How Seismic Use Group Classification affects your Water Storage Tank

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Courtesy of Superior Tank Co., Inc.

When purchasing a bolted steel tank or a welded steel water storage tank, one of the key criteria determining the design parameters of the tank is the Seismic Use Group (SUG) classification. The term Seismic Use Group is essentially the same as the terms Occupancy Category and Risk Category used by the International Building Code (IBC) and the American Society of Civil Engineers (ASCE). The Seismic Use Group classification is used to determine the magnitude of environmental loading applied to the bolted or welded tank structure. Despite the name, the Seismic Use Group classification is applicable to other environmental loads such as wind, snow, ice, and floods.

The Seismic Use Group along with the seismic accelerations at the site help determine the Seismic Design Category of a structure. Seismic Design Category is a letter rating from A to F with F being the worst case. There are special detailing requirements depending on what Seismic Design Category a structure is in. For example, with steel water storage tanks, the seismic design category impacts how the anchor bolts are designed.

The classification assigned to a bolted or welded water storage tank is based on the intended use and expected performance during and after a seismic event. Guidelines for the Seismic Use Group for welded and bolted steel water storage tanks are found in the American Water Works Association (AWWA) D103 Standard for Factory-Coated Bolted Carbon Steel Tanks for Water Storage and the AWWA D100 Standard for Welded Steel Tanks for Water Storage. The Seismic Use Group classification of a tank is based on the risk to human life, health, and welfare that could result if the welded or bolted steel tank is damaged or fails under environmental loading. SUGs consist of three categories denoted by the Roman Numerals I, II, and III.

Seismic Use Group III for is steel storage tanks having the greatest impact on life, health, and safety of the public. SUG III tanks service essential facilities as defined in the ASCE 7 as “buildings and other structures that must remain operational in the event of extreme environmental loading from wind, snow, or earthquakes”. Some examples of Seismic Use Group III applications are water storage tanks servicing hospitals, police stations, power generation facilities, emergency shelters, and water production facilities required to maintain water pressure for fire suppression. Bolted and welded tanks storing extremely toxic substances can also fall under SUG III.

Seismic Use Group II covers welded and bolted tanks that can have a significant economic impact or major disruption of day to day life. Examples of Seismic Use Group II storage tanks are ones servicing jails, power generation stations, primary water treatment facilities, wastewater facilities, and telecommunication facilities.

When neither SUG III or II are applicable, Seismic Use Group I can be used for bolted tanks and welded tanks. For water storage tanks serving several facilities, the facility having the highest Seismic Use Group shall be used to determine the seismic requirement. In addition, unless there is adequate information regarding the intended application of the bolted steel tank or welded tank to classify it in a lower Seismic Use Group, AWWA requires the storage tank to be assigned to SUG III. An engineer knowledgeable in Seismic Use Group classification should be able to provide the proper SUG classification for welded or bolted steel tanks.

As mentioned above, the SUG is roughly equivalent to the Risk Categories or Occupancy Categories defined in the International Building Code. Currently, the SUG numbers from the AWWA standards do not directly match the Risk Category numbers. For example, Risk Category IV equals Seismic Use Group III and Risk Category III is equivalent to Seismic Use Group II. However both Risk Categories I & II are combined into Seismic Use Group I.

The SUG selected for a bolted steel tank or a welded tank will have an impact in several aspects of the design. The SUG will determine how much of each environmental load is applied to the storage tank. Importance factors are applied to the design loading and a different value is assigned to each Seismic Use Group. For example, Seismic Use Group III has a seismic importance factor of 1.5 for seismic loads meaning that the calculated seismic loads are increased by 50%. The importance factor varies for each environmental load. In addition, the SUG selected for a bolted or welded tank will determine how much freeboard needs to be provided in the storage tank. Freeboard is the distance from the maximum operating capacity to the underside of the roof structure in the steel storage tank. The purpose of freeboard is to prevent the sloshing wave that develops in a seismic event from damaging the storage tank’s roof and roof support structure. Bolted and welded tanks with Seismic Use Group III classification in regions with high seismic acceleration can have significant freeboard requirement which can exceed 10’ in some cases. As design requirements become more stringent, the bolted or welded tank will increase in cost. Therefore, it is important to know which classification your new tank falls into.

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