The evolution of building code seismic requirements over the last 20 years.
Over the past 20 years, the seismic requirements in building codes adopted in the United States have changed significantly. These changes have been prompted primarily by observations of the performance of structural and nonstructural systems in major earthquake events, and have included additional seismic requirements for sprinkler systems that were beyond those specified in NFPA 13. The evolution of the building code seismic requirements over the past 20 years, and specifically the additional requirements for sprinkler systems beyond those found in NFPA 13, are described in this article along with the evolution of NFPA 13 to incorporate these requirements.
The current building code seismic requirements for nonstructural components (including sprinkler systems) are provided in Chapter 13 of ASCE 7-05,1 which are referenced by the International Building Code2 (IBC). The requirements for nonstructural components are primarily based on Chapter 6 of the 2003 NEHRP Recommend Provisions.3 Much of the requirements found in Chapter 6 of the 2003 Recommended Provisions were based on earlier developmental work primarily done between 1991 and 1997 by Technical Subcommittee (TS-8) of the NEHRP Provisions Update Committee (PUC). During this time frame, TS-8 was greatly influenced by the observations on the performance of nonstructural components during the 1989 Loma Prieta and the 1994 Northridge earthquakes. Both of these earthquakes caused significant nonstructural damage, including damage to sprinkler systems.
Until the IBC was published in 2000, building code seismic requirements in higher seismic areas were primarily based on those found in the Uniform Building Code4 (UBC). The UBC first introduced seismic lateral force requirements for nonstructural components in 1961. There was no significant change in the seismic force requirements for nonstructural components in the UBC until 1997. The 1997 UBC adopted the nonstructural force requirements that were provided in the 1994 NEHRP Recommended Provisions and included considerations from both the 1989 and 1994 earthquakes. These nonstructural force requirements are very similar to those found in ASCE 7 today.
Somewhat independently, the National Board of Fire Underwriters first developed non-mandatory seismic requirements for the hanging and bracing of sprinkler systems starting in 1947.5 In 1951, a specific requirement was added to the rules requiring that hanging and bracing be designed for a lateral force coefficient of 0.50 g. In 1980, the earthquake requirements were moved into a mandatory portion of NFPA 136 for areas subject to earthquakes. Although other improvements were made to the seismic provisions of NFPA 13, the lateral force coefficient of 0.50 g remained unchanged until the 2007 edition of NFPA 13. It should be noted that the NFPA 13 seismic forces were also meant to be used with the allowable stress design procedures. So through the 1994 UBC, the NFPA 13 seismic force coefficients and other design requirements were compatible with and slightly more conservative than those found in the building code (0.50 g versus 0.45 g). However, after 1994, building code seismic requirements and NFPA 13 seismic requirements started to diverge.
A report5 provided a comprehensive review of the observed sprinkler damage and comparison with existing codes and standards requirements. In this document, each type of damage was observed and suggestions were made as to what type of code change would be required in the 1996 edition of NFPA 13 to address the concern. Many changes were considered and adopted into the 1996 NFPA 13 based on the Northridge earthquake performance observations. However, some changes were not adopted and other concerns were not addressed. Among the changes not adopted or addressed were:
- Increasing the lateral seismic coefficient to greater than 0.50 g in some highly seismic areas and noticing that seismic accelerations were greater at the top of a building than on the ground
- Providing specific design requirements to avoid interference problems between sprinkler drops and suspended ceilings
- Providing specific drift criteria that needs to be accommodated by sprinkler drops in storage racks
Chapter 13 of ASCE 71 specifies two types of nonstructural demands. These are equivalent static lateral forces and relative displacement demands. It should be noted that nonstructural components located in buildings in low seismic areas are exempt from the seismic requirements of Chapter 13 of ASCE 7.
The equivalent static lateral forces are primarily used for design anchorage and bracing of a component. However, when the component is a designated seismic system, the component itself must be designed for the forces. Sprinkler systems are designated as seismic systems.
The relative displacement demand p is simply determined from the analysis of the structure in which the components are being attached. As a default, if the relative displacements are unknown, the relative displacement demands may be taken as the maximum allowable drift displacements allowed for the structure by ASCE 7. The relative displacement demand is used to determine the effects on displacement-sensitive components caused by relative anchor movements. For such components, inelastic deformations are acceptable, but failure of the component that can cause life-safety hazard is not.
In addition to the force requirements, ASCE 7 specifies that sprinkler drops that penetrate suspended ceilings in high seismic areas must satisfy at least one of the following criteria:
- The ceiling panels must have oversized penetration holes that provide 1 inch (25 mm) of clearance around the drops.
- The piping, HVACR system and ceiling system must be designed to act as an integral unit.
- Sprinkler drops must be designed with articulating connections that can accommodate at least one inch (25 mm) of displacement at the ceiling interface in all horizontal directions.
