Revisiting Earthquake Lessons - Nonstructural Components
Friday, July 19, 2019
Author: Brian Kehoe
Structure Type: Architectural and Mechanical Nonstructural Components
Earthquakes: 1987 Whittier Narrows, 1989 Loma Prieta, 1994 Northridge, 1992 Landers, 2014 South Napa
Lessons: Loss of life due to earthquake damage to nonstructural components is rare in the United States, but given the extent of damage to nonstructural components in past earthquakes, particularly to architectural components, there is a substantial onus to adequately design and install seismic bracing for nonstructural components to prevent future injuries due to failures of nonstructural components. Architectural nonstructural components represent the most substantial life safety hazard of nonstructural components to the public, particularly those on the exterior of the building, those along paths of egress, and those suspended above areas of public assembly. Other safety issues include the potential release of hazardous materials and ignition of flammable materials, such as natural gas. Mitigation of nonstructural seismic hazards requires adequate design of the bracing and connections of the components to the structure. The bracing must not only resist the seismic forces but also allow for building drifts. Just as important as the design is the need to adequately install the seismic bracing. This often requires verification by a qualified person that the bracing has been satisfactorily installed. Significant damage has occurred to fire sprinkler systems during past earthquakes. While these have not directly affected life safety, the resulting water damage has caused extensive property loss plus the loss of building functionality following the earthquake. This is particularly important for hospitals and other essential facilities.
Historic Background. The first seismic design provisions for nonstructural components was introduced in the 1959 SEAOC Blue Book, which was later incorporated into the 1960 edition of the Uniform Building Code (UBC). These provisions required a seismic force, Fp=CpWp, be applied to “parts and portions” of buildings other than structural components. The requirements for design of nonstructural components was largely unchanged through the 1985 UBC; although some additions were made regarding the types of components specifically addressed by the building code. In the 1988 UBC, significant changes were made to the design requirements, including the distinction between rigid and flexible or flexibly-mounted nonstructural components, the introduction of an importance factor for components containing hazardous materials or life safety systems, and a force reduction for components attached at or below ground level.
Concurrent with the building code changes for nonstructural components, the Federal Emergency Management Agency (FEMA) developed practical recommendations for mitigating nonstructural hazards. FEMA 74 Reducing the Risks of Nonstructural Earthquake Damage: A Practical Guide was first published in 1985. This was the first introduction to prescriptive and “do it yourself” techniques for bracing nonstructural components.
A major change to the seismic design of nonstructural components was introduced with the 1997 UBC. This revision provided a more comprehensive formula and tables for calculating nonstructural component design forces. These tables recognized and penalized bad performers such as equipment supported on isolation mounts and equipment with anchorage that include non-ductile materials. Based mainly on results of instrumented buildings in the 1994 Northridge Earthquake, the formula provided a variable seismic force along the height of the building. This was achieved by introduction of the term (1 + 3x/h) where x is the height of the component in the building and h is the total building height. This term was modified to (1+2x/h) in the 2000 International Building Code (IBC). Another significant code change that occurred with the 2006 IBC was the requirement for Special Certifications for Designated Seismic Systems. This requires shake table testing of components in essential facilities to verify that the components will remain operable.
The design philosophy for nonstructural components, as stated in the SEAOC Blue Book, has been to resist minor levels of earthquake shaking with no damage, allow some nonstructural component damage for moderate earthquake shaking, and to resist major earthquake ground motions with some nonstructural damage. Because the intent of the building code provisions is to safeguard against loss of life, the design requirements for nonstructural components has focused on those nonstructural components that are most likely to pose a threat to life safety.
Past earthquake experience has shown that there are some nonstructural components that either have resulted in actual or potential life safety hazards. Selected examples from several earthquakes are discussed below. Some of these observations have resulted in recent or proposed changes to design requirements for nonstructural components. Also observed in past earthquakes are instances where the required bracing or anchorage of nonstructural components was not installed or not properly installed. This has led to an increased emphasis on inspection and structural observation of nonstructural component installation, particularly for essential facilities.
The 1987 Whitter Narrows Earthquake. This earthquake resulted in a fatality due to the failure of a nonstructural component when an architectural precast concrete façade element fell from a parking structure at the California State University Los Angeles Campus. The connections of the precast facade element to the structural framing failed, causing the precast panel to dislodge and fall. The failure was the result of lack of sufficient anchorage embedment and excessive corrosion.
