2.3 Commodity Classifications & Storage Configurations

Commodity Classifications & Storage Configurations

High-piled storage represents the most severe fire challenge encountered in water-based fire protection engineering. As storage heights increase, the vertical surface area of combustible fuel accelerates fire growth exponentially. NFPA 13 classifies commodities into standardized categories based on the product itself, its packaging, and the pallet material, and establishes precise layout rules for rack, palletized, and solid-piled storage.

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NFPA 13 Commodity Classification Hierarchy

+-----------------------------------------------------------------------------+
|                     COMMODITY CLASSIFICATION FORMULA                        |
|       COMMODITY = BASE PRODUCT + PACKAGING MATERIAL + PALLET TYPE           |
+-----------------------------------------------------------------------------+

A common error in layout design is classifying a commodity solely by the base product. For example, noncombustible steel bolts (Class I) packaged in multi-wall cardboard boxes with heavy wood internal cradles (Class II), wrapped in polyethylene bubble wrap (Class IV or Group A Plastic), and stored on unlisted plastic pallets must be classified according to the highest hazard component of the total unit load.

+-----------------------------------------------------------------------------+
|                        COMMODITY CLASSES AT A GLANCE                        |
+-------------+---------------------------------------+-----------------------+
| Class       | Product Characteristics               | Allowable Plastics    |
+-------------+---------------------------------------+-----------------------+
| Class I     | Noncombustible in single-wall cartons | <= 5% unexpanded      |
| Class II    | Noncombustible in wood boxes/crates   | <= 5% unexpanded      |
| Class III   | Wood, paper, natural textiles         | <= 5% exp / 15% unexp |
| Class IV    | Combustible + moderate plastics       | 5-15% exp / 15-25% un |
| Group A     | Fast-burning polymers (PE, PP, PS)    | > 15% exp / > 25% un  |
+-------------+---------------------------------------+-----------------------+

1. Class I Commodity

• Definition: A noncombustible product that meets one of the following criteria:
  • Placed directly on wooden pallets.
  • Placed in single-layer corrugated cartons with or without single-thickness cardboard dividers, with or without pallets.
  • Shrink-wrapped or paper-wrapped as a unit load with or without pallets.
• Allowable Plastic Limit: Noncombustible products may contain no more than 5% by weight or volume of unexpanded plastic (e.g., protective plastic caps, friction plugs, or light plastic film wrapping).
• Representative Examples: Metal parts, noncombustible foods in glass jars or metal cans, frozen foods in waxed paper cartons, dry gypsum/cement in paper multi-wall sacks, empty glass bottles in cardboard dividers.

2. Class II Commodity

• Definition: A noncombustible product that is packaged in slatted wooden crates, solid wooden boxes, multi-wall corrugated wooden containers, or substantial combustible packaging, on or off wooden pallets.
• Allowable Plastic Limit: Same as Class I (<= 5% unexpanded plastic).
• Representative Examples: Incandescent light bulbs in multi-layer corrugated boxes, beer and wine in solid wooden crates, metal cooking appliances packaged in thick corrugated cartons with heavy wooden corner supports.

3. Class III Commodity

• Definition: A product manufactured from wood, paper, natural fibers (cotton, wool, linen), or Class I / Class II products containing combustible packaging or materials, on or off wooden pallets.
• Allowable Plastic Limits:
  • Contains <= 5% by weight or volume of expanded plastic (cellular foam), OR
  • Contains <= 15% by weight or volume of unexpanded plastic (solid plastic), OR
  • A combination of expanded and unexpanded plastic where:

$\frac{\% \text{ Expanded}}{5} + \frac{\% \text{ Unexpanded}}{15} \le 1.0$

• Representative Examples: Books, magazines, paper rolls, natural wood furniture, cotton textiles, grain/flour/sugar in paper bags, charcoal briquettes, mattresses without polyurethane foam.

4. Class IV Commodity

• Definition: A Class I, II, or III product that contains significant quantities of plastic, or products manufactured wholly or partially of Group B plastics or cartoned unexpanded Group A plastics.
• Allowable Plastic Limits:
  • Contains 5% to 15% by volume of expanded plastic, OR
  • Contains 15% to 25% by volume of unexpanded plastic, OR
  • A combination of expanded and unexpanded plastic where the total composite fuel load exceeds Class III limits but remains within Class IV thresholds.
• Representative Examples: Small household appliances with plastic housings in corrugated cartons, vinyl floor tiles, synthetic clothing and textiles (polyester, nylon blends), cartoned unexpanded plastic bottles/containers, upholstered furniture with light polyurethane foam.

