Seismic design tips for MEP engineers
Despite the incredible force of the March 11 earthquake in Japan, which had a magnitude of 8.9 and was followed by hundreds of aftershocks, most of Japan’s buildings and infrastructure had little structural damage. Most of the structural damage was a result of the tsunami and not the earthquake.
However, there was extensive damage to nonstructural components and systems, including building mechanical, electrical, and plumbing (MEP) systems. This is partly due to Japanese building codes, which set tougher standards for seismic design of structures than the United States, but generally do not regulate the seismic design of nonstructural systems and components. Fortunately for U.S. building owners, our codes do require seismic design of nonstructural systems.
Here are the top four things that MEP engineers should be aware of related to seismic design of mechanical and electrical systems:
Seismic design categories
Older U.S. building codes designated seismic design requirements by means of seismic zones. The codes specified five geographically based seismic zones, ranging from zone 0 (much of Texas and Florida), where earthquakes were not expected to occur and seismic design was not required, to zone 4 (coastal California and Alaska), where large earthquakes frequently occur and the most stringent seismic design and construction requirements applied.
The International Building Code (IBC), which forms the basis for most building codes enforced by state and local government today, no longer uses seismic zones. Instead, today’s building codes use the concept of a seismic design category. The six seismic design categories (A to F) are determined by geographic location, site soil type, and occupancy.
Seismic design category A encompasses buildings of ordinary occupancy that are located on sites with stiff soils and have little risk of experiencing earthquakes. Category F includes structures required to remain functional following a strong earthquake, such as hospitals and emergency communications centers, which are located very close to major active faults like the San Andreas fault in California or Wasatch fault in Salt Lake City. Like the old zone 4, category F requires extensive seismic-resistant design and construction requirements of both building structures and nonstructural systems.
Figure 1: Seismic Design Categories for Ordinary Occupancy Buildings on Firm Alluvial Soils (Courtesy: U.S. Geological Survey)
Some seismic-resistant design is required of all nonstructural systems in seismic design categories C, D, E, and F. Figure 1 is a map of the contiguous United States showing the locations in which the various seismic design categories apply to ordinary occupancy buildings on sites with typical soil conditions. For such sites and buildings, categories D, E, and F encompass areas shaded in yellow, orange, or red in this figure and are concentrated in a few states. For buildings with higher occupancy categories (e.g. schools, assembly buildings, hospitals, police stations, etc.) or buildings on soft soil sites, as are commonly present along the shores of rivers and estuaries, categories D, E, and F cover a larger portion of the map than shown in Figure 1.
Electrical and mechanical equipment
Most electrical and mechanical equipment for structures in seismic design categories C, D, E, or F must be anchored and braced to withstand seismic forces specified in Chapter 13 of the ASCE 7 standard “Minimum Design Loads for Buildings and Other Structures.” The seismic design force depends on the design spectral response acceleration, SDS, which is found by referring to maps contained within the standard and by applying coefficients related to site soil type, installation height within the building, building occupancy and the importance of the particular item to building function, and the specific characteristics of the equipment, as represented by the coefficients a and Rp.
The intent of these building code requirements is to assure that these components will not topple over or fall, potentially injure people during an earthquake, or block building egress after the earthquake. Generally, these requirements are not intended to assure that the equipment actually operate after a design earthquake.
The building code limits the types of post-installed anchor bolts (expansion bolts, epoxy anchors) that can be used to provide code-required seismic anchorage of equipment. Such anchors must be qualified by the manufacturer for use, using cyclic loading protocols that duplicate the stresses an anchor would experience in an earthquake.
The International Code Council (ICC) Evaluation Service published code evaluation reports for anchors that have been seismically qualified. If equipment is installed using unqualified anchors, building officials can require removal and replacement with appropriately qualified installations.
Designated seismic systems
The IBC identifies a limited class of MEP systems as “designated seismic systems.” These designated seismic systems comprise that equipment and their supporting utilities that are required to remain functional following a design earthquake in order to protect the safety of building occupants. This includes emergency power and lighting systems, fire suppression systems, and smoke exhaust systems associated with emergency egress paths. In addition to requirements for attachment of these systems to the primary structure, the building code requires demonstration, by the equipment supplier, that the equipment will actually operate after experiencing design shaking.
The code recognizes three means of demonstrating that equipment is capable of post-earthquake operation: shake-table testing, which involves mounting a prototype on a table in a laboratory that shakes the equipment as if it were in an earthquake, followed by demonstration that equipment will operate; experience data, which consists of documented evidence that similar equipment experienced real earthquake shaking of at least the intensity of the design earthquake without failure; and suitable analytical evaluation. Most equipment is qualified by shake-table testing. Many manufacturers of pumps, compressors, motor control centers, switchgear, and other equipment have specially developed models of equipment that meet these requirements.
Click here for a downloadable version of FEMA P-749 “Earthquake Resistant-Design Concepts, An Introduction to the NEHRP Recommended Seismic Provisions for New Buildings and Other Structures.” This guide of the building code requirements is written for people who are not structural engineers.
Click here for a Web-based application that calculates design ground motion at any location in the U.S.
Ronald O. Hamburger, SE, is a senior principal in the San Francisco office of engineering firm Simpson Gumpertz & Heger Inc.