#130 – Detector Layout Part 4: Beam Construction

by Shawn Lee

Next in our heat detector layout series is beam construction. For this post, we will assume that our beamed ceiling is level and the ceiling is 10 feet or less from the finished floor. There are some similarities between heat detector layout on joist ceilings and beamed ceilings with regards to reduced spacing at a right angle to the beams. However, the reduction is a little less. Therefore, we must reduce heat detector spacing by 50% at right angles to the joists — on a beam ceiling our heat detector spacing may have to be reduced to not more than around 66% of the listed spacing at right angles to the beams. This may seem a little confusing because that’s not how NFPA 72 phrases it. We will come back to that in just a bit. Here is what NFPA 72 has to say about heat detector location and spacing for beam ceilings:

• 17.6.3.3.1.1 A ceiling shall be treated as a smooth ceiling if the beams project no more than 4 in. (100 mm) below the ceiling.
• 17.6.3.3.1.2 Where the beams project more than 4 in. (100 mm) below the ceiling, the spacing of spot-type heat detectors at right angles to the direction of beam travel shall be not more than two-thirds of the listed spacing.
• 17.6.3.3.1.3 Where the beams project more than 18 in. (460 mm) below the ceiling and are more than 8 ft (2.4 m) on center, each bay formed by the beams shall be treated as a separate area.
• 17.6.3.3.2 Location. Where beams are less than 12 in. (300 mm) in depth and less than 8 ft (2.4 m) on center, detectors shall be permitted to be installed on the bottom of beams.

Let’s talk about each requirement. The first requirement, 17.6.3.3.1.1, simply tells us that beams that project 4 inches or less from the ceiling are not considered as beams. We treat it no differently than a smooth ceiling. As for where the detectors are mounted, make sure you review 17.6.3.3.2.

The second requirement, 17.6.3.3.1.2, tells us that if our beams do project more than 4 inches from the ceiling, then we must reduce our heat detector spacing at right angles to the beams. That reduced spacing is to be no more than 2/3 or about 66% of the detector’s listed spacing. The reason we say about 66% is because when doing the math, 2/3 of 100% = 66.66666…%. So, depending on whether you choose to round up or round down, you will have a maximum spacing of 66% or 67% of the detector’s listed spacing. That does make a difference in your layout. It is best to be more cautious and use 66%, as using 67% could result in a code violation. Therefore, using 66% or even 66.66666% vs. 67% is recommended. For example, in the case of our heat detector with 30-foot spacing here’s the difference:

• Option #1: S x 0.66
  30 x 0.66 = 19.8 feet spacing at right angles to the beam
• Option #2: S x 0.6666666
  30 x 0.66 = 19.999998 feet spacing at right angles to the beam
• Option #3: S x 0.67
  30 x 0.67 = 20.1 feet spacing at right angles to the beam

The difference between option #1 and #3 is 0.3 feet or a little less than 4 inches. It may not seem like a lot, but when you have multiple detectors in an area, 4 inches for each detector starts adding up and you will end up with a detector layout that is not code compliant. At the end of this post, there will be more discussion about the 2/3 reduction, but for now, let’s continue.

If you read the post on heat detector layout for solid joist ceilings, there was a brief explanation as to why the heat detector spacing is reduced at right angles to the joists. It has to do with the way heat will travel across the ceiling at right angles to the joists. The same is true of beam ceilings. Although the beams will be placed further apart than the joists, the heat traveling across the ceiling will still be slightly impeded by the beams. The space between the beams will “fill up” with heat before “spilling” into the next beam pocket. This can create a situation in which the detector response is delayed. Placing the heat detectors closer together will help to reduce the amount of time for the detectors to sense the heat and signal an alarm to the control unit.

Our third requirement, 17.6.3.3.1.3 tells us that if the beams project MORE THAN 18 inches from the ceiling AND are spaced MORE THAN 8 feet on center, then each bay formed by the beams is considered a separate area for heat detector layout purposes.

Our last requirement, 17.6.3.3.2, lets us know that if the beams project LESS THAN 12 inches from the ceiling AND are LESS THAN 8 feet on center, then we are permitted to install the heat detectors on the bottom of the beams. Pretty straight forward.

Having read through all of that, you may be wondering how it plays out on a layout. For ceilings that have beams that project 4 inches or less from the ceiling, the spacing and installation is exactly like a smooth ceiling which we covered in another post, therefore there’s no illustration for that. But let’s look at the other requirements.

Example #1:

1. Beams project 6 inches from the ceiling.
2. Beam spacing is 6-foot centers.
3. Ceiling height is 10 feet or less and ceiling is level.

