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Suspended or Floor Mounted Vertical Obstructions – NFPA 13

Blog Post created by boconnor Employee on Jul 17, 2020

There are many different requirements for obstruction in NFPA 13 Standard for the Installation of Sprinkler Systems  based on the type of sprinkler being used as well as the distance, type, and size of the obstruction. This blog will address suspended or floor mounted vertical obstructions requirements from the 2019 edition of NFPA 13 since this is a topic NFPA has recently received several technical questions on.

 

What are suspended or floor mounted vertical obstructions?


An obstruction is something that affects the discharge pattern of one or more sprinklers. An example of what a discharge or distribution pattern looks like is provided below:

 

(This is from the 2019 edition of NFPA 13, Figure A.9.5.5.1)

 

NFPA 13 section 10.2.7.2.2 gives a few examples of things that could be considered suspended or floor mounted vertical obstructions which include privacy curtains, freestanding partitions, and room dividers.

 

The basic rule for these obstructions is simple. There are tables in NFPA 13 which contains horizontal distances and the required minimum vertical distance that the obstruction must be from the sprinkler deflector. There are also figures to help you understand how the table should be used. Below are the table and figures for standard spray upright/pendent and sidewall sprinklers but the same table and figures are located in the extended coverage and residential sprinkler chapters.

 

                     

(This is from the 2019 edition of NFPA 13 Table 10.2.7.2.2 and Figure 10.2.7.2.2)

 

 

(This is figure 10.3.6.2.2 from the 2019 edition of NFPA 13)

 

You will notice that the relationship between the horizontal and vertical distances forms an umbrella shape similar to Figure A.9.5.5.1 above. The intent of this is to make sure the obstruction doesn’t block the development of the sprinkler pattern which occurs within the first 18 vertical inches (450 mm) of the sprinkler.

 

What about non-light hazard occupancies?


You’ll notice that the requirements for suspended or floor mounted vertical obstructions only apply to light hazard occupancies. These requirements shouldn’t be applied for anything except light hazard occupancies because the testing that was done to justify the addition of this code section only evaluated sprinkler performance in a light hazard environment.

 

Well, what do you do when you are in something other than a light hazard occupancy? The answer is that you should follow the general obstruction rules of NFPA 13. For obstructions below 18 inches for standard pendent and upright spray sprinklers this means that as long as the obstruction is less than 4ft (1.2 m) wide that it is not considered an obstruction. For obstructions less than 18 inches (450 mm) below the sprinkler deflector there are additional diagrams and tables you need to follow because of the potential to disrupt the sprinkler pattern development. A common rule that is followed for obstructions within 18 inches of the sprinkler deflector is the “three times rule”. This requires sprinklers to be positioned away from obstructions a minimum or three times the maximum dimension of the obstruction.

 

Are there any exceptions?

 

Have you ever noticed that in healthcare facilities the privacy curtains are mostly solid except for the top 22 inches (550 mm)? According to NFPA 13, those privacy curtains are not considered obstructions if they follow three rules: 

 

  1. Curtains need to be supported by fabric mesh on a ceiling track
  2. The openings in the mesh part of the curtain needs to be at least 70% of the area
  3. The mesh portion of the curtain needs to extend at least 22 inches (550 mm) from the ceiling

 

Those rules allow heat from the fire and sprinkler water discharge to pass through the mesh portion of the curtain without having a major impact on the sprinkler discharge pattern development or sprinkler activation time. Once again this exception to the rule can only be applied to light hazard occupancies.

 

With all of this being said, it is also important to understand how the building will look when it is finished by reviewing all of the architectural, structural, and MEP drawings. Changes in any one of those drawings can create an obstruction to your once properly designed sprinkler system.


Let us know what your experience is with suspended or floor mounted vertical obstructions in the comments below.

 

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