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CHAPTER 3

Basic Design Overview (Part 2)

THIS chapter begins with the assumption that you are by

now somewhat familiar with the building plans for theproject example, and that the initial drafting (blocking out)

of the building layout is complete. Your sheet now looks

something like the example on page 140 labeledFigure A-1.

As is the typical procedure, what you have traced for areas of

exposed construction is the architects floor plan, and what

youhave traced for areas with finished construction is a close

copy of the architects reflected ceiling plan.

Once the building is blocked out in scale, it is time to draw

the sprinklers and system piping on the plan. This is the most

critical engineering portion of your work. It requires a com-

plete knowledge of theapplicable codes, mental visualization

of theproposed building, anda recall of your own knowledge

and instincts that you attained from all your past experiencein this area.

As was touched on in the last chapter, the architectural

plans mayincludea preliminarysprinkler plan,complete with

many legal disclaimers. It may be wise or unwise to use any

part of this plan as your own. The preliminary plan may be

an exact depiction of what the architect and building owner

desire, or it may simply be a quick drawing made to satisfy

some municipal requirement. Often, it has been prepared by

anengineer (hiredby thearchitect)whosereal area of special-

ization is his accompanying drawings (i.e., the mechanical,

electrical, civil, and/or plumbing plans). If the quality of the

preliminary sprinkler plan does not make this possibility ev-

ident, a phone call to this engineer is always worthwhile: for

only then will you find out if his design is diagrammatic in

nature, or if you are expected to comply with his plan with

limitedlatitudefor deviation. In many of thesecases however,

thefiner pointsin theengineers plan maybe wayoff themark

of conforming to the fire sprinkler engineering code. If so, a

good question to ask during this phone call would be about

what, if any, requirements in excess of the NFPA Pamphlet

No. 13 requirements are being asked for.

Thereareprojectsin which systemdesign hasalready been

thoroughly researched, prepared, andeven reviewed. When acommercial insurance company is very involved, as with one

of their H.P.R. clients, a pre-construction meeting is held

during the planning stages of a fire sprinkler system project

to allow for recognition of the hazard, understanding of the

project requirements, and providing bid specifications [1].

This will take care of allthe majorengineeringissues after the

local fire prevention bureau has their say, although the level

of preconstruction document review may vary substantially

depending on location [2]. Most fire prevention bureaus are

only interested in reviewing the final shop drawings which

you will prepare.

* * *For our purpose in this chapter, we are assuming that none

of the above preplanning activities have taken place. We will

now move on to the task of laying out the system after first

determining the occupancy and hazard level of the building

tenant, and whether or not any chance of pipe freezing will

exist.The answer weget isthatthis tenantis in the businessof

manufacturing thermopane glass and aluminum patio doors,

and that the facility will be heated year-round.

Since we have established that there is no reasonable

chance of future pipe rupture caused by freezing tempera-

tures, we will immediately decide to install the most reliable

system available, which is the wet-pipe automatic sprinkler

system. Speed of operation, low installation cost, and ease

of maintenance account for the fact that 75% of all sprin-

kler systems in use are of the wet-pipe type. There is no

size limitation on wet pipe systems, except that the max-

imum area protected by any one system on any one floor

of a single fire section cannot exceed 52,000 square feet

[3]. Our example building in Figure A-1 is well below this

maximum limit in size, and can be protected by a single

system.

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We are not concerned in this chapter with gridded or looped

systems, and so will utilize the only alternative left, which is

called a tree system. In this layout configuration, the water

flows from the system riser in a direction down feed-main and

cross-main piping, and finally down smaller sized branch-line

piping to the operating sprinkler. Water discharged from that

sprinkler always comes from a single direction.

In the layout phase, we first establish where we would liketo run our cross-main piping. In all phases of sprinkler design,

bear in mind this golden rule:

DO NOT MAKE A MISTAKE ON THE CROSS-MAIN!

