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Chapter 2: Utilities

 

Mitigation Measures for Utility Equipment and Systems

 

A. Poor Equipment Anchorage/Stability

Problem: Utility equipment such as electrical transformers and generators are often not properly anchored or stabilized to carry earthquake forces and movements. As a result, such equipment can tip over or fall during an earthquake, damaging the equipment and creating additional hazards.

Mitigation Objective: Anchor or stabilize utility equipment to withstand earthquake forces and movements.


A.1 Anchor Electrical Transformers

Ground movement during an earthquake can cause inadequately anchored, pole-mounted transformers to fall, and slab mounted transformers to slide or topple. Damage caused by the movement of these transformers can be mitigated relatively inexpensively by properly anchoring the transformers to utility poles and the equipment to foundation slabs. Connections to the transformers should be flexible enough to help isolate the stresses from other sources. Unanchored electrical and instrumentation cabinets and motor control centers should also be anchored to prevent sliding or toppling.

Effectiveness:

  • Very effective.

    


A.2 Combine Equipment on One Foundation

Differential movement during an earthquake can result in the misalignment of mechanical equipment. Horizontal pumps, compressors, and other mechanical equipment that have connected motors, engines, and similar components should be mounted on a single foundation.

Effectiveness:

  • Very effective.
  • Secure equipment to foundation for maximum effectiveness.



B. Soil Movement/Settlement

Problem: Underground utility transmission lines and connections are often not strong enough to withstand soil movements or differential settlement triggered by earthquakes. Utility pipelines and connections located above ground may not be properly braced against earthquake forces and movements. As a result, transmission lines and their connections can crack, leak, or fail, even damaging other facilities.

Mitigation Objective: Reinforce, restrain, or improve utility transmission lines and connections to withstand earthquake forces, soil movements and differential settlements.


B.1 Install Expansion Joints

During earthquakes, ground motion can cause transmission lines to leak, crack or break. Expansion joints can be added to allow some movement. There are a variety of expansion joint configurations and materials available, from flexible, single-layer joints to complex, multi-layer composite constructions. Expansion joints are installed as flexible connections at various points along duct and pipe systems.

Effectiveness:

  • Somewhat to very effective.

Limitations:

  • Qualified professionals should design and install expansion joints.

Considerations:


B.2 Reinforce Well Shaft or Install Submersible Pump

Groundwater well casings, long shafts for pumps, and other well equipment can be damaged from ground movement. Two measures to reduce damage are: 1) Install a heavy well casing to protect the shaft and the pump from ground movement; and 2) Install a submersible pump as it does not incorporate a long shaft and so is not vulnerable to shaft damage. In areas with a risk for liquefaction, install a larger outside casing down to the liquefiable soil layer to protect equipment.

Effectiveness:

  • Somewhat to very effective, depending on displacement.
  • Allow space for movement between the casing and equipment slab to increase effectiveness.
  • Install a flexible connection between the casing and discharge piping to increase effectiveness (See p. 57).

Limitations:

  • Pumps constructed of steel are less prone to damage than cast iron pumps.
  • Consider using well screens rather than slotted casing to reduce damage to equipment from pumping sand.

    


B.3 Restrain Pipes

During an earthquake, soil movement along underground and above-ground pipelines can cause pipes to pull apart or fracture. Restraining rods installed loosely with rubber gaskets on existing pipes at bell and spigot joints can allow for extension, compression, and joint rotation. Replace pipe with flexible joint pipe, or ball and socket pipe with restraining rings. These pipes provide varying degrees of joint freedom while restraining the joints from separating.

Effectiveness:

  • Very effective.

Limitations:

  • May not be cost effective for undamaged underground lines.


B.4 Improve Pipe Materials

The performance of underground piping in earthquakes is largely dependent upon the construction material for the pipe. Pipes made of brittle materials, such as cast iron, are particularly vulnerable to breakage during an earthquake. Replacement of pipe made of brittle materials with pipe made of more flexible, ductile materials like steel, ductile iron, copper, and some plastics can mitigate pipe damage from an earthquake.

Effectiveness:

  • Somewhat to very effective, depending on ground displacement.
  • Strengthening ductile pipe by increasing the pipe wall thickness can also improve the viability of the pipeline.
  • Corrosion protection measures for buried pipelines in corrosive soils can maintain the pipe strength.

Limitations:

  • May not be cost effective for undamaged underground lines.


C. Damage to Tank Structures

Problem: Vertical tank structures or standpipes may be improperly anchored to their foundation. Tank wall sections may not be adequate to handle ground movements and the dynamic forces generated by water sloshing inside the tank. Horizontal tank structures may not be sufficiently anchored to their foundations to withstand earthquake forces, and elevated tank structures may not be adequately braced against lateral earthquake forces and movements. As a result, tank structures can move, leak, or collapse during an earthquake, destroying the tanks and creating additional hazards.

Mitigation Objective: Anchor or improve tank structures to withstand earthquake forces and movements.


C.1 Anchor Tank Structures at Base

Anchors installed at a tank base can reduce damage from earth movement.

For vertical tanks, anchor systems can be: 1) Composed of metal straps welded to tank and embedded in a concrete footing; and 2) Consist of vertical anchor bolts connected with chair anchors into the foundation.

Large horizontal tanks, both above and below ground, should also be securely supported and anchored. Horizontal tanks should have saddles or other supports to provide longitudinal support.

Effectiveness:

  • Very effective in reducing tank damage.
  • Install flexible connections to reduce pipe related damage.

Limitations:

  • Foundations need to be sufficient to support anchoring.

Considerations:

 


C.2 Install Friction Dampers on Elevated Tanks

Large, elevated tank structures are commonly damaged by ground movement. Friction dampers can be installed to help absorb tank movements and increase seismic resistance. The dampers are designed to slip at a predetermined load to reduce forces imposed on the tank and can be integrated into the cross bracing supporting the tank.

Effectiveness:

  • Very effective in reducing tank damage.
  • Install flexible connections to reduce pipe related damage.

Effectiveness depends on analysis of existing tank behavior and location of dampers.


C.3 Stiffen Vertical Tank Walls

Thin tank wall sections can buckle due to seismic forces and the dynamic forces of water sloshing inside the tank. To reduce the risk of future earthquake damage, damaged tank wall sections can be stiffened during repairs with steel beams that are welded to the inside of the tank.

Effectiveness:

  • Somewhat effective.

Limitations:

  • Repair of tank wall requires emptying, disinfecting, and relining the tank.

    

 

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


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