Chapter 12 of NFPA 13, Installation of Sprinkler Systems1 contains extensive information regarding the design of automatic sprinkler systems for storage occupancies. Along with the storage definitions in Chapter 3 and the classification of commodities in Chapter 5, a wealth of information is available to the designer. Additional information on commodity classification, storage arrangements, and sprinkler design is contained in FM Global Data Sheet 8-1, Commodity Classification,2 and Data Sheet 8-9, Storage of Class 1, 2, 3, 4, and Plastic Commodities.3 For many warehouses, use of these resources provides appropriate automatic sprinkler design guidance. There are a number of storage configurations, however, that do not fall within the parameters provided by these resources. A review of additional information and engineering judgment are both necessary to determine sprinkler designs that are appropriate to these storage situations.

Computer Models and Analytical Tools
Computer models and other analytical tools have been developed to assist the fire protection engineer in applying performance-based design techniques to a number of fire protection problems. Use of heat release rates (HRR), compartment and zone fire models, smoke generation models, and other analytical methods have enhanced the ability to predict fire development and its effects.

Most of the sprinkler protection criteria for stored commodities were developed from full-scale testing that predated the availability of the heat release rate calorimeter. Heskestad has analyzed a large series of storage tests, using mass-loss-rate data and values of effective heats of combustion to determine heat release rates, which are tabulated in The SFPE Handbook of Fire Protection Engineering. Unfortunately, a scaling factor does not exist "that would enable HRR values to be computed for stack/rack heights other than those tested."4

It is also worth recognizing that while hydraulic calculations are widely used in the design of sprinkler systems, these calculation methods only address water flowing through the piping. "There are only very rudimentary calculation methods available with regard to the most fundamental aspects of sprinkler systems, i.e., the ability of water to suppress fires."5

Modeling techniques have limited applicability to determine proper sprinkler design in warehouse buildings being designed today.

Storage Variables Affecting Sprinkler Design
Since most of the sprinkler design criteria have been developed by fullscale fire tests, a review of the various factors observed in these tests is useful in determining the expected fire in storage arrangements that do not fit into criteria outlined in NFPA 13 or other resources. There are many variables that need to be taken into consideration, including the commodity classification and packaging, storage configuration, aisle widths, storage height, pile stability, clearance between top of storage and sprinklers, type of sprinkler, as well as others.

In this discussion, three of the most significant factors will be considered:

Commodity Class
The commodity class consists of the material, its form, and its packaging, if any. The configuration of the material affects the fire intensity. For example, stacks of cut paper are considered a Class III commodity. The same paper stored on end in rolls is beyond the protection envisioned for commodity Classes I through IV and must be protected as required for roll paper. Similarly, polyethylene in pellet or granular form stored in fiberboard drums is considered "free-flowing" and may be protected as a Class IV commodity. The same plastic after it is extruded into a toy and packaged in a carton would have much different fire growth characteristics and would be protected as a Group A plastic.

The packaging can also influence the heat release rates in a storage array. Noncombustible products in slatted wooden crates are defined as Class II commodities by NFPA 13. In this case, the packaging is contributing to the combustibility of the commodity. By contrast, the packaging may limit the involvement of the material inside. Exposed plastic commodities, for example, require higher sprinkler densities than the same plastic contained in corrugated cartons since the cartons absorb sprinkler water and delay involvement of the plastic material.

Storage Arrangement
NFPA 13 clearly defines several types of storage. Palletized, solidpiled, bin box, and shelf storage generally are grouped together due to their similar fire development characteristics. Air spaces or flues exist only where there is imperfect contact or where a pile is close to, but not touching, another pile. The bulk of flame spread in these storage arrays is on the faces of the storage, although palletized storage provides horizontal air spaces which may assist the spread of fire within the pile, mostly out of reach of water from sprinklers. Shelf and bin box storage permit fire spread into areas shielded from sprinkler water. Even with these limited areas that are shielded from sprinkler water, densities outlined in NFPA 13 will provide adequate control for these types of storage arrangements.

Rack storage presents more of a challenge to control by automatic sprinklers because there are increased numbers of burning surfaces, combustibles are held in a stable arrangement that is ideal for fire growth, and there is greater difficulty for sprinkler water to penetrate through narrow vertical spaces in the rack. Control of rack storage fires is partially dependent on the size and location of aisles to provide breaks in the continuity of combustibles. NFPA 13 requires higher ceiling sprinkler densities in storage configurations with narrow aisles [less than 2.4 m (8 ft) wide]. In-rack sprinklers may be mandatory in multiple-row racks, depending on the rack depth, commodity classification, and storage height.

Solid shelving within racks further restricts sprinkler water-penetration into the racks. Depending on the size of the solid shelf, NFPA 13 may require in-rack sprinklers.

