Protecting nonstructural systems in earthquakes
The performance of nonstructural systems—like mechanical, electrical, and plumbing components; architectural components; and contents—is generally not given sufficient consideration, except in specific circumstances.
By Andrew C. T. Thompson, Arup, San Francisco
The design and construction communities have made great strides in protecting building occupants in earthquakes. Consideration is now shifting to usability and reparability—structural design for which (so-called performance-based design) is generally accepted in practice. However, the performance of nonstructural systems (mechanical, electrical, and plumbing components; architectural components; and contents) is generally not given sufficient consideration, except in specific circumstances such as hospitals. As was demonstrated in the 2010 Chile and Baja California earthquakes, nonstructural earthquake damage can significantly affect the continued operation of a building.
Although codes, standards, and guidelines do exist, the state of practice is inconsistent. For standard construction, the status quo is ineffective. For hospitals and other emergency centers, the design, review, and inspection requirements are effective but expensive, and hence seldom used for standard construction. The middle ground is often reserved for specific cases where business interruption (discontinued operation) concerns drive a more rigorous approach.
Codes, standards, and guidelines
The 2006 International Building Code (IBC)’s adopted reference ASCE 7-05 (ASCE 7-10 may now be adopted by some jurisdictions) contains design criteria for both force- and displacement-controlled nonstructural component anchorage. For the lateral force calculation, a component importance factor, Ip, is used. This is different from the building importance factor of previous editions of the code. Ip = 1.0 for typical components. Ip = 1.5 if a component (a) is needed for life-safety, (b) is needed for continued operation of an Occupancy Category IV facility (e.g., essential facilities), or (c) contains hazardous materials. Displacement demand is determined based on relative displacements calculated from elastic analysis. Several items are specifically exempt from these requirements, including most nonstructural components in normal occupancies in areas of moderate seismicity.
ASCE 7-05 also contains provisions related to post-earthquake functionality of the equipment itself. In such cases, certification based on shake table testing or earthquake-experience data are submitted to the jurisdiction authority. Shake-table testing is based on the International Code Council Evaluation Service’s ICC-ES AC 156, Acceptance Criteria for Seismic Qualification by Shake-Table Testing of Nonstructural Components and Systems.
The 2006 IBC contains further requirements for site inspection of some nonstructural systems in regions of moderate to high seismicity.
The 2006 IBC accepts other code criteria. For example, it allows seismic restraint of fire protection systems to be designed and verified in accordance with NFPA 13, Standard for the Installation of Sprinkler Systems.
In addition to these codes and standards, there are a number of guidance documents relevant to improving nonstructural earthquake design and construction practice, including the Federal Emergency Management Agency (FEMA)’s FEMA 74, Reducing the Risks of Nonstructural Earthquake Damage: A Practical Guide, Fourth Edition, 2010.
A list of codes, standards, and guidance documents related to nonstructural hardening can be found in the Applied Technology Council’s ATC-69, Reducing the Risk of Nonstructural Earthquake Damage: State of the Art and Practice Report.
Despite these code provisions, for standard construction, the extent to which nonstructural earthquake protection measures are realized in the field varies greatly from project to project. Similar projects in the same city, built to the same building code, may have significantly different levels of earthquake protection for nonstructural systems. This is mainly due to differences in construction inspection practices, particularly of anchorage. It is rare that nonstructural systems are subjected to comprehensive construction inspection by regulatory bodies or by design professionals. (A notable exception is fire-sprinkler piping, which is typically handled by the fire marshal.)
This is not the case for hospitals. The California Office of Statewide Health Planning and Development (OSHPD) requires a rigorous design, review, and construction inspection process. This is expensive and can add significantly to the cost of the hospital (although this cost is quickly repaid when lifecycle costs are considered). Existing hospitals must be upgraded based on Senate Bill 1953 (1994). By 2002, hospitals were required to anchor and brace communications, emergency power, and fire alarm components. By 2013, nonstructural components in critical-care areas must be anchored; by 2030 most nonstructural components in the hospital must be braced.
The middle ground between standard- and hospital-construction is often dominated by specific situations in which an organization relies on nonstructural components for business-critical operations (e.g., manufacturing facilities, laboratories, museums, etc.). In these cases, a tailored risk-based cost-benefit approach is used: The nonstructural systems are designed and constructed with specific post-earthquake critical operations in mind.
Necessary technological developments are ongoing for protecting against nonstructural earthquake damage, with significant research being undertaken internationally in the public and private sectors. In the United States, this research is mostly conducted at the national earthquake engineering research centers (including PEER and MCEER), and is often funded by the FEMA, the National Science Foundation (NSF), and the ATC.
The greatest improvement for standard occupancy buildings, however, will come from process changes, not technological changes. Of all the team members (owners, architects, engineers, contractors, equipment suppliers, plan reviewers, and inspectors), no one entity is charged with the responsibility or budget for tracking the design and construction of the anchorage for nonstructural systems. Any modest costs associated with one of the team members owning, and being paid for, this scope item would be well compensated by significantly improved performance. These additional costs could be shown to be recovered much sooner if lifecycle and insurance costs are considered.
Thompson leads the risk consulting practice for Arup Americas. He is coauthor of the book Peace of Mind in Earthquake Country and is a recipient of Consulting-Specifying Engineer’s 2010 40 Under 40 award. Thompson regularly travels to earthquake-affected regions to assist clients, most recently to Chile, Haiti, and Southern California.
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