Deep 18-inch Beams Obstructed or Unobstructed?

4/24/2023

I have a non-combustible shell building with 18-inch steel beams under corrugated metal deck.

Some of the beams are spaced in excess of 7'-6" apart, but not exceeding 9'-6" and creating an area exceeding 300 sq. ft.

To loosely summarize definitions, per NFPA 13, 2019 edition Section A.3.3.41.1 for Obstructed Construction, Beam and Girder Construction, the steel beams are to be spaced between 3 ft. up to 7½ ft.

Panel Construction requires that all cross-sectional areas be filled including the interface at the roof, (which this is not the case on my project), and not exceed the 300 sq. ft. area.

My question is this, since this building does not meet either criteria and is not combustible construction, (no wood is being utilized in the structure), there are no concrete tees being utilized and there are no bar joists present, would this then be considered as Unobstructed?

My thoughts are no, and that this is still Obstructed Construction, however I can't find a proper definition within the Obstructed Construction definition.

I am wanting to know if I can utilize the 1" to 6" below Obstructed Construction allowance however I am held up by the definition of the construction type and am unsure if I need to space within the bays or if I can space below the 18-inch steel beams.

Any thoughts or input on this would be greatly appreciated.

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15 Comments

Anthony

4/24/2023 07:08:03 am

If its not obstructed construction then it's not, there is no grey area.

You'll need to utilize Table 8.6.5.1.2 and FIGURE 8.6.5.1.2(a) (NFPA 13-2016) OR pocket protect each area.

Glenn Berger

4/24/2023 08:05:34 am

For obstructed construction construction, the requirement is 7'6" or less with deep beams.

The application that you describe is unobstructed.

Larry Appel

4/24/2023 08:26:52 am

Glenn is correct, you’re describing an unobstructed application!

Dan Wilder

4/24/2023 08:31:14 am

How deep is the pan deck?

The description of the building looks like it includes areas that will transition between obstructed (where the beams are less than 7'-6" on center) and areas spaced greater than 7'-6" that exceed the 300 sq. ft. allowance.

The descriptions are fairly clear and as described, I would go with unobstructed spacing requirements when the areas are not complying with the obstructed rules.

In the past I have been able to provide protection on a bay by bay basis, where I utilized both 1"-12" for some bays and 1'-6" below with the max at 22"....but I went with the more restrictive (12" max) for the one bay space that bounded the unobstructed/obstructed change simply due to the pan deck interface not being filled. It also took a talk with the AHJ prior to submittal to have their buy in along with an informal interpretation as a little back up. Depending on how large the area, it does add up.

Jeffrey Davis

4/24/2023 10:28:19 am

The deck is made up utilizing Robinson decking.
Thank you for the reply and I will run this by the AHJ for their approval.

4/24/2023 04:54:11 pm

Any pan decking greater than 3" deep you need to measure to that high point which can greatly affect the deflector distance clearances.

Todd E Wyatt

4/24/2023 08:47:18 am

2019 NFPA 13 references “obstructed” or “obstruction” (277) times.

Chapter 3 Definitions includes separate definitions for “Continuous Obstruction” (“ ... affect the discharge pattern of TWO OR MORE adjacent sprinklers ...”) and “Noncontinuous Obstruction” (“ ... affects the discharge pattern of a SINGLE sprinkler ...”).

Chapter 9 Sprinkler Location Requirements, 9.5.5 Obstructions to Sprinkler Discharge includes prescriptive requirements “to minimize the effect of obstructions through the use of specific criteria in 9.5.5, 10.2.7, 10.3.6, 11.2.5, 11.3.6, 12.1.10, 12.1.11, 13.2.8, and 14.2.11. The obstruction criteria for storage sprinklers in 13.2.8 and 14.2.11 is the most stringent.” (A.9.5.5.1)

Since your Project does not include concrete tees, then “Chapter 10 Installation Requirements for Standard Pendent, Upright, and Sidewall Spray Sprinklers, 10.2.6.1.2 Obstructed Construction (5) Installed with deflectors of sprinklers under CONCRETE TEE construction with stems spaced less than 7-1⁄2 ft (2.3 m) on centers, regardless of the depth of the tee, located at or above a horizontal plane 1 in. (25 mm) below the bottom of the stems of the tees and complying with Table 10.2.7.1.2)” or “Chapter 13 13.2.7.1.2 Obstructed Construction. (3) Installed with deflectors of sprinklers under CONCRETE TEE construction with stems spaced less than 7 ft 6 in. (2.3 m) but more than 3 ft (900 mm) on centers, regardless of the depth of the tee, located at or above a horizontal plane 1 in. (25 mm) below the bottom of the stems of the tees and in compliance with Table 13.2.8.1.2.” cannot be used.

The referenced standard (e.g. 2019 NFPA 13) shall be used based on the specific requirements of the Project. “A.3.3.41.1 OBSTRUCTED Construction” includes (10) examples of OBSTRUCTED Construction, including “Beam and Girder Construction”. “A.3.3.41.2 UNOBSTRUCTED Construction” includes (5) examples of UNOBSTRUCTED Construction. Design Professionals should not “cherry-pick” passages from the referenced standard that are not applicable to their Project.

4/24/2023 08:47:39 am

REFERENCES
2019 NFPA 13
Chapter 3 Definitions

A.3.3.25 Ceiling Pocket.
It is not the intent of this definition to be applied to structural and/or framing members otherwise used to define obstructed or UNOBSTRUCTED construction. Ceiling pockets can be protected or unprotected. A ceiling pocket where the upper ceiling is within the allowable vertical distance from the sprinkler deflector should be considered a protected ceiling pocket. Buildings with protected ceiling pockets are permitted to use the quick-response reduction of 19.3.3.2.3. Buildings with unprotected ceiling pockets greater than 32 ft2 (3.0 m2) are not allowed to use the quick-response reduction of 19.3.3.2.3.

