The ESFR trap
While supporting a design-build team preparing bids for a project to provide a new warehouse building, we almost fell into the ESFR trap. I feel very fortunate to have talented engineers on my staff that caught us before we walked into it.
The request for proposal (RFP) for the project stated:
“Sprinkler protection must be based on the maximum potential height of storage (ceiling height minus minimum clearance required for sprinkler head). Provide ESFR sprinklers in the high rack storage areas. Sprinkler protection to be based on Class IV commodities, unless a more severe class of storage is determined.”
No problem here, right? The client wants a warehouse that has maximum flexibility in terms of the storage commodities and storage arrangements, and clearly wants to avoid the need for in-rack sprinklers.
During preparation of pricing documents for the sprinkler contractor, we determined the maximum potential storage height would be 20 feet. From Table 188.8.131.52 of NFPA 13 (2013) for Class IV commodities and a maximum storage height of 20 feet and a maximum roof height of 25 feet, we selected the K = 16.8 ESFR sprinkler with a minimum operating pressure of 35 psi. The design area will include 12 sprinklers and the hose stream allowance will be 250 gpm. One nice thing about the ESFR is that same design criteria can be used for protection of Group A plastics (cartoned, nonexpanded), as well as these same commodities for palletized or solid-pile storage from Tables 14.4.1 and 15.4.1. No in-rack sprinklers, maximum flexibility right? Well, not so fast.
The RFP requires the roof and supporting walls and columns to be of reinforced concrete construction. It turns out that the structural engineer is also looking to give the client maximum flexibility and minimum cost. In this case, the structural engineer has determined that the most efficient system is to use precast concrete double-tees. The concrete tees are spaced on 5 feet centers. The tees have a depth of 24 inches and the tee thickness varies from approximately 8 inches at the plank to 6 inches at the bottom. The general contractor is very pleased with the structural engineer’s choice. The design is based on a precast system that is commonly available and the system can provide spans long enough to reduce the number of columns required. The trap is set.
According to NFPA 13 (2013), this is considered obstructed construction, see A.3.7.1(2). Though paragraph 184.108.40.206 of NFPA 13 permits ESFR sprinklers to be used in obstructed construction, paragraph 220.127.116.11.1 in combination with paragraph 18.104.22.168 puts the kibosh on the use of many ESFR applications using concrete double tees.
Per paragraph 22.214.171.124.1 where solid structural members exceed a depth of 12 inches, ESFR sprinklers must be installed in each channel. Therefore, in the case of the structural engineer’s precast design solution, ESFR sprinklers are required in every channel. Paragraph 126.96.36.199 imposes a minimum distance between ESFR sprinklers of 8 feet. Given that the double tees proposed for our project require sprinklers in each channel and given that the tees are spaced 5 feet on center, ESFR sprinklers would be in violation of the minimum distance between sprinklers.
Here we have a case where the RFP requirement to provide ESFR sprinklers for the warehouse eliminates what otherwise would be a very cost effective solution to the requirement that the structural system be reinforced concrete.
In our case, there are two options if double-tees are to be used: 1) install a smooth, flat ceiling, such as gypsum board, below the precast structural system; or 2) increase the spacing between the tees.
In the article, “ESFR Sprinklers and Obstructed Construction,” published in the March-April 2009 issue of the NFSA Sprinkler Quarterly, author Ken Isman discussed this same issue with double tee construction. He noted that one way to comply with the minimum 8 feet spacing between ESFR would be to stagger the sprinklers. In that case, the minimum 8 feet spacing can be met if the double-tees are increased to 5 feet and 4 inches on center.
For our project, the contractor opted to install the smooth ceiling below the structure. Hopefully, those reviewing the proposal for the client will understand that this more expensive solution is the best way to get ESFR protection in a building with an efficient concrete structural system.
Getting back to the more general discussion, if the special rules for solid structural members exceeding 12 inches do not apply, there is another set of rules you need to meet. Unlike the other sprinklers used for the protection of storage, standard and CMSA, NFPA 13 does not have special rules for the distance of sprinklers below ceilings for obstructed construction for ESFR sprinklers.
In the case of both standard sprinklers and CMSA sprinklers maximum clearances of up to 22 inches are permitted. There is also a special exception for the use of these sprinklers with concrete tee construction, paragraphs 188.8.131.52.2(5) and 184.108.40.206.2 (3), which allows the sprinkler deflector to be located one-inch below the bottom of the tees regardless of the depth of the tee.
The distance below ceilings for ESFR sprinklers is strictly based on the sprinkler orientation, pendent or upright; and k-factor. The maximum distances vary from 12 inches to 18 inches. ESFR sprinkler obstruction rules in paragraph 220.127.116.11 and 18.104.22.168 of NFPA 13 are also more severe and can result in adding additional sprinklers below obstructions.
The point I am trying to make is that one must be very careful when trying to use ESFR sprinklers on a project. One must understand the roof and roof framing that will be used and determine if the building construction is suitable for ESFR sprinklers.
Ken Isman sums it up well in his article stating:
“NFPA 13 does not prohibit the use of ESFR sprinklers when a building has obstructed construction, but it does make it more difficult to install the system. If you can't meet the rules discussed here, then you can't use the ESFR sprinklers unless the building owner is willing to put in a drop ceiling under the structural members to create a flat, smooth unobstructed ceiling situation. ESFR sprinklers are just not flexible enough to be used with every ceiling configuration, so you must understand all of the rules if you are going to try and make an ESFR sprinkler system fit into a building.”
When one is trying to avoid the use of in-rack sprinklers, ESFR sprinklers can be a great solution. But, unless you make sure the building structure is suitable, you may fall into the trap.