliers own and maintain their own equipment to construct the concrete pedestal and steel tanks. Though each company has constructed dozens of CETs and have been doing so for over 10 years, each company uses a slightly different form system for the pedestal construction; form material varies (steel, wood or composite) and the form height varies (4', 6' or more).
The specification should specify that form panels consist of curved, prefabricated segments designed for full height plastic concrete pressure and provide a uniform pattern of vertical and horizontal rustications for architectural relief. The ACI Standards ensure proper form construction and concrete placement for whatever form height a contractor uses.
The project specification should require that contractors own and maintain their own forming equipment in order to ensure quality construction. The intent of the project specifications should focus on final product and quality requirements and allow the contractors to use their own proven construction techniques.
Owners and engineers should consider the in-house technical capabilities of all potential suppliers. Contractors should be required to have Professional Engineers with CET design experience on-staff that are licensed in the state where a project is being constructed. Additionally, reasonable experience requirements should be stated in all specifications, being careful not to eliminate qualified bidders.
Most CETs constructed today are 500,000 gallons or larger and all three contractors mentioned above have completed tanks ranging from this size to 2.5 million gallons or more. The specification should be written to require experience constructing tanks of similar capacity to that which is being specified and not exact capacity. This affords the purchaser the flexibility to fairly evaluate the qualifications and experience of the contractors equally, without penalizing them for completing slightly larger or smaller CETs.
To summarize, there is much confusion and misdirection in the CET marketplace as contractors dictate project specifications because there is no standard. Until the AWWA Standards Committee 170 releases the standard for the design and construction of composite elevated tanks, owners and engineers assume the responsibility of establishing project specifications for a CET.
It is in the best interest of engineers, owners and ratepayers to allow qualified and experienced contractors to bid these projects without limiting contractor’s proven methods of construction. Project specifications should allow the flexibility for each contractor to construct a quality and safe product for the owner. Considering that all three manufacturers mentioned in this article are voting members of the AWWA CET Standards Committee, engineers and owners can draw upon the accumulated knowledge and experience of these manufacturers once the standard is issued.
About the Author:
John E. Kraft, P.E., is Vice President of Business Development for Caldwell Tanks Inc. in Louisville, KY. Kraft has been with Caldwell for 15 years, now being responsible for business development for the Tank Company and general management of Caldwell Energy Company. Prior to joining Caldwell, Kraft was an engineer for Allison Gas Turbine, division of General Motors. He is a graduate of Purdue University and a licensed Professional Engineer in several states. He may be reached via e-mail at firstname.lastname@example.org.