The beams for the illustration above project more than 4 inches from the ceiling, so 17.6.3.3.1.2 is applicable. We must reduce the heat detector’s listed spacing to no more than 66% at right angles to the beams. And you must also recall that a heat detector may not be any further than 1/2 or 50% of its spacing from the sidewall. That means at right angles to the beams, our heat detector spacing can be no more than S x 0.66 ÷ 2 from the sidewalls. In our case, this is 30 x 0.66 ÷ 2 = 9.9 feet. As you can see, in the illustration above, the distance is 9 feet from the sidewalls so the detectors could be mounted on the bottoms of the beams making 17.6.3.3.2 applicable because of the beam depth and spacing. However, I could have also placed the detectors inside a beam pocket, so long as I did not exceed 2/3 spacing (19.8 feet) between them as shown in the illustration below.

In this illustration, notice the detectors are in the beam pockets and moved 3 feet closer to the sidewalls. There is now 18 feet between the heat detectors (still less than 19.8 feet).

Now let’s look at a larger space with larger beams depths and spacing.

Example #2:

1. Beams project 20 inches from the ceiling.
2. Beam spacing is 15-foot centers.
3. Ceiling height is 10 feet or less and ceiling is level.

Because of beam depth and beam spacing, we must follow the requirements of 17.6.3.3.1.2 (2/3 spacing at right angles to the beams) and 17.6.3.3.1.3. (each bay formed by beams is a separate area). The maximum spacing allowed from any sidewall, or beam in this case, is 9.9 feet at a right angle to the sidewall or beam. In a direction parallel with the beam, the maximum spacing from a sidewall is 1/2 the detector’s listed spacing. In this case that would be a max of 15 feet from the sidewall. The maximum distance between any two heat detectors at a right angle to the beam is 19.8 feet, and in a direction parallel to the beams, there is no reduction. So that means the maximum heat detector spacing in the direction of the beam is the listed spacing of 30 feet.

In the illustration above, we have no code violations since all distances between the detectors and the sidewall or beams is less than 9.9 feet. In the direction of beam travel, our detectors are only 7.5 feet from the closest sidewall. In addition, the two heat detectors in each beam pocket are only 25 feet apart. Assuming there are no other obstructions or ceiling features for the illustration above that would require us to relocate our heat detectors, our layout is code compliant.

Now, let’s go back and re-address the 2/3 reduction requirement using 66% as the reduction factor vs. 67%. We’ll also expand a little more on using 2/3 as the factor vs. 66%. First, 66% vs. 67%: if we use 67% as our factor, we exceed the allowable spacing for our beam ceilings. On low ceiling heights, the spacing is small. If you recall from my explanation above, when we calculated the reduced spacing with 66% vs. 67% the two calculations produced 19.8 feet and 20.1 feet, a difference of approximately 4 inches. You may think that 4 inches isn’t that big of a deal, but keep in mind that may be compounded by the number of detectors you have. Also, keep in mind that as ceiling heights increase, our heat detector spacing must decrease. The ceiling height reduction must be performed prior to any other reductions. These reductions can have a significant effect on your layout. If you use the 67% vs. 66%, you may save yourself from using a few heat detectors, but you will be short-changing the occupants of the facility with a non-code compliant detector layout. That is assuming this is not figured out during the plan review.

There is another reason using 67% as your factor is incorrect. Let’s assume we are using a heat detector with 30-foot listed spacing as we have been for our layout examples. If you use “2/3” as stated by NFPA 72, 17.6.3.3.1.2, then you calculate your reduced spacing as such:

S x 2/3 which equates to: S x 2 ÷ 3
30 feet x 2 ÷ 3
60 feet ÷ 3
The reduced spacing = 20 feet max

So, as you can see, if we were to use the 2/3 max spacing requirement from 17.6.3.3.1.2 and apply that to a 30-foot heat detector, then we see that 20 feet is the maximum spacing we can use at a right angle to a beam. But if we use the 67% factor, then we end up with 20.1 feet which is a code violation.

At this point you may be thinking that if we use 66% then that is incorrect as well. The reason using 66% will work is because the maximum spacing allowed per NFPA 72 will not be exceeded. When using 66% for our 30-foot heat detector, we end up with 19.8 feet, which is less than the 20 feet max that NFPA 72 allows. Regardless of which way you choose to reduce the detector spacing for a beam ceiling, if you make sure you don’t exceed the allowable spacing as determined by your detector’s listed spacing and the requirements of 17.6.3.3.1.2, you can’t go wrong./span>

As you read through the requirements for detector spacing, you will see how complicated heat detector spacing can get. Once you learn the basics of detector layout, then applying the various reduction factors becomes a little bit easier. Keep reviewing NFPA 72 and applying what you review. Make up a few of your own examples to help reinforce what you learn. Before you know it, it will be second nature.

To read the previous article in this series, Link to: #127 – Fire Alarm Detector Layout Part 3: Solid Joists