Group A, Group B, and Group C Plastics

Plastics possess significantly higher heats of combustion and burning rates than cellulosic materials. Wood produces approximately 8,000 to 9,000 BTU/lb, whereas Group A hydrocarbon-based plastics produce 14,000 to 20,000 BTU/lb—comparable to liquid gasoline.

+-----------------------------------------------------------------------------+
|                        PLASTIC CLASSIFICATION MATRIX                        |
+---------+--------------------+----------------------------------------------+
| Group   | Heat of Combustion | Representative Polymers                      |
+---------+--------------------+----------------------------------------------+
| Group A | 14,000-20,000 BTU/lb| PE, PP, PS, PU, ABS, Polycarbonate, FRP, PET |
| Group B | 10,000-14,000 BTU/lb| Nylon, Silicone Rubber, Cellulosics, EVA     |
| Group C | 7,000-10,000 BTU/lb| Rigid PVC, PTFE (Teflon), Melamine, PVDC     |
+---------+--------------------+----------------------------------------------+

1. Group A Plastics (Severe Fire Hazard)

Group A plastics burn rapidly with intense heat release, heavy black soot production, and rapid pool-fire melting.

• Representative Polymers: Polyethylene (PE), Polypropylene (PP), Polystyrene (PS), Polyurethane (PU), Acrylonitrile-Butadiene-Styrene (ABS), Polycarbonate, Polyethylene Terephthalate (PET), Fiberglass-Reinforced Plastic (FRP), Plasticized Polyvinyl Chloride (PVC with > 20% plasticizer).
• Sub-Classifications:
  • Cartoned vs. Exposed: Cartoned plastics are enclosed within corrugated cardboard boxes on all sides and top, which delays plastic ignition. Exposed plastics are open to immediate flame contact.
  • Unexpanded vs. Expanded: Unexpanded plastics are solid, high-density polymers. Expanded plastics (cellular foams) contain air pockets, resulting in massive surface-area-to-mass ratios that produce explosive fire growth rates.

2. Group B Plastics (Moderate Fire Hazard)

Group B plastics exhibit moderate burning rates and heats of combustion.

• Representative Polymers: Cellulosics (cellulose acetate, cellulose acetate butyrate), Chloroprene rubber, Fluorinated ethylene propylene (FEP), Nylon, Silicone rubber, Ethylene-vinyl acetate (EVA).
• Layout Rule: Group B plastics are protected equivalently to Class IV commodities.

3. Group C Plastics (Low Fire Hazard)

Group C plastics are dense, char-forming, or self-extinguishing polymers with low heats of combustion.

• Representative Polymers: Fluoroplastics (PTFE / Teflon, PVDF), Melamine resins, Phenol-formaldehyde resins, Rigid Polyvinyl Chloride (unplasticized PVC), Polyvinylidene chloride (PVDC / Saran).
• Layout Rule: Group C plastics are protected equivalently to Class III commodities.

Pallet Factor & Plastic Pallet Deratings

Standard commodity classifications assume storage on standard wooden pallets. The use of plastic pallets introduces substantial fuel directly at the base of the rack:

• Unlisted Plastic Pallets: The use of unlisted (solid or structural foam) polypropylene or polyethylene pallets increases the commodity classification by one full class (e.g., Class I becomes Class II; Class III becomes Class IV). If the commodity is Class IV, the presence of unlisted plastic pallets upgrades the storage to Group A Plastic.
• UL-Listed Non-Propagating Pallets: Pallets tested and listed in accordance with UL 2335 or FM 4996 (fire-retardant treated polymers) do not require a commodity class upgrade and are treated the same as standard wooden pallets.

