The Tarheel Pipeline, Fall 2015

Fall 2015

feature

Water Works Association (AWWA), in accordance with its Seismic

Use Group (SUG) and site class. The SUG is a classification assigned

to a tank based on its intended use and expected performance;

Tanks that serve multiple facilities use the highest SUG. Site class

accounts for the effect of local soil conditions on the ground motion

and are based on the soil present and their engineering properties as

established by a geotechnical investigation. The SUG and site class

help determine the appropriate freeboard and the number of anchor

bolts needed. Freeboard is the distance from the Maximum Operating

Level (MOL) to the lowest level of the roof framing and is determined

by the sloshing wave height that could occur

. Freeboard is taken into

consideration to prevent a tank from overturning or causing roof

damage due to sloshing.

The design of the piping system connected to the tank should consider

the effects of foundation movements and potential movement of the

connection points during earthquakes. Sufficient flexibility should be

provided to avoid release of the tank contents due to failure of the

piping system. The piping system and supports shall be designed so

as not to impart significant mechanical loading on the attachments

of the tank. Mechanical devices that add flexibility, such as bellows,

expansion joints, and other flexible apparatus, may be used when

designed for the seismic displacements and defined operating pressure

WHAT TO LOOK FOR

Water tanks should be inspected regularly for proper working order

and stability before severe weather hits. Overhead obstructions, trees

and overgrowth that could puncture or damage a tank during severe

weather should be removed, and operators should routinely look for

foundation, wind, and earthquake damage. Such damage on tower-

supported tanks may be indicated by cracked coating or welds at the

tower connections; broken, bent, or sagging rods; buckled struts;

dented or twisted columns; or missing or loose rod pins. If any of these

conditions are observed, the tank should be professionally inspected.

In addition, tanks in areas at high risk for wind or earthquake damage

should be inspected more frequently than tanks in low risk areas

References

1. National Fire Protection Association, NFPA-22 Standard for Water

Tanks for Private Fire Protection. 2013

2. National Fire Protection Association, NFPA-780 Standard for the

Installation of Lightning Protection Systems. 2014.

3-4. American Water Works Association, D100-11 Welded Carbon

Steel Tanks for Water Storage. 2011.

5. American Water Works Association, M42 Steel Water Storage

Tanks. 2013.