Sprinkler Type
The development of sprinklers with larger k factors, largedrop sprinklers, and early suppression fast response (ESFR) sprinklers have provided improved protection of storage while allowing for a variety of commodities stored in different storage arrangements with the same sprinkler design. Nonetheless, there are situations where the use of these specialized sprinklers may be inappropriate. A review of the design criteria and limitations contained in NFPA 13 is necessary-to determine the appropriate sprinkler type for storage arrangements that are not specifically included in NFPA 13.

Examples of Storage Arrangements Not Addressed by NFPA 13

There are several storage situations that are not specifically addressed by NFPA 13. The following examples suggest judgments that should be included in determining appropriate sprinkler design criteria. For these examples, the basic stored material remains the same, with a variety of packaging and storage arrangements that affect proper sprinkler design.

Commodity Class
The stored material for these examples is household furniture, including mattresses and upholstered furniture. For purposes of the example, the storage consists of solid-core expanded polyurethane mattresses, and wood frame chairs and couches with expanded polyurethane cushions. Due to the many variables addressed by NFPA 13 to determine the proper sprinkler design, these examples are limited to storage height less than 7.6 m (25 ft).

NFPA 13 defines polyurethane as a Group A plastic (5.6.4.1), with further guidance in Annex A (Table A.5.6.3) that furniture with foam plastic cushioning and foam mattresses are classified as Group A plastics.

Palletized and Rack Storage
For palletized storage, control mode sprinkler protection criteria are provided for plastics in cartons or exposed (not in cartons). NFPA 13 does not provide ESFR sprinkler protection criteria for exposed expanded plastic (Section 12.2.3.3).

A furniture warehouse often uses a rack storage arrangement since palletized storage of upholstered furniture is often not practical. If the furniture is stored in typical singlelayer corrugated cartons, protection guidelines are provided in NFPA 13 for control mode and ESFR sprinklers.

If the furniture is not stored in cartons, NFPA 13 does not provide sprinkler design criteria for exposed, expanded Group A plastics stored in racks (Section 12.3.3.3). Use of FM Data Sheet 8-93 may therefore be appropriate, since protection criteria for exposed (uncartoned) expanded plastics are provided for control mode and ESFR (suppression mode) sprinklers.

Storage in Crates
If the same furniture is placed in substantial wood crates, as may be the case in a moving and storage company facility, with crates stacked two-to three-high, sprinkler protection indicated in NFPA 13 for solid piled/palletized storage would be appropriate. Since the commodity class of expanded plastic materials in wood crates is not addressed specifically by NFPA 13, judgments must be made based on the expected fire development. For palletized storage, Note A in Table 12.2.3.1.1 indicates that when a Group A plastic ( expanded or unexpanded) is contained in cartons consisting of multiple layers of corrugation or equivalent outer material that would significantly delay fire involvement of the Group A plastic, it may be treated as a Class IV commodity. It seems clear that crated upholstered furniture may be treated as a Class IV commodity rather than a plastic commodity. Assuming that the wood crates are of substantial construction and that no furniture is located in the facility that is not crated, the fire development will primarily involve the outside of the wood crates where sprinkler water can reach the majority of surfaces. A case could be made that the fire development and heat release rates will be more like a Class III commodity.

If the wood crates were slatted, early involvement of the plastics in the fire would be expected. A determination would have to be made if the fire development would more closely resemble cartoned or exposed plastics. Most likely, the storage would have to be treated as an exposed plastic.

Storage in Shipping Containers
Now suppose the crated furniture is placed in an overseas steel shipping container, which is sealed and located in a separate warehouse awaiting shipment. In this case, it is unlikely that the contents of the container would contribute to the fire development. Combustibility of the warehouse is similar to Class I commodity storage, or it possibly could be considered noncombustible storage.

Storage in Portable Self-Storage Containers
Several companies provide self-storage containers that are delivered to a home or business where they are packed by the client. The containers are then stored in warehouse buildings.

Containers are typically 2m to 2.5 m (7 ft to 8 ft) tall, and range in length and width from 1.5 m x 2.5 m (5 ft x 8 ft) to as large as 2.5 m x 6 m (8 ft x 20 ft). The containers typically are wood frame with plywood sides and floor. Larger containers may have wood, steel, or aluminum frames. A hinged or roll-up door is provided for loading and unloading. Weather resistance is provided by solid plastic tops, weather-resistant skins over the plywood sides, or containers are encapsulated with removable weather resistant covers.

Depending on the size, containers can accommodate at least one standard room of household furniture, while the larger containers can often accommodate the contents of 150 m2 to 250 m2 (1,500 sq ft to 2,500 sq ft) homes.

