fire protection

Seismic Protection for Fire Sprinkler Systems in Canada


Introduction:

To date, earthquakes remain one of the most unpredictable and devastating of natural disasters. While earthquakes cannot be predicted or prevented, it is possible to estimate the effects of future earthquakes by analyzing the historical data recorded from past earthquakes. This is the basis for seismic protection of the living spaces.

Structural engineers design buildings based on expected seismic forces. It is possible for a building to stand still during an earthquake if it has been designed and constructed to withstand the maximum lateral forces generated by that earthquake. However, even if a building structurally survives an earthquake there are still a number of concerns related to the non-structural components of a building, such as the state of the building's fire protection system. For example, what would happen if a small fire ignited and the sprinkler system or its piping had been damaged during the earthquake? This small fire could soon turn into a very large fire and engulf the entire building if allowed to grow without interruption. This would quickly develop into an untenable condition for the building occupants as well as eventually compromise the structural integrity of the building. However, if the building's fire sprinkler system had been designed to withstand such seismic forces it would remain strong and operate as intended if any fires were to occur in an earthquake-hit building.

The purpose of this article is to briefly discuss the seismic protection requirements for fire sprinkler systems in buildings and facilities in Canada.

Seismic Protection for a Building:

For the purpose of this article, the National Building Code of Canada 2010 (NBC) will be referenced, as the earthquake data in the Provincial Building Codes are directly adopted from the NBC.

Subsection 4.1.8. of the NBC is followed by a Structural Engineer to determine the seismic protection requirements for a building to be constructed at a particular site. According to Sentence 4.1.8.1.(1) of the NBC, the deflections and specified loading due to earthquake motions need not be considered in the design if S(0.2) [Design Spectral Response Acceleration for a period of 0.2 second], is less than or equal to 0.12.

This means that, if S(0.2) of a particular location is not greater than 0.12, a building to be constructed in that location is not required to be seismically protected. All non-structural building components, including fire sprinkler systems, are therefore not required to be seismically protected.

On the other hand, if S(0.2) is greater than 0.12, the seismic risk of the building is required to be determined, based on the following parameters: Site Class (Fa), Importance Factor (IE), and Spectral Response Acceleration (Sa(0.2)). These parameters play an important role in determining the level of seismic protection needed for the building. Every building will have a different level of risk based on these parameters. The parameters are briefly discussed below.

Site Class (Fa): The Site Class is a function of the type of soil and how it is expected to behave during an earthquake. It is classified as Class A (Hard rock) through Class F (such as peat or very high plasticity clays) [refer to Table 4.1.8.4.A. of the NBC]. The Site Class has a tremendous impact on the seismic evaluation.

Importance Factor (IE): The Importance Factor for earthquake loads and effects depends on the building importance category. It focuses on whether the building could either present a hazard or provide needed services to the community following an earthquake. It ranges from 0.8 (Low Importance) to 1.5 (Post-disaster) and is determined from 4.1.8.5. of the NBC.
A Post-disaster building is defined as a building essential to provide services in the event of a disaster. It generally includes hospitals, fire stations, police stations, radio stations, telephone exchanges, power stations, electrical substations, water pumping stations, fuel depot buildings and air traffic control towers and facilities.

Spectral Response Acceleration (Sa(0.2)): The Spectral Response Acceleration is defined as 5% damped spectral response acceleration, expressed as a ratio to gravitational acceleration, for a period of 0.2 second. This value is based on a 2% probability of being exceeded in 50 years and can be found in Appendix C, Table C-2 in Division B of the NBC.

Based on the above 3 parameters and other site-specific factors addressed in Subsection 4.1.8. of the NBC, the structural engineer determines the appropriate level of structural protection required for the building to resist the seismic forces. However, not all buildings requiring seismic protection will require seismic bracing for the fire sprinkler system and other mechanical systems.

Seismic Protection for Fire Sprinkler Systems:

Article 4.1.8.18. of the NBC “Elements of Structures, Non-Structural Components and Equipment” addresses the seismic protection requirements for all non-structural components located in the building. Table 4.1.8.18. does not have any specific category for the fire sprinkler systems. However, these systems would closely match with Category 15 – “Pipes, ducts, cable trays (including contents)”.

Sentence 4.1.8.17.(1) requires that, except as provided in Sentences 4.1.8.17.(2) and 4.1.8.17.(8), elements and components of buildings described in Table 4.1.8.17. and their connections to the structure be designed to accommodate the building deflections calculated in accordance with Article 4.1.8.13. and the element or component deflections calculated in accordance with Sentence 4.1.8.17.(10), and be designed for a lateral force, VP, applied through the centre of mass of the element or component

Sentence 4.1.8.17.(2) permits the omission of seismic protection for the non-structural building components in Categories 6 through 21, if IEFaSa(0.2) is less than 0.35 and the building is not designated as a post-disaster building. This means, the building satisfying the above criteria do not require seismic bracing for the fire sprinkler systems.

Seismic Design for Sprinkler Systems:

Once it is determined that the non-structural components (e.g., fire sprinkler systems) must be seismically protected, appropriate and acceptable procedures need to be followed to calculate the seismic forces, to design, layout, and install the seismic bracing.

It is recommended that the authority having jurisdiction (AHJ) be consulted at the early stage of a project to confirm the acceptable design, installation and layout guidelines for the seismic bracing. Part 4 of the NBC has specific requirements for lateral force and deflection calculations. However, it does not provide any guidance on the installation or layout of the seismic bracing. NFPA 13, “Standard for Installation of Sprinkler Systems”, the referenced fire sprinkler installation standard in the NBC, offers specific requirements for the calculation of seismic forces as well the proper installation and layout of seismic bracing for sprinkler piping.

Since both NBC and NFPA 13 provide procedures for calculation of the seismic forces applied on sprinkler system piping, the procedure acceptable to the local AHJ needs to be determined. Once the seismic forces are calculated, Section 9.3 of NFPA 13, “Protection of Piping Against Damage Where Subject to Earthquakes” is required to be followed.

Summary:
  • Obtain the data for S(0.2) from the Project Structural Engineer or Architect. If S(0.2) is less than or equal to 0.12, no seismic protection is required for the building. Otherwise, the building is required to be seismically protected.
  • Ask the Project Structural Engineer or Architect for the values of Site Class (Fa), Importance Factor (IE), and Spectral Response Acceleration (Sa(0.2)). If the product of these parameters is less than 0.35 and the building is not designated as a post-disaster building, fire sprinkler systems are not required to be seismically protected. Otherwise, the seismic protection is required to be provided.
  • Consult with the local AHJ for an acceptable method of calculating, designing and installing the seismic bracing for the sprinkler systems.
REFERENCES
  1. Kalafat, E., 2007. Seismic Protection of Fire Sprinklers and Other Mechanical Systems, Proceedings of Clima 2007 Wellbeing Indoors.
  2. Huggins, R.J., Jan/Feb 2005. Sprinkler shake-up, NFPA Journal
  3. Osburn, M., Do you need Seismic Bracing for your Sprinkler System?, CASA.

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Fire sprinkler systems that are designed to withstand the seismic forces of an earthquake have a greater chance of remaining strong and operating as intended if any fires were to occur in an earthquake-hit building.
Obtain the data for S(0.2) from the Project Structural Engineer or Architect. If S(0.2) is less than or equal to 0.12, no seismic protection is required for the building. Otherwise, the building is required to be seismically protected.