Researching cost-effective protection arrangements for solid shelf rack storage

NFPA Journal, Jul/Aug 2003 by Fleming, Russell P

THE FIRE PROTECTION RESEARCH

Foundation is currently looking into a new project, tentatively named the Solid Shelf Rack Storage Protection Research Project, to develop possible rack configurations and cost-effective protection arrangements as an alternative to the current criteria in NFPA 13, Installation of Sprinkler Systems.

Why now? Because the 2002 edition of NFPA 13 more definitively spells out the protection criteria for racks with solid shelves, thanks to a task group that reviewed all available test data relating to sprinkler performance with solid shelves.

Racks with solid shelving are different from traditional shelf storage, in which the shelves, by definition, are limited to a depth of 30 inches (0.76 meters). Shelves are usually spaced 2 feet (0.6 meters) apart vertically and back-to-back shelves are generally separated by a vertical barrier. Storage racks are typically about 4 feet (1.2 meters) deep, and, in a typical double-row configuration, present a considerable challenge to sprinklers, especially when the normally open racks are covered with some form of shelving.

The first thing the task group did was define what a solid shelf is. Group members agreed that the area of a solid shelf is bounded by the perimeter aisle or flue space on all four sides. Flues are either longitudinal or transverse. In typical double-row rack storage, the longitudinal flue space is the long, narrow area between the racks, and the transverse flue spaces are those between pallet loads as well as those between the bays formed by the rack structure. Flues allow heat from a fire to rise to activate ceiling sprinklers and allow water from the overhead sprinklers to penetrate the rack to stop the spread of a fire. Without open flue spaces, fires have a greater tendency to spread laterally throughout racks, becoming large enough to jump the aisles to adjacent racks.

The task group agreed that solid shelves less than 20 square feet (1.86 square meters) between flue spaces are not a concern, since this is basically the size of a standard pallet Task group members also agreed that racks with solid shelves made of wire mesh, slats, or other materials more than 50 percent open can be considered open racks, provided flue spaces are maintained. This means that racks surfaced with such materials that extend to the limits of the longitudinal and transverse flues can be considered open racks, even if the shelves' surface is bigger than 20 square feet.

The sprinkler protection criteria for rack storage dates back to the 1971 edition of NFPA 231C, Rack Storage of Materials, which was merged into NFPA 13 in the 1999 edition. NFPA 231C was based on an extensive series of fullscale fire tests, some of which revealed the poorer performance of ceiling sprinklers in racks using solid shelves, even shelves 27 square feet (2.5 square meters) in area. In those early editions, NFPA 231C always required that "sprinklers be installed both at the ceiling and beneath each shelf in doubleor multiple-row racks with solid shelves that obstruct both longitudinal and transverse flue spaces." Another section of the standard traditionally allowed one to eliminate the longitudinal flue for double-row racks without solid shelves up to 25 feet (7.6 meters) high. However, it was never clear what really constituted solid shelves or what was to be done if there were longitudinal flues but no transverse flues.

The new criteria, based on the areas of solid shelves, distinguish between solid shelves 20 to 64 square feet (1.86 to 5.95 square meters) and those larger than 64 square feet. For the larger shelves, the requirement for sprinklers below each shelf has been retained. For the intermediatesized shelves, sprinklers are required at maximum vertical intervals of 6 feet (1.83 meters), but not necessarily under every shelf. For intermediate-sized solid shelves spaced 2 feet apart vertically, a level of sprinklers is only required under every third shelf.

There are some exceptions. For example, slatted shelves are generally considered the same as solid shelves. However, a special section of NFPA 13 allows slatted-shelf storage arrangements involving specific types of ceiling-only sprinklers based on the sprinklers' effectiveness in full-scale fire tests.

The proposed research program may also lead to specific alternate arrangements by which sprinklers can provide adequate protection for storage on solid shelves. In the meantime, users of NFPA 13 have a better understanding of how to protect shelf storage in racks.

RUSSELL P. FLEMING, P.E.