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Storage Configurations & Rack Layouts

NFPA 13 defines five primary storage arrangements, each presenting distinct thermodynamic behavior:

STORAGE ARRANGEMENT SCHEMATICS:

1. SOLID PILE / PALLETIZED:       2. OPEN WIRE RACK:           3. SOLID SHELF RACK:
   +---+ +---+ +---+                |   |   |   |                +===================+
   | P | | P | | P |                |===+===+===|                |   | Solid Shelf   |
   |---+ |---+ |---|                | P | P | P |                +===================+
   | P | | P | | P |                |===+===+===|                |   | In-Rack Req'd |
   +---+ +---+ +---+                | P | P | P |                +===================+
   (Direct floor stacks)            (>=50% open mesh)            (Blocks ceiling water)

1. Solid Piled Storage: Unit loads, boxes, or bales stacked directly on the floor or on pallets placed directly adjacent to one another without intervening aisles or rack frames.
2. Palletized Storage: Unit loads placed on pallets, with pallets stacked directly on top of lower pallets in tiered tiers without structural rack framing.
3. Bin Box Storage: Storage in small, open-faced or compartmentalized boxes not exceeding 30 in. (762 mm) in width, depth, or height, supported on structural frames.
4. Shelf Storage: Storage on shelves not exceeding 30 in. (762 mm) deep, with aisles not less than 30 in. (762 mm) wide.
5. Rack Storage: Storage on structural steel framing assemblies:
   • Single-Row Racks: Racks with a depth not exceeding 6 ft (1.8 m), separated by aisles of at least 3.5 ft (1.1 m).
   • Double-Row Racks: Two single-row racks placed back-to-back, with a total combined depth not exceeding 12 ft (3.7 m), separated by aisles of at least 3.5 ft (1.1 m).
   • Multiple-Row Racks: Racks with a total depth exceeding 12 ft (3.7 m) or continuous back-to-back racks without separating aisles.
   • Portable Racks: Moveable steel cantilever or pallet frames that can be stacked on top of one another.

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Shelving Types & Flue Space Dynamics

1. Solid Shelving vs. Open Wire Decking

• Solid Shelving Definition: Shelving that is solid (e.g., plywood, particle board, sheet metal) or slatted with less than 50% open area, where the individual shelf area exceeds 20 sq ft (1.9 m²).
• Fire Protection Consequence: Solid shelves act as horizontal umbrellas, shielding burning pallet loads beneath them from ceiling sprinkler water spray. When solid shelving exceeds 20 sq ft, NFPA 13 mandates the installation of in-rack sprinklers beneath every solid tier.
• Open Wire Decking: Wire mesh decking or steel bar grating with >= 50% uniform opening across the entire horizontal surface area allows ceiling sprinkler water to penetrate downward through multiple tiers, eliminating the solid shelf in-rack penalty.

2. Longitudinal and Transverse Flue Spaces

Flue spaces are clear, continuous vertical openings maintained throughout high-piled rack arrays. They perform two critical engineering functions:

1. Convective Heat Chimneys: Flues allow hot combustion gases to rise freely to the ceiling, rapidly fusing overhead sprinklers directly above the fire plume.
2. Water Distribution Channels: Flues provide open vertical channels through which downward-discharged sprinkler water penetrates between pallet loads to suppress the core of the fire.

RACK FLUE SPACE GEOMETRY (Plan View of Double-Row Rack):
+---------------------+  ===  +---------------------+
| Pallet Load A1      |   |   | Pallet Load B1      |
|                     |   |   |                     |
+---------------------+   |   +---------------------+
| <--- Transverse --> |   |   | <--- Transverse --> |
|   Flue (3"-6")      |   | L |   Flue (3"-6")      |
+---------------------+   | o | +---------------------+
| Pallet Load A2      |   | n | | Pallet Load B2      |
|                     |   | g | |                     |
+---------------------+   | i | +---------------------+
|                     |   | t | |
|   AISLE SPACE       |   | u | |   AISLE SPACE
|   (>= 3.5 ft)       |   | d | |   (>= 3.5 ft)
|                     |   | i | |
|                     |   | n | |
|                     |   | a | |
|                     |   | l | |
|                     |   |   | |
|                     |  (6") | |
+---------------------+  ===  +---------------------+

• Transverse Flue Spaces: Clear vertical spaces between pallet loads perpendicular to the aisle, having a minimum width of 3 in. (76 mm) to 6 in. (152 mm).
• Longitudinal Flue Spaces: Clear vertical spaces between back-to-back rack rows parallel to the aisle, having a minimum width of 6 in. (152 mm) throughout double-row and multiple-row racks.
• Flue Keeper Bars & Load Stops: Mechanical stops installed on rack beams to prevent forklift operators from pushing pallets into the longitudinal flue space, ensuring vertical flue continuity.