Using elevated walkways in storage configurations may create configurations that fall outside the scope of published automatic sprinkler standards. Just adding sprinklers under elevated walkways may not provide effective fire protection.

Unless a storage configuration with elevated walkways is proven by full-scale fire tests, provide a solid mezzanine at each walkway level, and then provide automatic sprinklers below each mezzanine.

Introduction Storage configurations may be fitted with elevated walkways in aisles to allow access to goods by personnel

who manually store and retrieve the goods. This configuration may be used by facilities such as customer

order fulfillment centers, distribution centers, and records retention warehouses.

This document provides guidelines for storage configurations with elevated walkways in aisles which are

aligned with published third-party automatic sprinkler standards. In addition, Zurich loss experience is

consistent with the guidelines.

Discussion Scope of this document

This document applies to storage configurations where two or more adjacent aisles have elevated walkways

and the aisles do not exceed 2.4 m (8 ft.) wide, and may be subject to change based upon either research or

loss experience.

Storage configurations

Elevated walkways may be used with a variety of storage configurations including:

• Rack storage

RiskTopics Automatic sprinklers – Storage & elevated walkways in aisles July 2017

• Shelf storage

• Shelf storage with vertical bulkheads (bin-box)

Appendix A offers additional information on each of these storage configurations.

Elevated walkway configurations

Elevated walkways may have solid or open grate decking.

• Solid decking may include walkways with a solid surface such as plywood

• Open grate decking may include walkways formed with open metal grate

View below an open metal grate mezzanine covered with an engineered wood deck

(Photo source: Rich Gallagher, The Zurich Services Corporation)

Elevated walkways and aisle fires

Elevated walkways can interact with heat rising from a fire in the aisle way below.

• Solid walkways may form a ceiling and capture heat from a fire occurring in the aisle below. The captured

heat may then be directed horizontally in the form of a “ceiling jet” towards the nearest sprinklers under

the solid walkway.

• Open grate walkways may allow heat from a fire below to pass through the walkway and continue rising

vertically. A ceiling jet may not be formed, and heat may not be directed horizontally to the nearest

sprinklers under the walkway. If a fire is not located directly below a walkway sprinkler, there may be a

delay in the operating sprinklers below the walkway.

Elevated walkways and ceiling sprinkler discharge

Elevated walkways may interfere with water discharged from ceiling sprinklers.

• Solid walkways may form obstructions to the water discharged from ceiling sprinklers.

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Open grate walkways may not fully obstruct the passage of water; however, they may adversely disrupt a

descending water spray distribution. In addition, NFPA’s Automatic Sprinkler Systems Handbook (2016)

points out that fire experience shows burning goods may fall onto a walkway and form an obstruction to

water spray even if the walkway has an open grate deck.

Operation of sprinklers – In-racks vs. under elevated walkways

As indicated in NFPA’s Automatic Sprinkler Systems Handbook (2016), sprinklers located in racks operate once

they are immersed in the heat plume of a fire. Since the heat plume of a rack fire is expected to rise up the

rack flues, sprinklers located outside of rack flues are not likely to operate in a timely manner. Sprinklers

located under elevated walkways in aisles between racks are examples of sprinklers located outside of rack

flues.

Automatic sprinklers and storage

Full-scale, rack storage fire tests are generally based upon configurations where:

• Transverse and longitudinal flue spaces are provided within racks

• Open aisles (with no elevated walkways) are provided between racks

The typical rack fire scenario may have a sequence such as:

• A fire initiates in a rack and spreads vertically up through a rack flue

• In-rack sprinklers operate once they are exposed to sufficient heat within the vertical heat plume within the

rack flue

• Where the fire spreads to the top of the rack, the vertical heat plume exits the top of the rack, travels

vertically to the ceiling, forms a ceiling jet, reaches nearby ceiling sprinklers, and heats the nearby ceiling

sprinklers until they operate

• Discharge from operating in-rack and ceiling sprinklers penetrate down into the rack flues to help control

or suppress the fire and help limit horizontal fire spread along the rack

• Discharge from ceiling sprinklers also pre-wet goods across aisles to help limit horizontal fire spread from

rack to rack

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Example of full-scale rack fire test with open aisle

(Photo source: Rich Gallagher, The Zurich Services Corporation)

Benefits of open aisles

When a fire occurs in a rack, open aisles may help ceiling sprinklers pre-wet goods in adjacent racks and

reduce the likelihood of fire jump across the aisle. When pre-wetting is compromised, fire jump across an aisle

may occur. When conditions are present that may allow fire to spread across aisles, there is a concern

sprinklers may fail due to uncontrolled horizontal fire spread.

