While being located geographically in the middle of the United States, it may not seem like seismic bracing would be a major concern. After all, we don’t have the frequency of intense earthquake movement that covers news headlines like the west coast experiences.
Despite the (fortunately) absent frequency, the New Madrid fault line runs near Memphis, Tennessee up to the bootheel of Missouri.
[Note: Yes, we Missouri-folk actually describe a portion of the state as a “bootheel”. When you say it aloud, though, you have to add a little twang.]
Back to seismic – this fault line has the potential for very strong seismic activity just as much as portions of California and the Pacific Northwest. As a result, seismic bracing is common for us in southeastern Missouri, in St. Louis, and even into central Missouri and southwestern Illinois.
As we move away from the fault line, at some point, seismic movement would be less severe – even to the extent that bracing isn’t necessary.
Where is that point?
How do we determine when seismic bracing is necessary for fire suppression systems?
Today’s article is covering just that. It’s an exercise I practice commonly as I essentially live on the boundary of where seismic is and is not required by code.
International Building Code References ASCE 7
Seismic bracing has roots in NFPA 13. As is the case between a “code” and a “standard”, however, NFPA 13 as a standard only tells us how to design and install the system. Code tells us when and where systems and components are required.
The International Building Code Section 1613 for Earthquake Loads requires that “every structure… including nonstructural components that are permanently attached to structures and their supports… shall be designed and constructed to resist the effects of earthquake motions in accordance with ASCE 7”. [2015 Edition 1613.1]
There are a few exceptions, most notably detached one- and two-family dwellings in some areas.
ASCE 7 Requirements Based on Seismic Design Category
ASCE 7, Chapter 13 (2010 Edition), for Seismic Design Requirements for Nonstructural Components, states:
ASCE 7 Chapter 13 addresses Seismic Design Requirements for Non-Structural Components. Its scope covers the minimum design criteria for nonstructural components (like fire suppression systems) that are attached to the structure.
ASCE 7 Chapter 13 suggests that seismic bracing is required for all structures, unless they meet an exemption. Section 13.1.4 specifically lists exemptions from seismic design requirements.
These Exemptions include:
Additionally, ASCE 7 Section 11.7 states that Seismic Design Category A need only comply with Section 1.4 (not Chapter 13).
So What is a Seismic Design Category?
A Seismic Design Category is a “classification assigned to a structure based on its Risk Category and the severity of the design earthquake ground motion at the site.” (ASCE 7 Chapter 11 Definitions)
In short, it’s a classification on the entire structure, ranging from A (least risk) to F (greatest severity).
Seismic Design Category A structures encompass buildings of ordinary occupancy located on sites with stiff soils and have little risk of experiencing earthquakes.
Seismic Design Category F, on the contrary, are required to remain functional following a strong earthquake, such as hospitals and emergency communication centers, and are located very close to major active faults.
What Impacts Seismic Design Category?
Several contributing factors are combined to give the seismic design category. They principally include:
Structures that are of high importance following an earthquake, such as a hospital, are of greater importance and carry a higher risk category.
Soil conditions greatly impact the ability of the building to response to motion. Stiff soil or rock conditions generally allow the building to better respond to an earthquake. Loose, stiff soil, or soft clay don’t give buildings the ability to move with the ground, and therefore create worse seismic reaction forces within the building.
Lastly, and probably the most obvious, the building’s proximity to a major fault line. The closer to a fault line, the greater the seismic forces from an earthquake event for the same earthquake.
How to Determine Seismic Design Category?
The International Building Code Section 1613 allows the Seismic Design Category to be determined either by IBC 1613 or ASCE 7.
The International Building Code Section 1613 details the step-by-step process to determine the Seismic Design Category. This involves using data from site soil testing, the Risk Category, and earthquake severity parameters from provided maps.
ASCE 7 has similar provisions in Chapter 11, detailing similar inputs of Risk Category, Mapped Response Parameters, and site conditions.
In practice, however, there are third-party tools that help make this process much easier.
Here are a few available, for free online:
With the Risk Category, Address, and Site Information, a Seismic Design Category can be quickly determined for a building. These reports also give important design parameters that will be used for Seismic Calculations for the design of the system.
What if I Don’t Know The Site Class?
With new construction, structural foundation design requires geotechnical testing and reports which give these values. The structural engineer will assess the report, and typically place the building’s Seismic Design Category in their front-sheet notes or in structural specifications.
This isn’t the case with interior remodels or work within existing buildings. No soil testing is done and sometimes no structural work is done.
When this is the case, the International Building Code requires a Site Class D to be used (IBC 1613.3.2). This could be made more conservative by the building official if geotechnical data determines that Site Class E or F soils are present at the site.
So Does My Building Require Seismic Design?
Back to the original question – once we know the Seismic Design Category, it’s easy to determine where fire suppression systems require seismic design.
If the Seismic Design Category is A or B, then Seismic Design Criteria does not apply. If the Seismic Deign Category is C, D, E, or F – then Seismic Design Criteria applies. Under this later scenario, all the requirements of NFPA 13 for Seismic Design now become an enforceable requirement for the system design.
Here’s a summary of the code path:
Seismic Design Criteria for Fire Suppression Systems depends upon the Seismic Design Category for the Building. This Seismic Design Category incorporates the importance of the building, it’s proximity to seismic fault lines, and soil conditions at the site.
While the determination path through codes & standards might not be as clear as other system requirements, seismic design is nonetheless a crucial component for the performance of a fire suppression system and an important consideration in the design of the system.
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Joseph Meyer, PE, is a Fire Protection Engineer in St. Louis, Missouri. See bio on About page.