By Dave Ornduff and Jennifer Walker
Established more than 175 years ago, the city of Washington, Iowa, has a rich history and is home to several historical sites, including vintage homes and buildings that are listed on the National Register of Historical Places. One particular structure, although not deemed a historical site, is a 100-year-old, aboveground concrete reservoir. The reservoir was adorned with murals painted by local artists and has long been cherished as a local landmark.
In its earlier years, the top of the tank was used for city-wide dancing events, but in recent years, it fell into disrepair and the access to the roof was closed in the interest of public safety. Due to the extent and quantity of the defects, renovating the historic tank was beyond the scope of the facility improvements plan and beyond the limits of local funding sources.
The city opted to replace the reservoir with a new 1.0-million-gallon (MG) tank and selected FOX Engineering Associates Inc. of Ames, Iowa, as the engineering firm. After considering several alternatives, FOX hired DN Tanks to design and construct an AWWA D110 Type III prestressed concrete tank for the new reservoir.
One of the main concerns with the old tank was the failing roof. The old roof was a flat, column-supported concrete covering with internal concrete beams. The concrete was spalling due to years of freeze and thaw damage, and a significant amount of the roof's original reinforcing steel was exposed on both the inside (underside) and exterior. Further, there were large cracks in the old tank that seeped water into the bottom third of the side walls.
One of the benefits of the prestressed concrete tank is its proven cold-climate performance. The new tank incorporated a free-span concrete dome roof with no internal columns or supports. Since the new prestressed tank was designed to keep the wall concrete in compression, it was less likely to develop cracks in the same manner as the old reservoir.
The new reservoir also provided the city with operational flexibility equivalent to two separate tanks. FOX designed the reservoir as a "tank-within-a-tank" by means of an inner, independent 500,000-gallon concentric cell within the total tank capacity of 1.0 MG (100' ID x 17.5' SWD). Each cell provides a half million gallons of storage and operates independently or in tandem.
The underslab piping was designed so that the water could pass through the inner cell first and then the outer cell, operating in series for better circulation (short of installing a mixing system). Alternatively, the cells can also run in parallel. The "tank-within-a-tank" design gave the city the flexibility to run only one cell during low demand periods to improve water quality, while the two-cell design provided redundancy for inspections and cleaning.
While a new tank would meet the long-term needs of the city of Washington, its citizens were concerned with the aesthetics of a new structure that would replace a cherished landmark embellished with murals. The finish of many cast-in-place tanks often appears very utilitarian and isn't enhanced with attractive architectural elements. This project, however, demanded an exterior appearance with a positive aesthetic without being obtrusive.
The smooth gunite finish of the tank -- along with the elegant arches and pilasters -- was a great fit. The successful completion of the project provided the city with a reliable prestressed concrete tank-within-a-tank as well as a new structure that the community is proud to display.
DN Tanks is an exhibitor at WEFTEC.15 and can be found at Booth 1717. For more information, visit www.dntanks.com.
