Common Fire Sprinkler Design Issues [Illustrated]

Illustrated Solutions for Common Sprinkler Design Issues

Answers to the most common questions and concerns for fire sprinkler system design.

This book is not intended to be a comprehensive manual on fire sprinkler design, rather it is a brief guide that serves to answer the most common questions and concerns regarding the design of fire sprinkler systems. This book provides the fire plan reviewer or inspector with easily accessible answers to the most important questions for approval and acceptance of a fire sprinkler system.

• Does the structure require a fire sprinkler system?
• Is the occupancy and commodity properly classified?
• Are plastics properly identified and protected?
• Does the rack storage need in-rack sprinklers?
• Will there be any obstructions to the discharge pattern?
• How are the cloud ceilings protected?
• What is the "small room" rule?

This book is written by Aaron Johnson from, TheCodeCoach.com and illustrated by Joseph Meyer from, MeyerFire.com.

An illustrated guide for fire sprinkler design. Create better educated fire protection professional for the promotion of fire protection and life safety.

Kindle - $4.99

Print - $9.99

Fire Sprinkler Design: An Illustrated Guide for AHJ's [BOOK]

After twelve years in the fire service and ten years of writing on fire prevention and fire protection topics, there are a handful of sprinkler design elements and questions that continue to show up. This book is not intended to be an exhaustive or fully comprehensive manual on fire sprinkler design, rather it is a brief guide that serves to answer the most commonly seen questions regarding fire sprinkler systems and design. Often times, reading the codes and standards language as written can be somewhat confusing. Our attempt in this manual is to provide clarity on a selection of these complicated design rules. Always, the main goal being a better educated fire protection professional to promote fire protection and life safety.

It has been my great privilege to partner with the premiere fire protection engineering author and illustrator, Joseph Meyer. His drawing and illustration talents bring a fresh dimension to the fire protection system design principles we want to share in these pages. Through the combined efforts of our written content and Joe’s illustrations we hope to bring a better understanding to the art of fire protection and sprinkler design.

Learn the most critical design elements related to:

• Documentation and installation requirements
• Obstructions, discharge patterns, and construction features
• Storage protection, occupancies, and commodity classifications
• Small spaces and the “small room” rule

An illustrated guide for fire sprinkler design. Create better educated fire protection professional for the promotion of fire protection and life safety.

Kindle - $4.99

Print - $9.99

What is the "small room" rule?

In conducting plan reviews or fire sprinkler field inspections you may have heard, or seen, the invocation of the “small room" rule. Though not specifically stated in NFPA 13 as the “small room" rule, it is a combination of several code sections within the standard that can be utilized to provide advantages in hydraulic calculations and flexibility in sprinkler spacing.

To apply the small room rule, the space must meet all the requirements of a small room as defined in NFPA 13. A “small room” is a space that is classified as light hazard, has unobstructed ceiling construction, and is no greater than 800 sq.ft.  Application of the small room rule will affect two critical system design elements, maximum area of protection and sprinkler spacing.

The normal method for determining the area of protection per sprinkler head is to use the formula:

As = S x L

In this case the distance between sprinklers along a branch line (S) and the distance between branch lines (L) is determined and then multiplied to establish the “maximum allowable protection area of coverage for a sprinkler” (As).  The small room allowances of Section 8.6.2.1.2 permits the sprinkler designer to ignore the rules and requirements of the S x L formula. Instead, an alternate method is allowed. If the space meets the requirements for a small room, then maximum coverage area per sprinkler can be determined by simply dividing the area of the room by the total number of sprinklers in the room.

Sprinkler spacing and distance between individual sprinklers is calculated using the tables and guidance in Section 8.6.3. In these tables, the maximum spacing distance for sprinklers is 15 feet between sprinklers and 7.5 feet from a wall. Utilizing the small room rule requirements, allows sprinklers to be spaced up 9 feet from any single wall.

The small room rule can be stated as:

In a room of 800 sq.ft. or less, having unobstructed ceiling construction, and light hazard contents, sprinklers may be located up to 9’ from any single wall with the maximum area of coverage being determined by dividing the size of the room by the number of sprinklers.

Code summary for small room rule:

NFPA 13:3.3.22 - small room defined

NFPA 13:8.6.2.1 - max. area of coverage

NFPA 13:8.6.3..4 - sprinkler spacing from a wall

Sprinkler Obstructions and Discharge Patterns

photo source: pmengineer.com

Important to fire sprinkler effectiveness is sprinkler discharge pattern development. A fire sprinkler system should be designed in a manner that provides full water coverage from the fire sprinkler system. Inherent building construction or design elements can pose potential obstructions to the full coverage or discharge pattern of the sprinkler system.  However, these buildings and elements must still be protected. NFPA 13, Standard for the Installation of Sprinkler Systems, provides sprinkler coverage guidance for these obstructed areas.

To determine the proper application it must first be determined if the construction is obstructed or unobstructed.  Then it must be determined if the obstruction is continuous or noncontinuous.

Is this obstructed or unobstructed construction?

To be considered obstructed, the construction elements must be composed of “beams, trusses, or other members” that will impede heat flow, water distribution, or otherwise affect the ability of the sprinkler to control or suppress the fire. [13:3.7.1]

Construction types that may create obstructions, include:

• Beam and girder
• Concrete Tee
• Composite wood joist
• Panel construction
• Bar joist with fireproofing
• Steel purlin

Unobstructed construction includes any “beams, trusses, or other members” that do not impede heat flow, water distribution or the ability of the fire sprinkler to control or suppress fire. This generally consists of construction members that are not solid.[13:3.7.2]

Unobstructed construction elements may include:

• Bar joist
• Open-grid ceilings
• Smooth ceilings
• Standard mill (heavy timber)

Is the obstruction continuous or noncontinuous?

If the obstruction affects the sprinkler discharge pattern of two or more adjacent sprinklers then the obstruction is considered to be continuous. However, if only one sprinkler discharge pattern is affected then the obstruction is noncontinuous.

Different sprinkler head types and orientations have different requirements for designing to protect obstructed areas.  These are addressed in the following sections of NFPA 13.

• General guidance: 13:8.5.5
• Pendant and upright sprinklers: 13:8.6.5
• Sidewall sprinklers: 13:8.7.5
• ESFR sprinklers: 13:8.12.5

A great reference tool for quickly determining the design criteria and requirements for obstructed areas is this ceiling obstructions reference tool/calculator provided by Meyer Fire.