3.3.140 OBSTRUCTION.
ENHANCED CONTENT
The terms continuous OBSTRUCTION and noncontinuous OBSTRUCTION are used throughout 9.5.5. Guidance is provided in Chapters 10 through 15 on how to locate various types of sprinklers in regard to such obstructions.
In assigning whether a configuration is a continuous obstruction, the determining factor is not the individual OBSTRUCTING component but rather the impact on adjacent sprinklers. For instance, a run of hanging lights that has a 24 in. (600 mm) gap between light fixtures would affect the discharge of adjacent sprinklers, presenting a continuous OBSTRUCTION similar to that of a solid duct.

A.3.3.41.1 OBSTRUCTED Construction.
The following examples of OBSTRUCTED construction are provided to assist the user in determining the type of construction feature:
(1) Beam and Girder Construction. The term beam and girder construction as used in this standard includes noncombustible and combustible roof or floor decks supported by wood beams of 4 in. (100 mm) or greater nominal thickness or concrete or steel beams spaced 3 ft to 71⁄2 ft (900 mm to 2.3 m) on center and either supported on or framed into girders. [Where supporting a wood plank deck, this includes semi-mill and panel construction, and where supporting (with steel framing) gypsum plank, steel deck, concrete, tile, or similar material, this includes much of the so-called noncombustible construction.]
(2) Concrete Tee Construction.
(3) composite Wood Joist Construction.
(4) Panel Construction.
(5) Semi-Mill Construction.
(6) Wood Joist Construction.
(7) Bar Joist Construction with Fireproofing.
(8) Steel Purlin Construction.
(9) Truss Construction (Wood or Steel). The term truss construction refers to parallel or pitched chord members connected by open web members supporting a roof or floor deck with top and bottom members greater than 4 in. (100 mm) in depth. [See Figure A.‍3.3.41.1(c).]
(10) Bar Joist Construction (Wood or Steel).

A.3.3.41.2 UNOBSTRUCTED Construction.
The following examples of unobstructed construction are provided to assist the user in determining the type of construction feature:
(1) Bar Joist Construction.
(2) Open-Grid Ceilings.
(3) Smooth Ceiling Construction.
(a) Flat slab, pan-type reinforced concrete
(b) Continuous smooth bays
(c) Smooth roof or floor decks
(d) Smooth monolithic ceilings
(e) Open-web-type steel beams, regardless of spacing
(f) Smooth shell-type roofs
(g) Suspended ceilings of combustible or noncombustible construction
(h) Smooth monolithic ceilings
(4) Standard Mill Construction.
(5) truss Construction (Wood or Steel).

3.3.140.1 Continuous OBSTRUCTION.
An OBSTRUCTION located at or below the level of sprinkler deflectors that affect the discharge pattern of two or more adjacent sprinklers. (AUT-SSI)
3.3.140.2 Noncontinuous OBSTRUCTION.
An OBSTRUCTION at or below the level of the sprinkler deflector that affects the discharge pattern of a single sprinkler. (AUT-SSI)

danefre

4/24/2023 09:21:33 am

That ceiling pocket part is important. You cannot apply the ceiling pocket section for bays created by structural members. Ceiling pockets are architectural features!

SCHULMAN

4/24/2023 08:49:24 am

Unobstructed for the win ... but I am curious so I want to expand the discussion if that is fair ... what if the ultimate goal is to have lay-in (ACT) ceilings throughout. (creating a non-combustible ceiling space). He did say "shell" space afterall ... do you place upright sprigs and have the tenant contractor run the flexheads? Would you then disregard beam spacing with exception of sprigs that end up too close to beams ... so you'd add one or two along a run to re-space for that? ... For a code compliant shell space that won't be used as installed ... ? Any contractors out there who care to weigh in ?

4/24/2023 08:56:16 am

We usually do both shell and tenant work for our contracts. We put the sprigs on 1" tees with a plug on the bull. That outlet will later be used for the flexhead.

A lot of jurisdictions here require the uprights to remain in the non-combustible concealed space above the ACT. So all we have to do is take the plug out.

If we receive the tenant plans before we fabricate the shell building, then we "double dip" and design the shell building outlets to be as close as possible to final indented flex locations.

4/24/2023 09:53:06 am

Thank you sir.

4/24/2023 08:50:33 am

You can have beams spaced more than 7'-6 and still have obstructed construction as you indicated. Some of the above responses are misleading.

Panel construction does require "all cross-sectional areas be filled including the interface at the roof". This is most often achieved with K13 thermal/acoustic insulation sprayed on the beams. That's a common situation where beams are spaced more than 7'-6 OC and it's still obstructed construction. Obviously still abide by the 300sqft limitation.

To answer your question: If you don't meet the requirements of obstructed construction then you have to abide by the more stringent 1"-12" deflector and protect every bay or however your spacing works out to be.

Jesse

4/24/2023 09:25:58 am

As you have described it, I see this as unobstructed construction.

4/24/2023 10:37:37 am

Thank you guys for your help clarifying this for me. It has helped give me more to think about in regards to obstructed and unobstructed construction.
I was stuck on the idea of not wanting to bay space in the effort to reduce head counts however, in the end, we ended up going with the bay spacing. Thanks again.

Deep 18-inch Beams Obstructed or Unobstructed?

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