Storage configurations

Most warehouse storage is either solid pile on floor, on pallets, or on racks. A small but growing fraction of warehouse storage is in open top bins or wire baskets. Automated Storage and Retrieval is another small but important storage configuration. Fire protection considerations inherent in each configuration are described here.

Solid piled storage of cartoned commodities is storage without any spaces between cartons in each stack. The only exposed surfaces in solid piled storage are the outer walls and top surface of the storage pile. Large, structurally stable commodities can be solid piled quite high, but many smaller cartoned commodities cannot be solid piled higher than about 3.7 m (12 ft). The relatively

Table5.3.Representativeverylargelosswarehousefires(atleast$20millionperloss) YearCompanyLocationStoreditemsStorage area (1000sqSt) Building height(ft)Storage height (ft)

Storage configurationProperty damageContributingfactors 1977FordMotorCologne,W.G.AutoParts10003017–20BasketsIn Racks>$100MAisleStorage,Plastics, MotorOil 1978Montgomery WardBensonville,ILGen.Merchandise200???$30MAisleStorage 1979Supermarket GeneralEdison,NJGen.Merchandise290?20Rack$30MAerosols 1981KMartFalls Township,PAGen.Merchandise12003015Rack, Palletized>$100MAerosols,IneffectiveFire Wall 1984HanworthLondonComputerEquipment78??Rack$65MNoSprinklers 1984Cricklewood TradingLondonGen.Merchandise750a25–50a20–30?$100MPartiallySprinklered,Cocoa Butter 1983Multi-OccupancyBradford, EnglandGen.Merchandise330FourStories?Rack$25MNoSprinklers,LPG Cylinders 1983BritishArmyDonnington, EnglandArmySupplies4403330Rack$330MNoSprinklers,RoofVents 1985MTM(Mitsui)Elizabeth,NJAerosols,Gen. Merchandise5002417Rack$150MAerosols,IneffectiveFire Walls 1986SandozBasel, SwitzerlandChemicals5026–4015–20Palletized Drums$20MNoSprinklers,Flammable Liquids 1987Service MerchandiseGarland,TXGen.Merchandise2002524Rack$52MDelayedDetection, High-PiledStorage 1987Sherwin- WilliamsDayton,OHPaint,Solvent1803014–16Palletized, Rack$49MFlammableLiquids, Aerosols aTheCricklewoodwarehousecomplexfireinvolvedninebuildings

Up to 4 layers or 5 m in height

1.2 m

1.2 m

Figure 5.1. Palletized storage with 1.2 m (4 ft) wide aisles

small exposed surface area and restricted air access in solid piled storage restrict burning rates compared to other storage modes.

Palletized storage entails stacking and handling of pallet loads on top of each other as illustrated in Figures 5.1 and 5.2. The pallets are typically 1.1 – 1.5 m² (12 – 16 ft²) and about 15 cm (6 in) deep with slatted upper and lower surfaces. Pallet surface areas as well as commodity wall surfaces are exposed to flame, and the pallets allow air access to burning horizontal surfaces. Fire growth rates in palletized storage are comparable to those in solid piled storage. This implies that the early fire development is primarily governed by the exposed vertical surfaces available for flame spread rather than the exposed horizontal surfaces. Indeed, tests have shown that doubling the number of vertical exposed surfaces by providing transverse flue spaces as well as longitudinal flue spaces between storage stacks, does cause the rate of fire growth to increase substantially.

Palletized storage is usually more stable than solid piled storage and has somewhat greater air access during burning because of the pallet spacing. These comparisons, which imply an enhanced flammability of rack storage, are summarized in Table 5.4.

Rack storage entails placing each unit load on a structural steel rack with open, slotted, or solid shelves. The rack structures provide greater load carrying capacity and stability than either solid piled or palletized storage. Consequently, greater storage heights can be achieved with rack storage. Figure 5.3 illustrates how narrow aisle, side loading high lift trucks allow access to upper tier storage at heights up to 12.2 m (40 ft). Since rack storage tiers are often 1.5 m (5 ft) high, this corresponds to unit load storage up to eight tiers high. Rack supported building structures can go significantly higher than the eight tiers.