Storage configurations with elevated walkways

The success of sprinklers may be adversely affected by the introduction of elevated walkways in the aisles.

While vertical openings may continue to support the vertical spread of heat and fire, it is not clear the

appropriate sprinklers will operate or be effective.

A number of sprinkler arrangements have been proposed or installed to protect storage with elevated

walkways. These arrangements have included:

• Sprinklers in the storage racks only (no sprinklers under the elevated walkways)

• Sprinklers under elevated walkways only (no sprinklers in the storage racks)

• Sprinklers in the storage rack and under the elevated walkways

These arrangements should be validated by full-scale fire tests to help ensure their appropriateness for the

specific storage and walkway configuration.

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Guidance For storage configurations within the scope of this document, protect storage configurations with elevated

walkways using a design proven by full-scale fire test. Where a design is based upon full-scale fire tests, see

the Risktopic titled Zurich Recognized Solutions for Property Risks for additional guidance. When storage

configurations are not backed up by full-scale fire tests, consider the following guidelines:

Provide solid mezzanines

Provide solid horizontal mezzanines at each walkway level. Providing solid mezzanines specifically includes:

• Closing all openings between solid walkways and solid shelves where the shelves form part of the

mezzanine ceiling.

Example of a storage array with solid elevated walkways and solid shelving at the elevated walkway levels

(Image source: Rich Gallagher, The Zurich Services Corporation)

• Closing openings between rack uprights.

Example of opening at the rack uprights (Photo source: Malcolm Davies, Zurich Risk Engineering)

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As a note, no publically available research has been identified that establishes the allowable size of gaps

around each rack upright. Floors and shelving should be installed without gaps; however, until research is

available, consider limiting gaps to 12 mm (0.5 in.) around each individual rack upright.

Provide sprinklers below mezzanines

Provide sprinklers below each mezzanine in accordance with third-party automatic sprinkler standards

recognized by Zurich (see Appendix A of the Risktopic titled Zurich Recognized Solutions for Property Risks for

additional guidance). The sprinkler design will be based upon typical factors such as:

• Categorization of the goods in storage (commodity classification)

• Configuration of the storage (open-frame rack, solid shelf, solid shelf with vertical bulkheads)

• Storage height and mezzanine ceiling height

• Clearance from the top of storage to the sprinkler deflectors below the mezzanine

Loss Scenario – Solid mezzanine floors with gaps between rack uprights

A five-level pick module was formed around single-row racks. Rack depths and aisle widths both were

estimated at about 1.2 m (4 ft.). Mezzanine floors were solid except for 80 mm (3 in.) wide gaps between

the rack uprights.

A fire started on the second mezzanine level. The fire spread to the third, fourth, and fifth mezzanine

levels via the gaps in the mezzanine floor between the rack uprights. One sprinkler operated in the second

mezzanine level, four sprinklers operated in the third mezzanine level, six sprinklers operated in the fourth

mezzanine level, and six sprinklers operated at the ceiling above the fifth mezzanine level. In all, 17

sprinklers operated.

It is anticipated this fire could have been more challenging had the fire started on the first mezzanine

level.

It is anticipated the fire damage may have been limited to one mezzanine level and smoke and water

damage would have been reduced if the mezzanine floors did not have gaps between the rack uprights.

Loss scenario – Solid mezzanine floors without gaps

This case involved a three-level pick module with solid mezzanine floors. Shelves included vertical

bulkheads (bin-box).

A fire started on the second mezzanine level. The fire was contained to the second mezzanine with one,

control mode sprinklers operating.

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Provide sprinklers of appropriate response time index (RTI)

Sprinklers installed below a mezzanine should have a RTI equal to or less than the RTI of the building ceiling-

level sprinklers.

Where roof level sprinklers have a fast RTI (such as ESFR sprinklers), also install sprinklers with a fast RTI below

each mezzanine.

Where roof level sprinklers have a fast RTI (such as ESFR sprinklers) but standard RTI sprinklers have been

installed below the mezzanines, provide a draft curtain at least 0.6 m (2 ft.) deep around the perimeter of the

mezzanines.

See Appendix B for further information regarding sprinkler response time index (RTI).