Why? Because if branch-line (normally 12) piping can-

not be routed where you show it, an installer can correct that

without much fanfare. But rerouting the big cross-main pipe

is a different matter altogether. Not only is it expensive and

time-consuming, and not only does it affect all other down-

stream piping, but it can halt the installation completely while

the installers scratch their heads and attempt to achieve an en-

gineering solution on the job. This wastes time and time ismoney.

Again, the cross-main location and elevation is the first

thing that you lay out. The elevation must be low enough to

cross below structural beams, which usually means: lower

than the branch-lines. Naturally, the cross-main should be

hung high enough so as not to interfere with future building

operations. The system pipe elevations should be designed so

that all or most of the system can be drained when necessary

at the base of the system riser.

When steel bar-joists support the roof of the structure,

the direction of the branch-line runs are always perpendic-

ular to the joists, for ease of pipe hanging. Hence, to feed

the branch-lines, the cross-main must run parallel to the

joists.1

ALWAYS KNOW WHAT YOURE HANGING

THE PIPE TO!

Specifically, for optimum cost efficiency, the main should

be routed beneath a single joist, hung from one side of that

joist or the other.2 Look at the HVAC (Mechanical) drawings

to make absolutely certain that the joist location that you

select has the least chance of obstructional interference. And

dont select a joist (obviously) that rests directly above a

column, or the pipe will be run smack into a structural columnor I-beam. In order to avoid ductwork, it may be necessary

to offset the main once, or several times, or even change

elevation. Offsets, though, shall be minimized to reduce labor

andmaterialinstallation cost. Varying depths of I-beamsmust

be checked (on the steel drawings),3 but the highest priority

consideration must be that there is reasonable space available

for the piping to fit where it is shown. This space includes

space above the pipe as well: for example, there may be space

available to run pipe below some ductwork, but then it is

economically impractical to hang the pipe.

* * *

On concealedjobs (where all piping is concealed above

a drywall or drop-ceiling), the main piping must be hung at a

low elevation, beneath all other mechanical tradework. Many

designers routinely set cross-main elevations at 7

to 9

(pipecenterline) above the highest drop-ceiling height in these

instances. This is done because it is generally observed that

the sprinkler trade, being the last of the mechanical trades to

man a job, hang their pipe on concealed jobs below all

the ductwork, storm piping, electrical cable trays, and the

like; but also that the main must lay high enough to avoid

hindrances such as ceiling speakers and recessed lighting

fixtures. On tightjobs, where space available above ceiling

is at the bare minimum, it may be necessary to run the main

at an even lower elevation, being careful to route this pipe to

avoid hitting all the recessed lights.

* * *Before you lay out your branch-lines and fix their ele-

vation, you need to spot all the sprinkler-head locations.

The information youll need for this task is detailed more

thoroughly in Chapter 10, and requires knowledge of spac-

ing limitations. As previously mentioned, our example job

is of a glass-door manufacturer, an ordinary hazard occu-

pancy. The maximum head spacing for this tenant would be

130 sq. ft. The sprinklers must be positioned high enough

to fuse from a collection of heat, so factory and warehouse

sprinkler branch-line piping always run high, through the bar

joists. The pipe will run through either an A or V space

in these joists (Note Figure B-2 on page 151). To avoid run-

ning into joist bracing material, your safe bet is to space the

sprinkler lines an even number apart from one another (i.e.,

10, 12, or 14). The spacing between sprinklers in light and

ordinary hazard occupancies is only allowable up to a 15

maximum.6

Onceyouhave determined thedistancefrombranch-line to

branch-line, you can set the number of sprinklers on a line. In

the Figure A-2 example, the branch-lines are spaced 12apart,

andthesprinkler-headsare107 apart on-center, givingyoua

coverage per sprinkler of (12 107) 127 square feet. Your

distance from the sprinkler-head to a wall is not to exceed

one-half the distance between sprinklers in any direction (as

noted in NFPA #131999 ed.-5-5.3.2).