Figure 5.2. Warehouse palletized storage

Table 5.4. Comparison of storage configuration burning characteristics Solid piled Palletized Rack storage Exposed surfaces per unit

load

4 4+ pallet 5+ pallet

Air access Restricted Less restricted Least restricted

Load stability Least stable More stable Most stable

Water spray access Depends on longitudinal and transverse flues

Storage racks are termed single-row, double-row, or multiple-row, depending on the row spac-ing. Single-row racks have aisle spacing of at least 1.1 m (3.5 ft) on both sides of each row.

Double-row racks are separated by a relatively narrow longitudinal flue on one side and by a wider aisle on the other side as illustrated in Figure 5.3. Multiple-row racks are racks greater than about 3.7 m (12 ft) wide or single-row racks or double-row racks with aisle spacing less than 1.1 m (3.5 ft). The relatively narrow spacing between rows in multiple-row racks usually promotes more rapid fire spread across rows.

Unlike solid piled and palletized storage, the unit load in each tier of rack storage has an exposed top surface. This exposed top surface may not necessarily increase the early fire devel-opment rate, but it does contribute to the burning rate eventually and allows for greater air access in ventilation limited fires. (If the racks contain solid shelves, sprinkler spray access to the lower burning tiers will be obstructed unless in-rack sprinklers are employed.) The overall effect is that rack storage is a greater fire protection challenge than either solid piled or pal-letized storage. This increased challenge has resulted in the average property damage for rack storage fires being more than twice as large as the average property loss in solid piled and palletized storage.

Most warehouse storage racks have open or slatted shelves to support the stored commodities.

This is a critical feature for protection by ceiling sprinklers because it allows water to reach

Aisle width

Tier height

L

Figure 5.3. Palletized rack storage with indicated aisle width, tier height, and longitudinal flue space, L

the lower storage tiers. Racks with solid or slatted shelves provide a much greater challenge, and often need in-rack sprinklers to supplement ceiling sprinkler protection. One example of the effect of solid shelving is a pair of comparison fire tests conducted with 20 ft high rack storage of plastic commodity protected by K = 11.4 gpm/psi^1/2 ceiling sprinklers at a discharge density of 0.60 gpm/ft²(Troup, 1994). In the test with slatted wood shelves, only four sprinklers actuated and controlled the fire with only three pallet loads of commodity consumed. The test with solid wood shelves resulted in the fire spreading to the ends of the storage array, with 15 pallet loads consumed and 15 ceiling sprinklers opening.

Storage in open top bins is far less prevalent than solid piled, palletized, and rack storage, but it is gaining in popularity, particularly for small parts storage in electronic assembly facilities.

Fires in the open top bins are usually slow in developing but can eventually become quite intense.

This occurred in the fire tests conducted at the British Fire Research Station (Field and Murrell, 1988) using a bin storage configuration with 1.2 m wide by 2.3 m high (3.9 ft wide by 7.5 ft high) aisles covered with a chipboard. There were a variety of bins fabricated from fiberboard, steel boxes, metal shelf trays, cardboard boxes, and timber and cardboard. Storage within the bins included small metal parts, paper, leather and plastic. Test results indicated that automatic

Figure 5.4. Large open automated storage and retrieval system

Figure 5.5. Enclosed ASRS photo with CO2 suppression system. Reproduced by permission of White Systems, Inc.

sprinklers installed centrally beneath the walkway at a maximum spacing of 2 m (6.6 ft) would provide the best available fire protection.

Bins are often used in small Automated Storage and Retrieval Systems (ASRS). In-rack sprin-klers are recommended by Field and Murrell (1988) for ASRS with combustible containers stored over 3.6 m (12 ft) high. The larger ASRS consist of a large unenclosed rack storage system with a motorized picker installed on rails in the aisle as shown in Figure 5.4. Fire protection choices for these systems are equivalent to those for high-rise rack storage. The smaller ASRS are sometimes comprised of enclosed modular units such as the one shown in Figure 5.5. The particular ASRS shown in Figure 5.5 is protected by a carbon dioxide system, where the CO2 is stored outside the unit and piped to discharge nozzles within the module.