Conclusion Elevated walkways in storage have become common place. Unless the selected sprinkler design for the

specific walkway and storage configuration is proven by full-scale fire test:

• Provide solid mezzanines at each walkway level

• Provide automatic sprinklers below each mezzanine using a design suitable for the storage below

References Risktopic. Zurich Recognized Solutions for Property risks. Zurich, Switzerland: Zurich Insurance Group Ltd.,

2016. Web. Web accessed 20170228.

http://www.zurichservices.com/zsc/reel.nsf/9f359b3938a6bdd448257a4f001c4596/72e45b8bedeaf19dc1257

ffc002df342/$FILE/rf_RecognizedSolutions_Property.pdf

LPC Rules for Automatic Sprinkler Installations Incorporating BS EN 12845. United Kingdom: Fire Protection

Association (FPA), 2015. Print

CEA 4001en. Sprinkler Systems: Planning and Installation. Brussels: European insurance and reinsurance

federation, 2009. Web. Web site accessed 20140306. http://www.denikoo.com/wp-

content/themes/GoMag/doc/zakoni/CEA%204001%20-%201237304692_cea4001s.pdf

NFPA 13. Standard for the Installation of Sprinkler Systems. Quincy, MA; NFPA, 2016. Online.

Klaus, Matthew J. Automatic Sprinkler Systems Handbook. Quincy, MA: National Fire Protection Association,

2016.

Richards, Gwynne. Warehouse Management: A Complete Guide to Improving Efficiency and Minimizing

Costs in the Modern Warehouse. London: Kogan Page, 2011. Print.

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Appendix A This appendix captures definitions for terms used in this document. Most definitions are based upon guidance

from the Zurich Property Protection Principles listed in the following table.

Source abbreviation Full name of reference

BS EN 12845 LPC Rules for Automatic Sprinkler Installations Incorporating BS EN 12845. United Kingdom: Fire Protection Association (FPA), 2015.

NFPA 13 NFPA 13. Standard for the Installation of Sprinkler Systems. Quincy, MA; NFPA, 2016.2

The following table summarizes storage configurations likely to be found in pick modules addressed by this

document. The table offers a comparison of storage designations and descriptions as found in the LPC Rules

(specifically technical bulletin TB234) and NFPA 13.

BS EN 12845 TB234

designation

BS EN 12846 TB234 description NFPA 13 description

ST4 • Rack storage

• Beam pallet racking

• Single-row beam racking

• Double-row beam racking

• Rack

• Single-row rack

• Double-row rack

Note: The ST4 configuration typically consists of goods on a pallet, and the pallet is supported on the beams of an open-frame rack. In a pick module, shelving may be added to allow goods to be supported without pallets. Where suitable flues are not maintained, the ST4 configuration may become a ST15 or ST16 configuration. See additional information in the definition for “shelving”.

ST15 Shelf storage with a depth not exceeding 0.8 m (30 in.)

Shelf storage

Note: This configuration consists of shelves without vertical bulkheads.

ST15 Shelf storage with open fronted, 5-sided, metal bin box

Bin box storage

Note: This configuration includes transverse and longitudinal vertical bulkheads.

ST16 Shelf storage with a depth exceeding 0.8 m (30 in.)

Rack storage with shelves

Note: This configuration consists of shelves without vertical bulkheads. This configuration is similar to ST4 (a rack) due to its depth; however, this configuration includes shelves that introduce a need for in-rack sprinklers.

NA NA Back-to-back shelf storage

Note: This configuration is only addressed in NFPA 13. Protection guidance is based upon full-scale fire tests.

8

Back-to-back shelf storage

“Back-to-back shelf storage” is a term defined in NFPA 13 term. It is a storage configuration that is formed by

two shelves separated by a vertical, longitudinal bulkhead.

The shelf on either side of the vertical, longitudinal bulkhead is limited to a depth of 760 mm (30 in.). This

means the overall depth of the back-to-back shelf storage system is not to exceed 1,520 mm (60 in.).

Guidance for back-to-back shelf storage was based upon full-scale fire tests.

Bin box

“Bin box storage” is a term defined in NFPA 13. See “shelf storage with vertical bulkheads”.

Flues

The typical double-row rack configuration includes longitudinal and transverse flues as shown in the following

figure.

Longitudinal and transverse flues in racks (Image source: Rich Gallagher, The Zurich Services Corporation)

Flues are nominally 150 mm (6 in.) wide, and transverse flues normally occur at 1.2 to 1.5 m (4 to 5 ft.)

intervals along the length of a rack.

