Posts Tagged ‘Earthquake Hazard Reduction’

Seismic Design For Fire Sprinkler Systems – Part 1b: IBC Requirements and Exemptions

January 23rd, 2009

Part 1: Using the Seismic Design Category to determine the need for earthquake bracing.

Continued from Seismic Design For Fire Sprinkler Systems – Part 1a: The Seismic Shift

IBC Requirements and Exemptions
Now that we are working in this new era, you must understand the “if” of the requirement before discussing the “how.” The information used to determine design standards includes data that is collected and tested by the National Earthquake Hazard Reduction Program (NEHRP). IBC, NFPA 5000: Building Construction and Safety Code, and others all use the data collected by this organization to create the criteria that should be followed.

Let’s first take a look at how the IBC deals with seismic. The text about earthquake protection in the IBC is based in large part on criteria found in ASCE 7. This separate document is published by the American Society of Civil Engineers. It includes design criteria for seismic restraint of architectural, mechanical, and electrical components and systems. The first edition of IBC in 2000 introduced the requirement for seismic design for fire sprinklers but did not directly reference ASCE 7 at that time. IBC Section 1614.1 states, “Every structure, and portion thereof, shall as a minimum, be designed and constructed to resist the effects of earthquake motions and assigned a Seismic Design Category as set forth in Section 1616.3. Structures determined to be in Seismic Design Category A need only comply with Section 1616.4.” The 2003 edition kept this requirement in place but revised the exemptions that followed.

The first exemption says, “Structures designed in accordance with the provisions of Sections 9.1 through 9.6, 9.13 and 9.14 of ASCE 7 shall be permitted.” This exemption allows the use of ASCE 7 in lieu of IBC.

The referenced sections that deal specifically with fire sprinklers are found in the body of Section 9.6. Within this section are six exemptions that detail when seismic is not required. It is within these six exemptions that the “if” can be determined. The first exemption allows you to exclude all aforementioned components if the Seismic Design Category is A. The second allows architectural components that are in a Seismic Design Category B with some exceptions concerning parapets and wall types. The third exemption is where fire sprinkler systems are addressed. This exemption allows mechanical and electrical components that are a Seismic Design Category B to be excluded. This section will prove to be the most referenced section in the process. After several years of dealing with this process, I have found that the majority of the country will be classified as a Seismic Design Category B. The fourth exemption appears to affect fire sprinkler systems as well. It allows mechanical and electrical components that have a Seismic Design Category C to be excluded; however, they must have an Importance Factor (Ip) that is equal to 1.0. Fire sprinkler systems have been assigned an Ip of 1.5 (ASCE 7-9.6 1.5) because they are considered life safety systems. Therefore, this exemption cannot be applied. The fifth and sixth exemptions, while applying to mechanical and electrical components, both include a requirement for an Ip equaling 1.0, meaning fire sprinklers are not allowed to be excluded.

Five other exemptions in IBC Section 1614.1 can be applied if the first one does not apply. The second exemption states, “Detached one- and two-family dwellings as applicable in Section 101.2 in Seismic Design Categories A, B and C, or located where the mapped short-period spectral response acceleration, Ss, is less than 0.4g, are exempt from the requirements of Sections 1613 through 1622.” No specific language in NFPA 13D: Standard for the Installation of Sprinkler Systems in One- and Two-Family Dwellings and Manufactured Homes requires seismic design for structures of this type; however, be advised, this does not include multifamily structures. NFPA 13R: Standard for the Installation of Sprinkler Systems in Residential Occupancies Up to and Including Four Stories in Height requires systems to follow the requirements of NFPA 13 in their entirety. There are no exceptions to this. If your project is a one- or two-family detached dwelling, seismic design is not required. However, if you are working on a multifamily structure that most likely falls into a R1 type of occupancy, it will be subject to seismic design if it cannot meet any of the other exemptions.

The third IBC Section 1614.1 exemption states, “The seismic-force-resisting system of wood frame buildings that conform to the provisions of Section 2308 are not required to be analyzed as specified in Section 1616.1.” This exemption deals more with the structure itself rather than the portions thereof. I doubt this section ever could be applied in an effort to exempt fire sprinkler systems. The fourth exemption states, “Agricultural storage structures intended only for incidental human occupancy are exempt from the requirements of Sections 1613 through 1623.” The reasoning behind this exemption seems self-explanatory. Obviously these types of structures would have a very low occupancy load and most likely would not require very extensive life safety systems. Hence, it stands to reason that system protection would be minimal.

The fifth and sixth exemptions are really the only other viable exemptions where seismic design for fire sprinkler systems is allowed to be excluded. Exemption five allows you to use the seismic maps that are included in Section 1615. “Structures located where mapped short-period spectral response acceleration, Ss, determined in accordance with Section 1615.1 is less than or equal to 0.15g and where the mapped spectral response acceleration at 1-second period, S1, determined in accordance with Section 1615.1 is less than or equal to 0.04g shall be categorized as Seismic Design Category A. Seismic Design Category A structures need only comply with Section 1616.4.” The contour lines shown on these maps are based on two different time periods. Without delving too deep into the world of seismology, we will accept these maps as a guide to determining the anticipated g-forces that are expected over a given time period.

Finally, exemption number six allows you to use a calculation procedure to determine the values to be compared with the allowed minimums. It states, “Structures located where the short-period design spectral response acceleration, SDS, determined in accordance with Section 1615.1, is less than or equal to 0.167g and the design spectral response acceleration at 1-second period, SD1, determined in accordance with Section 1615.1, is less than or equal to 0.067g, shall be categorized as Seismic Design Category A and need only comply with Section 1616.4.”

According to these two exemptions, if you look at the two different maps—short period and long period—and interpolate your location on each, and the values you determine are less than those listed in these exceptions respectively, then you do not have to provide seismic
restraint for the system.

For example, consider a single-story office building being built in Tampa, Fla. You would look at the short and long-term spectral response maps, IBC Figures 1615-1 and 1615-2, and interpolate as exactly as possible the closest g-force percentage. (Keep in mind that these values are presented as percentages. This will become useful when we actually do the calculations.)
Seismic Figure 1
The maps in the code book itself are very small and somewhat difficult to read. Several resources are available that provide these maps as .dwf files, which are a type of AutoCAD viewing file similar to a .pdf or Acrobat file. Most manufacturers that provide components that are listed for seismic restraint have these files available. A software program available through the International Code Council also provides a more useful and accurate way to evaluate these maps. You also can purchase the maps as one large foldout that has the short-term period on one side and the long-term period on the other. I highly recommend
this investment.

Figure 1 is an enlargement from the electronic version of the map that came from ICC. It helps you better determine the short- and long-term values for our example. Figure 1 is the short-term map. The upper contour line is 10 (the number is not visible) and the lower contour line is 5 as shown.

As you can see, Tampa, which is in Hillsborough County, falls nearly between the short period response percentages lines of 5 and 10. Depending on the project’s location in the county, you can further define, or interpolate, between these two lines; however, for the sake of this example we are going to assume a value of 7.5.

The long-period map (Figure 2) has contour lines 2 and 4 showing in the same area (2 is the lower contour line and 4 is the upper contour line). Here again, you can interpolate between the contour lines or simply choose the higher of the two.

Keep in mind that you must satisfy both the short- and long-term values in order to use the fifth exemption.

Continued at Seismic Design For Fire Sprinkler Systems – Part 1c: Determining the Seismic Design Category of a Building