Active Design in Active Seismic Zones

02/01/2006


We are all familiar with scenes of catastrophic earthquake damage from reports in the newspaper and on the evening news. The release of energy during seismic events is enormous and can incur significant loss of human life. The physical damage resulting from these events can be staggering to buildings, utility and transportation infrastructure systems. It often requires years of repair and reconstruction to return these systems to their pre-earthquake condition.

In an attempt to minimize the potential damaging effects from seismic events, design standards have been created and incorporated into building codes in seismically active areas. California and Japan in particular have benefited greatly from input from the building design and construction industry on this matter. Chapter 16 of the Uniform Building Code (UBC) prescribes structural design requirements based on the site seismic zone assignment (UBC, Chap. 16, Figure 16-2) and the structure zone factor, Z (UBC, Chap. 16, Table 16-I). These classifications are used to demonstrate compliance of the structural connections and attachments of other elements, i.e., equipment supports, to the primary structural system.

As far as M/E/P systems, the Sheet Metal and Air Conditioning Contractors National Organization, Inc. (SMACNA) and the National Uniform Seismic Installation Guidelines (NUSIG) have developed a set of design and installation guidelines utilizing the same design criteria found in the UBC. Compliance is voluntary, but many local and state jurisdictions have adopted them and accept their application as compliance with the seismic sections of their respective building codes. Additionally, the National Fire Protection Assn. (NFPA) has codified the seismic restraint requirements for fire sprinklers and equipment since 1939. Furthermore, insurance underwriters such as Factory Mutual impose seismic criteria more stringent than many building codes to further reduce their risk from seismic events. Compliance with the more stringent insurance criteria is mandatory for many projects in order to secure financing from lenders.

So what does this mean for M/E/P systems? The design and construction of buildings meeting the code-minimum requirements for seismic design in active seismic areas (Zones 3 and 4) typically trigger several considerations related to the support of most of the building system types. For starters, lighting fixtures in lay-in tile ceiling systems must be independently supported directly from the building structure and must not rely on support from the ceiling grid system. Both the lighting and ceiling grid system must be sway-braced from at least two points of connection.

HVAC systems and ductwork is another affected area. Rectangular ducts with cross-sectional areas of 6 sq. ft. and larger, and round ducts with diameters of 28 in. and larger, must be braced. Unbraced ductwork must be installed with at least 6 in. of clearance to ceiling-hanger wires. Spring-isolated equipment—i.e., fans, pumps, etc.—is required to be equipped with seismic housings or "snubbers" to resist the lateral load forces from an earthquake. Anchor bolts supporting equipment must be properly sized to resist pull out from the vertical forces from an earthquake. Most design professionals, in their specifications, require manufacturers of vibration-spring isolators to include stamped calculations as part of their submittal to demonstrate that these loads have been taken into account for the specific piece of equipment.

Of course, seismic considerations in relation to plumbing are paramount. The following systems must be braced: for fuel oil, gas, medical gas and compressed air, all piping 1 in. nominal diameter and larger; all piping 1-1/4 in. nominal diameter and larger in boiler rooms, mechanical equipment rooms and refrigeration equipment rooms; and all piping 2-1/2 in. nominal diameter and larger. Conduit must be braced the same as the equivalent weight pipe. An exception is that bracing is not required for piping and ductwork systems suspended by hangers 12 in. or less in length as measured from the top of the pipe or duct to the bottom of the support where the hanger is attached.

Gas piping is also a crucial area for seismic design considerations. Many owners opt to install seismic gas valves that automatically close in the event of an earthquake to stop the flow of natural gas to the structure. This would significantly reduce the possibility of fire.

In addition to the requirements above, flexible connections must be provided for all piping and electrical system elements where they cross a seismic joint in large structures. The structural engineer must be consulted to determine the expected differential movement between the two structures across the joint. This differential movement can be as much as 18 in. in all directions.

Most design professionals who practice in Seismic Zones 3 and 4 also include provisions in their designs for the expected differential movement between the building and the ground during a seismic event by providing utility entrance vaults for cold water and sprinkler water systems. The vaults contain piping arrangements with multiple elbow swings and/or flexible piping sections to account for this movement and maintain these services to the building after an earthquake. This is particularly important when structures are built in areas with suspect or substandard geotechnical characteristics. Examples include buildings built in landfill areas that are subject to liquefaction during an earthquake. The building structure itself will likely be supported on piles on bedrock up to 150 ft. below ground, but the adjacent grade may slump along with the utilities buried within it.

These issues represent an abbreviated overview of the seismic design features that will result in a building design that provides a minimum level of occupant safety. The building and systems may not remain operational after a moderate to severe earthquake but will not collapse or result in falling hazards or obstructions prohibiting the safe evacuation of building occupants.





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