Three Times Rule

[FireFreak08]

I have a silly questions that has been bugging me.

In situations where there is a 24" wide column located against a wall. Do you still need to follow the 3 times rule. I'm a little confused about the comment that states "The maximum clear distance required shall be 24"

I'm not sure if no matter what, I would need to be 6' away in order to spray on the two sides. In a situation where being 6' away, it may also put me further than 7'-6" from the wall.

Can someone please clarify the maximum clear distance comment. Refer to 8.6.5.2.1.3 (2002 edition)

 

[cdafd]

in my non professional non engineering opinion

you need to read the entire book

if you look at 8.6.5.2.1.2 and 8.6.5.2.1.3 tells you there are other options and the 3x is one of them.

seems in your scenario it I would look at where other heads are in relation to the column, and even though I do not like shadows may approve it as is. No I would not approve a head more than max allowed off a wall, would tell you to add.

Than I keep hearing from some companies that the water is going to bounce and cover the shadow. Still looking for testing of the "bounce" factor of water.

 

[firepe]

In my professional Engineering opinion, application of the "three times" rule to a 24-in. wide column (whether located aginst a wall or not)requires that a head be NO FURTHER than 24-in. away from one side of the column, MAXIMUM. It does NOT require 3 times the 24-in. column width, or 72-in.

Regardless of the non engineering opinions or preferences that may be offered, it is clear that the intent is a MAXIMUM of 24-in. away from the column or obstruction.

This is one of a few instances in NFPA 13 where shadows are allowed, and regardless of plan reviewers or others that do not like or prefer it, it is allowed.

 

[cdafd]

missed the maximum 24 inches

thanks

but with that in mind if the column is against the wall, with no space behind it seems the 24 inch max rule would not apply in that seems like the max spacing for the head in use would apply, such as 7'6"???

 

[FireFreak08]

It does appear that it's 24" max from the column but then in a later paragraph it states sprinklers can be on either side of the obstruction (column) as long as you are within the 7'-6" to centerline of column.

With that, you could be more than 24".

So, the way I understand it is if I don't have coverage on both sides, I can't be more than 24". If I need to be more than 24" with a head on one side, then I need the 3 times rule?

Am I way off here?

 

[FireFreak08]

Didn't mean to post yet...

As for the column being against the wall. I guess it's a judgement call. The column is deeper than 24" so no matter what, if I'm less than 24" from the face of the column there will be a dry shadow. I usually put a head on each side in a case like this but It seems rediculous. I have a hard time believing that small of a dry shadow will create a huge hazard.

Lastly, why can't NFPA put a sq.ft number on (their words) "significant dry shadow"

 

[FFP1]

One man's effort to clear up the confusion:

"The maximum clear distance required shall be 24 in."

This essentially means NFPA 13 requires the same "miminum clear distance from the obstruction" for any and all obstructions which have an 8 in. wide dimension or larger. As long as the sprinkler head is at least 24 in. away from an obstruction, the obstruction is no longer considered an obstruction for that particular sprinkler (regardless of the size of the obstruction; unless the obstruction is >48 in. wide of course then we have a totally different issue).

Examples:

Obstruction dimension 6 in. => 18 in. minimum
Obstruction dimension 7 in. => 21 in. minimum
Obstruction dimension 8 in. => 24 in. minimum
Obstruction dimension 9 in. => 24 in. minimum
Obstruction dimension 10 in. => 24 in. minimum
Obstruction dimension 11 in. => 24 in. minimum
Obstruction dimension 16 in. => 24 in. minimum
Obstruction dimension 22 in. => 24 in. minimum