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Chapter 2:

Utilities

A:  Introduction

 

Utilities include water, wastewater, fuel, electricity, gas, and telecommunications systems. The basic components of utilities include supply and storage equipment, transmission lines, and the connections between these components. Utility components may be located above ground or underground, and rely on poles, grade level foundations, or soils for support.

Utilities commonly suffer earthquake damage for two reasons:

  1. Above ground utility equipment, tanks, pipelines, and connections are often inadequately braced or inadequately secured to their foundation structures. Like buildings and other facilities, utilities tend to be designed for vertical gravity loads. As a result, the equipment anchorage and pipeline bracing may not be strong enough to carry the large lateral forces associated with earthquakes.

  2. Underground utility pipelines and connections are often too weak or inflexible to withstand earthquake ground movements and differential settlements, causing them to crack or fail. Materials that are too flexible, however, also cannot handle additional displacements from earthquake forces.

Types of Damages to Utilities. Typical types of utility damage are described below:

  • Supply equipment. Supply equipment such as electrical transformers, pumps, or generators are typically located on grade level foundations or elevated support structures. When this equipment is not supported or anchored properly it may topple or fall from its supports during an earthquake. Supply equipment mounted on separate foundations can also be damaged by differential settlements or movements between the foundations. Porcelain components of electrical transformers are brittle and can break during an earthquake.

  • Utility transmission lines. Utility transmission lines include pipes with joints for water, wastewater, fuel, gas, and electrical conduits that run underground or above grade level. Damage to above ground transmission lines typically occurs along unsupported line sections when lines crack, leak, or fail. Damage to underground transmission lines usually occurs in areas of soil failure where the line sections cannot withstand soil movements or differential settlements.

  • Connections. Damage to connections between utility pipeline sections and/or between utility transmission lines and equipment occur where the connections can not withstand soil movements or differential settlements.

  • Tank structures. Tank structures may be oriented vertically, horizontally, at grade, or elevated. Tall vertical tank structures or standpipes are often damaged by a combination of the structure's reactions to ground shaking and dynamic forces generated by water sloshing inside the tank. Tank foundation supports fail and denting of thin tank wall sections often result. The most serious type of vertical tank damage occurs when the tank walls crush near the base, triggering tank leakage or collapse. Horizontal tanks are often damaged when tanks are not securely anchored to the foundations. Elevated tank structures may be damaged due to buckling of the cross braces between the tank legs.

In addition to the types of damage listed above, damage to utilities can trigger secondary damages that affect the community at large. Leaking or broken utilities can cause water damage, fire or explosion. Since these systems are interconnected, a loss of one utility system (such as electrical power) can often lead to a loss of other systems.

A structural engineer may need to be consulted to identify whether certain mitigation measures are appropriate. Some measures included in this section are not appropriate for all utilities. Be aware that choosing the wrong measure may cause more problems than not doing any retrofit at all.

 

Continue to UTILITY MITIGATION MEASURES

 
 

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Randolph Langenbach

M-Arch (Harvard), Dipl.Conservation (York, England)


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