Earthquakes and life safety systems

Published:  12 September, 2013

Earthquake protection for fire sprinkler systems, by Daniel C Duggan, VP Seismic Design Group.

Earthquake protection of fire sprinkler systems has been addressed in NFPA-13 since the 1940s. While it is commonly thought of as a requirement for earthquake sway bracing, NFPA-13 actually requires flexibility and/or clearance to relieve the strain from seismic loading in the piping, in addition to sway bracing.

While NFPA-13 provides criteria for earthquake protection when it is required, NFPA-13 does not determine when it is required. Rather, the earthquake protection requirement is determined by building codes, insurance companies, design professionals or others.

Determination of whether or not to provide earthquake protection is dependent on a variety of factors, the most obvious of which is the geographic location, which is generally defined on seismic activity maps. The geographic locations of concern are not always obvious though. The State of California in the US is always associated with earthquakes but the worst earthquake in the contiguous 48 US States was on the New Madrid Fault in the Southeast corner of the State of Missouri in the 1800s.

Years ago the determination of whether or not earthquake protection was required was based on seismic zone maps and in which seismic zone the building was located. With the passage of time, the mapping of seismic risk became more sophisticated and changed to the mapping of Av (velocity-related peak acceleration) contour lines, and the requirement for earthquake protection was based on the building proximity to those contour lines.

Now the determination of whether or not earthquake protection is required has become even more sophisticated and takes into account known fault lines, the probability of damaging earthquakes over different time spans, soil conditions, building usage and other factors. This new method is called the Two-Point Method, and it uses one map with Ss values and another map with S1 values.

The US Geological Survey has prepared seismic activity maps that include data for the entire globe. These maps are referenced by the IBC (International Building Code), which contains a calculation procedure for determining the SDC (seismic design category) based on formulas using two different maps. The first map is commonly called the Short Period Response Map and is used to determine the value of Ss that is used in the first SDC formula. The second map is commonly called the Long Period Response Map and is used to determine the value of S1 that is used in the second SDC formula. The higher letter value (A, B, C, D, E or F), resulting from these two formulas, is the seismic design category.

The IBC requires earthquake protection of non-structural building components, such as pipes, ducts, conduits, cable trays, air handling units, etc., which can be floor, roof or wall mounted.

These non-structural building components can be exempted from the earthquake protection requirements under certain conditions. All non-structural building components are exempted if they are in seismic design category A or B.

Qualification for exemption becomes more complicated for SDC ‘C’ and higher, because additional factors apply. The component importance factor (Ip) is one of those additional factors. The IBC assigns an Ip of 1.5 to life safety and hazardous components, and to components required for essential facilities to remain functional after an earthquake.

Fire sprinkler systems are obviously Ip of 1.5, because they are the preeminent life safety system in a building. Gas piping obviously has an Ip of 1.5, because its rupture would release flammable material that would support fires in the aftermath of an earthquake.

Non-structural components in hospitals, emergency shelters, fire and police stations, water treatment facilities, power generating facilities, etc. that are required for continued operation of the facility are also assigned an Ip of 1.5.

This means that some non-structural components may be exempted in one building but not in another, due to the building’s location and usage, along with the importance of the component and its usage. For example, plumbing piping in an office building at one location may be exempt but the same plumbing piping in a hospital may not, because it is essential to the continued operation of the essential facility. So, earthquake protection of a fire sprinkler system in a hospital, warehouse, office building or any other type of building depends on the calculated seismic design category and other factors.

In addition to exemption from earthquake protection based on seismic design category, there are also exemptions based on small component sizes and methods of attachment. One of these methods of attachment pertains to short hanger rods. The IBC permits exemption from its requirements for seismic sway bracing of pipes with an Ip of 1.0 (non-hazardous, non-life safety and non-essential), if the length of the hangers from the top of the pipe to the point of connection to the structure is less than 12”, and the hanger is detailed to avoid bending moments, because it only takes two or three cycles of bending of the rod to cause it to break. NFPA-13 has an exemption based on 6” hanger length but restricts its applicability to lateral sway braces or restraints, which are those oriented perpendicular to the pipe run.

Horizontal earthquake load

In addition to calculation of the SDC, the Ss value from the Short Period Response Map referenced above is also used in the calculation of the horizontal earthquake load that is to be resisted. In earlier years a single seismic map was used for everything but now there is a second map with S1 values that are used in the Two-Point Method for seismic calculations. The numbers on the S1 map are lower than the numbers on the Ss map, which could lead to an assumption that SDC calculations based on the Ss mapped values are more conservative. For these reasons, the second SDC calculation based on the S1 value is often overlooked. This is a mistake, because it is often the latter calculation that controls, due to the fact that the two SDC formulas have other variables that differ and either formula could control.

ASCE 7, which is referenced by the IBC, FEMA, CEGS, UFGS, NAVFAC, NFPA-13 and many other codes, standards and guidelines, specifies the Fp formulas for calculating the horizontal earthquake load.