The 1989 Loma Prieta Earthquake. Widespread failure of exterior brick masonry cladding was experienced throughout the San Francisco Bay Area. Most of this damage occurred in older structures that had not been designed under building codes that included requirements for seismic design of nonstructural components. Failure of inadequately anchored brick masonry parapets was also common, causing fatalities of pedestrians adjacent to a building in San Francisco. Another potentially hazardous failure occurred to a heavy ceiling in a theatre in San Francisco, which collapsed onto a section of the seating.
The 1994 Northridge Earthquake. A significant number of residential buildings with unreinforced masonry chimneys throughout the affected region experienced failure of the chimneys. This experience lead the City of Los Angeles to prohibit the replacement of the damaged chimneys with masonry construction. The replacement chimneys were required to be constructed with light gage steel framing around the flue. Other types of damage that were observed were the toppling of masonry fences and damage of fire sprinkler piping at a modern that forced evacuation of the hospital.
The 2014 South Napa Earthquake. Several buildings in the area affected by the South Napa Earthquake sustained significant nonstructural damage. Damage to exterior façade elements, including glazing, curtain walls, and storefronts was widespread. Damage to the glazing at the Napa County Air Traffic Control tower resulted in the tower being temporarily out of service. A section of precast concrete cladding on the penthouse of a telephone operations facility dislodged and fell, severing the building’s utility connection. Failures were observed of both anchored and adhered veneer. Widespread damage was observed to piping systems, including several buildings with damage to fire sprinkler piping due to inadequate clearance between the sprinkler heads and adjacent equipment that resulted in flooding. The only reported fatality during the earthquake was caused by an unanchored television that fell off a shelf onto the head of a victim. Another significant source of damage was toppling of stacked wine barrels. Not only did this cause an economic loss and a potential for a life safety hazard to occupants, but the toppling of the barrels also resulted in structural damage as the barrels impacted the adjacent structural framing.
Current Status. The desire for more resilient structures has resulted in an increase in the attention to the importance of the seismic performance nonstructural components. This recognition has led to development of new protocols for shake-table testing of nonstructural components, resulting in a significant increase in such testing. A large research project involving full-scale buildings outfitted with actual nonstructural components at the University of California, San Diego, as well as similar testing in the U.S. and Japan has led to a significant increase in the understanding of performance states for many types of nonstructural components. More testing of nonstructural components will continue in the future to supplement our understanding of their seismic behavior. In addition, changes in the building code requirements for designing and anchoring nonstructural components are in process based largely on recent studies funded by the National Institute of Standards and Technology. These changes are intended to better incorporate the impact of the building period on the performance of nonstructural elements.
FEMA 2015, Performance of Buildings and Nonstructural Components in the 2014 South Napa Earthquake (FEMA P-1024), Federal Emergency Management Agency, 2015.
FEMA 2015, Reducing the Risks of Nonstructural Earthquake Damage-A Practical Guide (FEMA E-74), https://www.fema.gov/fema-e-74-reducing-risks-nonstructural-earthquake-damage, Federal Emergency Management Agency, 2015.
ICBO 1961, Uniform Building Code, International Conference of Building Officials, Whittier, California, 1961.
ICBO 1985, Uniform Building Code, International Conference of Building Officials, Whitter, California, 1985.
ICBO 1988, Uniform Building Code, International Conference of Building Officials, Whitter, California, 1988.
ICBO 1997, Uniform Building Code, International Conference of Building Officials, Whitter, California, 1997.
NIST 2018, Recommendations for Improved Seismic Performance of Nonstructural Components, NIST GCT 18-917-43, Applied Technology Council, 2018.
NIST 2018, Seismic Analysis, Design, and Installation of Nonstructural Components and Systems - Background and Recommendations for Future Work, NIST GCT 17-917-44, Applied Technology Council, 2017.
SEAOC 1997, Recommended Lateral Force Requirements and Commentary, Structural Engineers Association of California, Sacramento, California, 1996.
Taly, N. 1988, The Whittier Narrows, California Earthquake of October 1, 1987 - Performance of Buildings at California State University, Los Angeles, Earthquake Spectra: May 1988, Vol. 4, No. 2, pp. 277-317