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Rules for Openings in Obstructed Construction Apply Only to Small Beam Pockets?

5/7/2021

2 Comments

 
Curious if others interpret the obstructed construction rule the same way I do.

I've seen and heard quite a bit of dispute on how Obstructed Construction should be applied to solid beams with corrugated metal deck above (when applied perpendicular to the beams in question). So the definition of obstructed construction is really defined by applying what unobstructed construction isn't.

In summary, obstructed construction is solid members (or members less than 70% open) that are spaced less than 7-1/2 on center. That would lead you to believe that openings of 29.99% are permitted as long as members are less than 7-1/2' on center.

If you go to the appendix, it offers a LOT more information on types of obstructed construction, one of those being panel construction. There it mentions that (pockets) 300 sqft or less can also be obstructed construction, even with members more than 7-1/2' on center, as long as NO openings are allowed, including at the beam to ceiling interface.

I take this opening rule as only applying to when the 300 sqft exception is being used and where beams are more than 7-1/2' on center. Is this how you interpret this section? 

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2 Comments
Craig Hanson link
5/7/2021 08:28:37 am

Here are two comments that I use when determining the 70% rule:

Unobstructed Construction- Sectional shows wood joist construction that may be considered obstructed if the open space between members is less than 70% [NFPA 13: 3.7.2*]. These are 30” panels on 24” centers which gives an area of 720 sq. Inches. The total area of the chords and webs exceeds 70% then it is obstructed. Orientation of the webs and chords make a difference to the equation. Please provide information on wood joist construction and whether this is obstructed or unobstructed.

The NFPA 13 handbook 8.6.5.2.16 & .7 (Exhibit 8.22 and 8.23) specifically address this issue on coverage and gives some examples on how coverage can be maintained without adding more heads. This method deals with not using the 3 times rule but running the piping in a manner that reduces the obstruction to the heads.

Reply
Dan Wilder
5/7/2021 12:13:09 pm

I would agree with you based on both conditions being met.

The NFSA Expert of the Day Handbook from August 2018 #4 (for 13-16 Ed) reinforces that there shall be no unfilled penetrations in the cross section area of the bounding structural members including the interface at the roof (per the annex). This would be specific to Panel Construction and its definition so if you were defining the construction as one of the other examples it wouldn't apply.

I will throw in this little nugget, if the sprinkler distance below corrugated metal decks (8.5.4.1.2 13-16ed) to deflector changes based on the depth of the pan (<3" = bottom of pan, >3"=top of pan) then doesn't that indicate the ability to trap heat and then negate the filling requirement when the pan is <3" because NFPA bases that dimension on activation?

I understand that the Annex indicates intent/opinion without it being a requirement but if the standard shows that deflector distance requirements, the appendix definition and additional commentary leads to the "ability to trap heat", doesn't that show that pan decks less than 3" deep trap heat and that additional beams below would only increase that ability?

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