sprinklernotes
Spacing Deviations with ESFR Sprinklers
You asked the following question: “When designing an ESFR system for a building
of beam and girder construction where the solid roof beams are spaced at 30 ft
6 in. with 9-in. deep purlins and a maximum roof elevation of 45 ft, is it
allowable to apply the “shift rule” by spacing the branch lines at 10 ft 2 in.
intervals within each bay while spacing the sprinklers along the branch lines
at 9 ft 10 in.; maintaining a maximum 100 ft² coverage?”
In response to your question, we have reviewed NFPA 13, Standard for the
Installation of Sprinkler Systems, 2019 edition. Our informal interpretation is
that the proposed arrangement would not be permitted.
While the exact arrangement proposed would not be permitted per the
installation standard or the sprinkler’s listing, there are other options that
would minimize the necessary revisions to the design of the sprinkler system.
The “shift rule” refers to the requirements of sections 14.2.8.2.3 and
14.2.8.2.4 in NFPA 13 and permits the maximum spacing requirements to be
exceeded. In this case, since the ceiling height exceeds 30 ft, ESFR sprinklers
are limited to 10-ft spacing before applying the “shift rule.”
14.2.8.2.3* Deviations from the maximum sprinkler spacing shall be permitted to
eliminate obstructions created by structural elements (such as trusses, bar
joists, and wind bracing) by moving a sprinkler along the branch line a maximum
of 1 ft (300 mm) from its allowable spacing, provided coverage for that
sprinkler does not exceed 110 ft2 (10 m2) per sprinkler where all of the
following conditions are met:
(1) The average actual floor area protected by the moved sprinkler and the
adjacent sprinklers shall not exceed 100 ft2 (9 m2).
(2) Adjacent branch lines shall maintain the same pattern.
(3) In no case shall the distance between sprinklers exceed 12 ft (3.7 m).
14.2.8.2.4 Deviations from the maximum sprinkler spacing shall be permitted to
eliminate obstructions created by structural elements (such as trusses, bar
joists, and wind bracing) by moving a single branch line a maximum of 1 ft (300
mm) from its allowable spacing, provided coverage for the sprinklers on that
branch line and the sprinklers on the branch line it is moving away from does
not exceed 110 ft2 (10 m2) per sprinkler where all of the following conditions
are met:
(1) The average actual floor area protected by the sprinklers on the moved
branch line and the sprinklers on the adjacent branch lines shall not exceed
100 ft2 (9 m2).
(2) In no case shall the distance between sprinklers exceed 12 ft (3.7 m).
(3) It shall not be permitted to move a branch line where there are moved
sprinklers on a branch line that exceed the maximum sprinkler spacing.
While the provisions of sections 14.2.8.2.3 and 14.2.8.2.4 permit sprinklers
and branch lines to be shifted 1 ft, the application of the rules only allows
for a single sprinkler or a single branch line to be moved. Since this is the
case, having all of the branch lines spaced at 10 ft 2 in. would not be
acceptable. To conform with the requirements of these two sections, a single
branch line may be spaced at 10 ft 6 in. with the other two spaced at 10 ft to
cover the 30 ft 6 in. bay. The spacing of the sprinklers along the branch line
would have to maintain maximum spacing requirements. The proposed 9 ft 10 in.
spacing would be allowed as the area of coverage on the 10 ft 6 in. branch line
does not exceed 110 ft2.
Another issue with this proposed spacing deviation is that it does not conform
with the charging language of sections referenced above. Both section
14.2.8.2.3 and 14.2.8.2.4 premise their requirement by stating that the intent
of the provision is “to eliminate obstructions created by structural elements.”
Again, that is not the case for this installation as the lines are being
shifted to provide coverage for the bay.
In a footnote to the maximum spacing requirements of Table 10.2.4.2.1(c),
standard spray uprights and pendants maximum spacing requirements are permitted
to be exceeded in 25 ft bays so that an additional branch line is not required
to be installed:
*In buildings where solid structural members create bays up to 25 ft (7.6 m)
wide, maximum spacing between sprinklers is permitted up to 12 ft 6 in. (3.8 m).
While this note is not applicable to ESFR sprinklers, a case could be made that
the basis for this allowance for extra hazard occupancies should also be
considered for ESFR sprinklers to eliminate the need for a fourth branch line
to be installed in this bay that exceeds 30 ft by a small margin.
Notwithstanding the arguments of this interpretation, the sprinkler must still
be installed per its listing and the manufacturer’s recommendations. It is
suggested that this deviation is discussed with the specific sprinkler
manufacturer as ESFR sprinkler requirements are derived from specific fire
modeling and full-scale testing. If acceptable to the sprinkler manufacturer,
the deviation should then be approved by the AHJ prior to install.