* * *

Noticethat we aresupplying seven sprinklers from a single

riser nipple, as opposed to six, or eight. The east half of

the building, framed by the I-beam, creates an area4 of

(23 74) 1702 sq. ft. Since our sprinkler spacing cannot

exceed130 sq. ft., and 1702 130 = 13.1, we know that we

will needat leastfourteenautomaticsprinklers to protect that

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23 74area. Twelve sprinklers would not be enough (for

any ordinary hazard occupancy). If sixteen sprinklers were to

be designed, the heads would be spaced 93apart, center-to-

center, giving you a coverage per sprinkler of (120 93)

111sq. ft.Butundernormalcircumstancesa contractor would

much prefer to install fourteen sprinklers in this area rather

than sixteen, for the same reason that one would only install

one unit heater (and not two or three) in a room if just onecould adequately do the job.

* * *

To set the elevation of the piping, we need to collect the

following facts: the building inFigure A-2 has a flat roof,5

the underside of which is 15

high. Its bar joists are all 62

apart and 20deep. The supporting I-beams all have a depth

of 24.

We have an obligation to keep our cross-main as high as

it can reasonably be situated, to maintain clearance for the

building owner. Our main piping would be safely installed at a

centerline elevation 6below the bottom of the solid I-beam.

This would translate to an elevation of (150 24 6)126. Hanging the branch-lines to run through the center of

the bar joists is also a safe bet, so we would be inclined to

have these installed at an elevation of (15 0 10) 142.

Attention must always be given to the HVAC contractor, the

no. 1 suspect in all sprinkler installation cases of conflict.

If his large airhandling equipment is to be installed on the

roof, we must be careful to avoid running sprinkler piping

below that area or areas. A basic goal of the sprinkler de-

signer is to avoid situations where field labor must cut pre-

fabricated piping because it will not fit where it is intended

to go.

Referring again to the example sketched in Figure A-2, the

fire sprinkler branch-linescould be placed116apart inorder

to reduce the overall sprinkler head spacing to (116107)

121.7 sq. ft. It may even be that this arrangement will work

well for installation, and the piping may be hung in that

fashion without field adjustment. In fact, the only field ad-

justment necessary for a branch-line obstruction, consisting

of steel bar-joist bracing in this case, would be to lengthen or

shorten the riser nipple (fed by the cross-main) to an eleva-

tion without obstruction for the line path.But our job is not to

take chances. Running the lines 116 apart would increase

thepossibility of positioning the sprinkler within 3 of a joist

web, which is a code violation as well as a sprinkler spraydischarge interference. Making use again of the information

derived from Figure B-2, I would recommend the 120spac-

ing to reduce your odds of running into structural members.

CITATIONS

1. Bruce A. Edwards, Fire Sprinkler Plan Review Process, FireProtection Contractor, Feb. 1995, p. 13.

2. Michael A. Crowley, Meeting Hospital Fire Codes: MissionImpossible? Consulting-Specifying Engineer, Feb. 1995, p. 57.

3. Sprinkler System Guide, Viking Fire Protection Associates In-corporated, Hastings, Michigan, 1981, p. 10.

ENDNOTES

1 As a general rule, 1 1/2 to 3 from the centerline of the joist is theusual spot for the pipe to hang. See Piping Methods and Details,Chapter 13.2 An installer will appreciate a note stating from which side of the

joist you intend to hang the cross-main.3 The optimum location for the main piping is to cross under theshallowest I-beam.4 Such an area created by ceiling-level beams is sometimes called atrave, or a bay.5

You will never encounter a single-story building with a perfectlyflat roof without pitch. This is strictly a hypothetical example.6 Sprinklersmay be positionedfurther apart only if specialextended-coverage sprinklers are used, in accordance with the manufacturersspecifications for installation.

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