Following a short erection time of only 4 1/2 months, the Alderwood Water District in Lynnwood, Washington is now home to the world's largest circular water reservoir cover. The 375' diameter, low-profile Temcor Aluminum Dome covers a 28 million-gallon reservoir.
The dome is part of a $4.5 million improvement project that included a seismic upgrade and protection from environmental contamination. "Covering the reservoir not only protected the water, but drastically reduced the required maintenance. Before the cover was installed, the reservoir had to be drained and cleaned yearly," said Tom Fahey, Temcor?s project manager. "Now that the water is protected from leaves and other debris, that costly process will need to be done maybe only every five years." The all-aluminum construction also means the roof itself will be virtually maintenance-free for years to come.
For aesthetic reasons, the dome roof had to be very low profile, so Temcor designed the cover with a rise of only 15' 8" (4 percent) rise. To accomplish that low rise, 85 stainless steel support columns were required. "Temcor could have designed the dome to be clear-span, without the supports, but it would have required a 70' rise which was not acceptable in that location," continued Fahey.
"Two companies bid on the dome portion of the improvement project," said Lowell Warren, project manager for Tetra Tech/KCM, the consultant engineering firm. Temcor?s bid was not the lowest, but they were awarded the contract based on their experience in erecting large-scale domes. "And we made the right decision. Temcor met or beat every deadline along the way."
Temcor built the reservoir cover using their patented center erection tower. For a roof of this size the tower was raised to a height of 218 feet, then the roof?s structure was assembled, within the drained reservoir, from the center out. With the completion of each strut ring, the dome was lifted five to six feet off the reservoir floor until the frame was complete.
The center tower erection method was used for this project for a number of reasons: There was limited available space at the site, so the tower method allowed the roof to be built within the reservoir; and the support columns had to be loaded-in and stored on the reservoir floor during the erection process, so once the assembly process was complete, the tower could hold the dome in place above the reservoir while the 18" diameter (average length 45') columns were erected. When the columns were in place, the roof was simply lowered into place on the reservoir wall and columns, and secured. Using the center tower to build the dome requires a relatively small crew?an average of only 12 people used on this project?that can work safely on the ground without the need for scaffolds or man-lifts. "And that small crew size contributes to lower construction costs," added Fahey.
Source: Temcor