The containers may contain diverse contents, typical of a household. Contracts may prohibit the storage of hazardous or flammable materials in the containers, as is typical in self-storage buildings. Commodities can range from Class I to Plastics. To continue with the previous examples, the following discussion will assume the contents to be solid-core expanded polyurethane mattresses, and wood frame chairs and couches with expanded polyurethane cushions.

In a warehouse, the storage units may be stacked two-or three-high. This storage array is similar to solidpiled/palletized storage of crated furniture discussed previously and the involvement of the contents in a fire starting outside the unit would be delayed, permitting sprinkler water to control a fire before involving the contents.

There are several differences with this storage arrangement, however, than was the case with crated furniture. Containers with doors and/or plastic tops may permit fire extension into the interior of the storage unit more quickly than would be the case with a solid wood crate. A fire originating or spreading to the inside of a container can be divided into two subcategories. One of these scenarios would include a fire involving a container stacked only one high or a container at the top of a pile in a multistacked arrangement. A fire can be expected to be controlled by a sprinkler system designed with the appropriate density for the commodity class.

On the other hand, a fire originating or extending within a container which is located at the bottom of a multi-stacked arrangement is shielded from sprinkler water, permitting fire growth to the entire container contents. Furthermore, the probability of fire spread to adjacent containers, particularly those located immediately above or stacked immediately adjacent to the container, becomes more prevalent.

Fire development within the storage unit may more closely resemble fire development in a rack storage arrangement. Although the sides of the storage container will act as a barrier to horizontal fire development, the fire characteristics may be similar to a doublerow 2.4 m (8 ft) wide rack section 4.9 m (16 ft) or longer with solid shelves [ resulting in 12 m2 (128 sq ft) of shelves]. NFPA 13, Section 12.3.1.9, indicates that solid shelving greater than 1.9 m2 (20 sq ft) requires in-rack sprinklers. As further amplified in Annex C.11, the limited full-scale testing of racks with solid shelves protected by ceiling sprinklers resulted in a large number of sprinklers operating and extensive spread in the rack array. Bulkheads within the racks and other items may influence proper sprinkler protection; however, insufficient data was developed by the testing program to determine a comprehensive protection standard. FM Data Sheet 8-93 provides criteria for rack storage with solid shelves, requiring rack sprinklers for all commodity classes when stored higher than 3 m (10 ft) in racks with solid shelves exceeding 6 m2 (64 square feet) in area, and for many configurations of storage height and building height when solid shelves exceed 1.9 m2 (20 sq ft) Obviously, sprinklers cannot be installed within the storage containers. Appropriate ceiling sprinkler protection must consider the shielded conditions.

Based on the preceding information, the use of ESFR sprinklers seems inadvisable due to the potential for shielded fire conditions. ESFR sprinklers are particularly sensitive to obstructions. Therefore, control mode sprinklers designed to meet two separate and distinct sprinkler densities and remote areas may be appropriate. One point would include a higher sprinkler density with a smaller remote area. This strategy would serve to control fires outside of containers or within containers which are at the top of a pile. The second point would include a lower sprinkler density with a large remote area. This strategy would serve to address the shielded fire condition occurring within a container located at the bottom of a pile in a multi-stacked arrangement. This criterion would provide wetting of adjacent containers so as to minimize the potential for fire spread and support a larger number of sprinklers operating.

Consideration should be given to providing increased hose demand for firefighting operations. Where water supplies permit, an increased hose demand will allow further flexibility to fire departments to be able to control fires for these storage configurations. Each facility will need to be evaluated individually in order to determine appropriate measures.

It is also worth examining the probability for fire ignition particularly within the containers. The potential for autoignition would be expected to be remote given that hazardous and flammable materials are not to be stored in the containers pursuant to the leasing contracts. One of the more credible sources of ignition, however, would be arson. A purposely set fire would expect to be originated in a container that is lowest to the ground, particularly where containers are stacked several high.

Thomas Allen and Renato Molina are
with Rolf Jensen & Associates, Inc.

References

  1. NFPA 13, Installation of Sprinkler Systems, National Fire Protection Association, Quincy, MA, 2002.
  2. FM Global Data Sheet 8-1, Commodity Classification, FM Global Johnston, RI, 2004.
  3. FM Global Data Sheet 8-9, Storage of Class 1, 2, 3, 4 and Plastic Commodities, FM Global Johnston, RI, 2005.
  4. Babrauskas, V., "Heat Release Rates," The SFPE Handbook of Fire Protection Engineering, Third Edition, National Fire Protection Association, Quincy, MA, 2002.
  5. Fleming, R., "Automatic Sprinkler System Calculations", The SFPE Handbook of Fire Protection Engineering, Third Edition, National Fire Protection Association, Quincy, MA, 2002.