Roof Top Firewater Tank Prevents Building Sway in Bay Area Winds

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One Rincon Hill, San Francisco Firewater Tank Dual Purpose Design

There is a unique feature of the building at One Rincon Hill in the city of San Francisco, which is its Bay Area firewater tank, situated atop the high-rise building. Although it’s not the largest building in that city, One Rincon is definitely the tallest building outside the main business district, and it can easily be seen against the skyline, with its rounded cylindrical top. This building stands nearly 700 feet above San Francisco Bay, and it contains a feature that makes this high-rise tower very unique, the water tank which holds 50,000 gallons.

One of the primary purposes of this tank is to stabilize the building, in case of strong winds which could come from an unexpected direction, and additionally to be used as a source of water to suppress fires when needed. Although engineers consider the likelihood of this particular type of wind direction to be minimal, if such swaying ever did occur, it could induce motion sickness in the occupants of the condo below. Thus the justification for including the enormous water tank with unique design and engineering at the top of the building.

How the design works

Any swaying which might develop as a result of strong winds from the wrong direction will be theoretically counteracted by the swishing of water from one side of the tank to the other. For instance, if the wind would blow from the south, the water inside the tank would be swished to the north side, thus negating any pressure exerted by the wind. This kind of engineering solution is known as a ‘tuned liquid damper’, and it has been used in Hong Kong and Korea on several buildings. It has yet to be used anywhere on the West Coast until now, and if successful, it would likely trigger replication of the engineering process in other cities and buildings new and existing buildings requiring water tanks in San Francisco.

Testing the tuned liquid damper

During the testing phase of construction, it was determined that winds which blew in from San Francisco Bay toward the city from the East were likely to cause the building to sway. The winds affecting San Francisco are predominately westerly winds, and these can reach 25 to 30 miles an hour for several days consecutively.

During the winter, winds generally blow into the Bay Area from the north, and during the fall season occasionally from the East. The most unusual wind direction for the bay area is a strong southeasterly wind coming in off San Francisco Bay, and this is the one that engineers found to be rather dangerous for the building at One Rincon Hill. A fire suppression water tank like the one installed at One Rincon would not only eliminate the swaying, but could also be used to safeguard the condo against the breakout of any kind of fire.

What test results showed

This particular type of wind produced movement in the building, and the type of movement it produced is known as ‘vortex shedding’ to engineers, because it refers to airflow which impacts a cylindrical body. Testing showed that the building could move as much as 16 inches, and had the capability of inducing motion sickness to any occupants.

San Francisco Condo Firewater Tank Success

Since this is an upscale condo building, it was deemed highly desirable to prevent the possibility of this kind of motion sickness, and that resulted in the cylindrical water tank being positioned at the top of the building. The 50,000 gallons of water included in the tank weighs a total of 416,500 pounds, and that is sufficient to prevent any swaying, under even the strongest of rogue San Francisco winds. This condo firewater tank could also provide a fair amount of protection in the event of a fire in the high-rise, since the water could be released to douse the flames.