Except for criteria for different materials, the above items were all issues that were recognized in the NISTR report but were not incorporated into NFPA 13 in 1996.
Between 2004 and 2007, members of the NEHRP PUC TS-8 subcommittee and ASCE 7 nonstructural subcommittee worked with the NFPA 13 subcommittees to incorporate changes to NFPA 13 that made them consistent with ASCE 7-05. The result was to modify the influence tables in NFPA 13 (2007) so that the hangers, bracing and piping design itself satisfied ASCE 7. In addition, the term "flexible device" was added as an option for sprinkler drops to allow that concept as one means of satisfying articulating connections. Because NFPA 13 (2007) was compatible with ASCE 7, NFPA 13 (2007) was deemed to comply with ASCE 7 in the 2009 IBC and 2009 NEHRP Recommended Pro visions. NFPA 13 (2010) provided further enhancements to improve compatibility with ASCE 7.
Members of the NEHRP PUC TS-8 subcommittee and ASCE 7 also worked closely with the ceiling industry to update the ASTM E 5807 standards, which provide a "Standard Practice for Installation of Ceiling Suspension Systems for Acoustical Tile and Lay-in Panels in Areas Subject to Earthquake Ground." The updated standard includes a requirement that where the ceiling panels are penetrated by sprinklers, they must satisfy one of the criteria specified above.
In ASCE 7 (2010), because both NFPA 13 (2007) and ASTM E 580 (2009) have been adopted, there are essentially no amendments to these documents in the area of seismic requirements for sprinklers and interfaces between sprinklers and ceilings. So, if both documents are satisfied, ASCE 7 (2010) is considered satisfied.
FLEXIBLE SPRINKLER DROP DEVICES AND SUSPENDED CEILING SYSTEM SEISMIC PERFORMANCE
One of the options provided in Chapter 13 of ASCE 7 is to permit nonstructural systems and components to be evaluated by shake table testing. A shake table testing protocol8 has been developed by the International Code Council Evaluation Services (ICC-ES). It provides shake table testing criteria that is tied to the nonstructural force equation. Basically, floor (or ceiling) test motions have been derived from the parameters that are used to construct the nonstructural force equation. The greater the test motion and higher the value of floor motions, the greater the seismic qualification value.
Suspended ceilings are particularly difficult to confidently analyze by normal structural analysis procedures. For this reason, a number of ceiling manufacturers have performed shake table testing to objectively evaluate the seismic performance of their ceiling system product lines.
As part of the shake table testing that was performed on Armstrong ceilings in early 2006, suspended ceiling systems were tested with flexible sprinkler drop devices (provided by FlexHead Industries) in lieu of hard-armover sprinkler drops. The devices were connected to a full installed and operational sprinkler system. A photograph of the testing is provided in Figure 1. A variety of low seismic and high seismic ceiling systems were tested for a range of shake table test motions.
It was concluded that FlexHead's flexible devices in conjunction ceiling systems performed excellently together and offered no downside to the performance of the ceiling system. These types of flexible devices are what was envisioned when the provision for articulating connections was conceived by seismic code developers in the NEHRP Recommended Provisions in the early 1990s.
Furthermore, these flexible devices also eliminate the problem of sprinkler drops jumping out their penetration holes because of vertical motions and snapping the sprinkler when the drop tries to reinsert itself. This particular problem with sprinkler drops was highlighted in the NISTR report.5 It is worth noting that flexible devices are produced differently by each manufacturer. One should not make the broad assumption that every flexible device will perform the same way in a seismic event. There is no substitute for testing data.
Robert E. Bachman is with R. E. Bachman Consulting Engineers.
- SCE/SEI Standard 7, Minimum Design Loads for Buildings and Other Structures, American Society of Civil Engineers, Reston, VA, 2005.
- International Building Code, International Code Council, Washington, DC, 2009.
- NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures, FEMA 450-1/Building Seismic Safety Council, Washington, DC, 2003.
- Uniform Building Code, International Conference of Building Officials, Whittier, CA, 1997.
- Fleming, R. "Analysis of Fire Sprinkler System Performance in the Northridge Earthquake," NISTGCR-98-736, National Institute of Standards and Technology, Gaithersburg, MD, 1998.
- NFPA 13. Installation of Sprinkler Systems, National Fire Protection Association, Quincy, MA, (edition as noted).
- ASTM E 580, Standard Practice for Installation of Ceiling Suspension Systems for Acoustical Tile and Lay-in Panels in Areas Subject to Earthquake Ground, ASTM International, West Coshohocken, PA, 2009.
- Acceptance Criteria for the Seismic Qualification Shake-Table Testing of Nonstructural Components and Systems, AC-156, International Code Council Evaluation Services, Country Club Hills, IL, 2007.