“Nominally 150 mm (6 in.)” means the flue is at least 75 mm (3 in.) wide. This width may be formed due to

the non-uniform alignment of flues vertically from tier-to-tier, or it may be formed due to the careful

placement of goods (e.g. the case of an automatic storage and retrieval system).

In multiple-row racks, longitudinal flues are not required; however, where they are provided, they are to be

nominally 150 mm (6 in.) wide. This nominal width is needed to allow the fire plume to grow vertically (rather

than horizontally) and to allow water from sprinklers to penetrate down into the rack.

Rack

A rack is a structure used to support goods.

A rack may consist of an open frame used to support pallet loads of goods. The pallet loads are placed in an

organized manner to maintain longitudinal and transverse flues. See “flues” for further information on

longitudinal and transverse flues

A rack may include shelves to provide support to goods stored with or without a pallet.

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Many full-scale fire tests are based upon goods on pallets stored in double-row, open-frame racks with open

aisles at least 1.2 m (4 ft.) wide. The following photo is an example of a rack storage fire test array.

Example of open frame rack storage

(Photo source: Rich Gallagher, The Zurich Services Corporation)

Shelf storage

This is a term used in BS EN 12845 and NFPA 13.

Within the scope of BS EN 12845, shelves may have any depth. In TB 234, shelf depths not exceeding 0.8 m

(30 in.) are designated ST15, and in-rack sprinklers are only recommended. Where shelf depth exceeds 0.8 m

(30 in.), the storage system is designated ST16, and in-rack sprinklers are required.

Within the scope of NFPA 13, shelf storage is limited to shelves with a depth of 0.8 m (30 in.). Where shelves

are wider, the storage system becomes a rack.

Example of shelf storage (Image source: Rich Gallagher, The Zurich Services Corporation)

Shelf storage with vertical bulkheads

This is a term used in BS EN 12845. It is similar to the term “bin box” used in NFPA 13. The term “shelf

storage with vertical bulkheads” is more descriptive of the storage configuration.

This storage system uses solid shelves; however, in addition, longitudinal and transverse bulkheads are also

provided. This configuration provides 5-sided compartments where goods are stored. The open side faces an

aisle.

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NFPA 13 defines bin boxes as being made of metal, wood, or cardboard. Zurich interprets this to mean all five

sides of the storage system is to be made of the same material. For example, if the compartments are made of

cardboard, the shelves are to be made of cardboard. Applying this interpretation means the compartments

will often be formed entirely of metal or they will be treated as shelf storage.

BS EN 12845 requires bulkhead material to meet the requirements of Euroclass A1 or A2. This essentially

means the bulkheads are to be noncombustible.

Example of shelf storage with vertical bulkheads or bin-box

(Image source: Rich Gallagher, The Zurich Services Corporation)

This storage configuration may be desirable as it tends to direct a fire out to the aisle of the storage system

before a fire can spread horizontally or vertically. This tends to allow sprinklers to control a fire to a more

limited volume.

Shelves

Shelves may be provided to support goods in place of pallets.

Whether a shelf is solid, slatted, or open grate, storage may be placed across the entire shelf area. As such,

any shelf should be treated as a solid shelf unless controls are provided to maintain longitudinal and

transverse flues.

Example of solid shelves (Image source: Rich Gallagher, The Zurich Services Corporation)

11

Example of a slatted shelf (Photo source: Stuart Lloyd, Zurich Risk Engineering)

Example of an open grate shelf (Photo source: Rich Gallagher, The Zurich Services Corporation)

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Appendix B Response time index, or RTI, is a measure of the thermal sensitivity of a sprinkler’s fusible element.

Sprinklers are typically rated as fast response or standard response. As explained in the NFPA’s Automatic

Sprinkler Systems Handbook (2016), fast response sprinklers have an RTI of 50 (meter-second) 1/2 or less

while standard response sprinklers have an RTI of 80 (meter-second) 1/2 or more.

The fast response sprinkler, whether of the quartzoid bulb or fuse link type, will have a fusible element of

lower mass compared to a similar standard response fusible element. The lower mass promotes faster

operation in response to heat.

For any given set of conditions, the fast response sprinkler is expected to operate sooner than the standard

response sprinkler.

Two quartzoid bulb-type sprinklers, fast response on left and standard response on right

(Photo source: Rich Gallagher, The Zurich Services Corporation)

Two fuse link-type sprinklers, fast response on left and standard response on right

(Photo source: Rich Gallagher, The Zurich Services Corporation)

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