The Fp formulas include Sds (spectral response acceleration), which is calculated using Ss and the site class information, along with Ap (component amplification factor), Rp (component response factor), Ip (component importance factor), Wp (component operating weight), and adjustment for the component anchorage elevation in the building.

By removing Wp from the Fp formulas and making some conservative but realistic assumptions with the other variables, it is possible to calculate an HLF (horizontal load factor) that can then be used to calculate the horizontal earthquake load. This is done by simply multiplying Wp by the HLF (component operating weight x horizontal load factor).

The NFPA-13 earthquake load calculations and associated Tables are based on the ASCE 7 criteria. However, ASCE 7 is an LRFD (strength design) standard and NFPA-13 is ASD (allowable stress design) standard. So, the horizontal load factor in NFPA-13 is represented by the seismic coefficient Cp, which has been adjusted from LRFD to ASD. The assumed values for the variables in the Fp formulas to determine Cp are generally based on ductile piping with threaded or grooved connections that is located at the top of a building that is located in Ste Class D, which is the ASCE 7 default site class. The calculated Fpw (horizontal earthquake load) in NFPA-13 is equal to Cp x Wp (seismic coefficient x component operating weight).

Seismic bracing

The earthquake sway bracing in NFPA-13 is intended to keep the sprinkler system piping fairly rigid and moving with portions of the building that are expected to move as a unit, such as ceilings or roofs. This is accomplished through the use of a system of lateral braces (oriented perpendicular to the pipe), longitudinal braces (oriented parallel to the pipe) and 4-way braces (braces resisting loads 360° in the horizontal plane).

In general lateral sway brace spacing is limited to 40 ft, while longitudinal sway brace spacing is limited to 80 ft. This is because the sprinkler piping is expected to act like a beam, when transferring the earthquake load to lateral sway braces, but the piping is expected to act like a column, when transferring the earthquake load to longitudinal sway braces. 4-way sway brace spacing is usually limited to the more conservative 40 ft, because they are acting as both lateral and longitudinal braces that occur at the same location.

These spacing limitations are maximums and are further limited by both the strength of the sway brace assembly and its anchorage to the structure. Lateral sway brace spacing is further limited by the bending strength of the sprinkler piping itself to prevent rupture and to act like a beam transferring the earthquake load to the braces. This is a requirement of ASCE 7, which is addressed in the NFPA-13 with the Maximum Fpw Tables that are based on the strengths of different types of pipe.

ASCE 7 requires concrete anchors to have passed the AC355.2 seismic pre-qualification testing. The ICC-ES (International Code Council Evaluation Service) publishes ES reports for concrete anchors that have passed AC355.2 testing. The allowable horizontal earthquake loads at various sway brace angles that appear in the NFPA-13 Fastener Tables, are conservative values that are based on anchors that have passed AC355.3 testing and include the effects of prying due to the geometry of the sway brace fitting used at the connection to the structure.

Automated calculations

All of this requires a significant amount of work just to perform the required calculations, even though NFPA-13 contains tables that can be used to look up much of the information. However, there are programs available to perform the required calculations.

The SCoPe Seismic Calculation Program performs all of the calculations described in this article and for any suspended non-structural building component, including NFPA-13 fire sprinkler piping. SCoPe performs these calculations in a couple of minutes for projects located anywhere in the world and refines the calculations for the most efficient result. It even has a button click to the USGS site to retrieve the seismic mapped values on the fly. It is so fast that it can be used as a valuable estimating tool and can even get a quick calculation of the seismic design category, which can be used to prove whether or not seismic sway bracing is required at all. For additional information about SCoPe go to www.seismicdesigngroup.com

Photos, top to bottom: Izmit earthquake, 1999 (US Geological Survey); National Seismic Hazard Maps (US Geological Survey); Loma Prieta earthquake, San Francisco Bay Area, 1989.


About the author

Daniel C. Duggan is President of Fire Sprinkler Design and VP of Seismic Design Group, LLC. He has 45 years of experience in the design of all types of fire sprinkler systems. He is a member of the NFPA-13 Committee on Hanging and Bracing, submitted proposals resulting in significant changes and additions to the Earthquake Protection Section of NFPA-13.

Duggan is the co-author of Loos & Co Manual of Code Compliance Guidelines for Earthquake Bracing of Non-Structural Building Components and Systems. The Manual is the culmination of over 20 years of research into the requirements of all of the model codes and standards used throughout the United States and received the 2004 National Earthquake Conference Award of Excellence. He has also created the SCoPe Seismic Calculation Program, as well as lectured and conducted seminars on earthquake bracing in various locations in the United States and internationally, along with being awarded several patents. Readers having questions or suggestions regarding this article may contact Duggan by e-mail at fsd@earthquakebrace.com.

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