Mezzanines are commonly used to maximize building space and ergonomics in both production and storage areas. The need for automatic sprinkler protection beneath a mezzanine with a solid deck/floor is rarely questioned when the occupancy below includes combustible loading of some sort. The solid mezzanine floor would obviously shield the ceiling sprinklers from the fire below and would allow the fire to grow unchecked except for manual response efforts.
The need for automatic sprinkler protection beneath non-solid (grated or slatted) mezzanines was a different issue. There was a time when automatic sprinklers were not required or recommended beneath grated or slatted mezzanines if sprinklers were provided at the roof level. The thought was that a fire involving the storage or production equipment below the mezzanine would be protected and eventually controlled by the ceiling sprinklers. However, the reduced effectiveness of ceiling sprinklers caused by non-solid floors was shown in a pair of fire tests by FM Global. The tests were done using shelf storage of paper records in corrugated cartons to 13 ft. high with 18 in. clearance to the ceiling sprinklers.
The two test arrangements were identical (sprinkler system design/type, storage types/heights, etc.) except one test used a metal grated catwalk at the 7 ft. elevation with storage above to 13 ft. from the floor level. The test that used the catwalk opened twice as many sprinklers. The fire spread, damage, and duration of high ceiling temperatures was significantly worse in the test with the catwalk than in the test without. The obstruction from the catwalk and spillage of paper onto the catwalk, which caused further obstruction, were the contributing factors in reducing the effectiveness of the ceiling sprinklers. These tests proved that non-solid floors obstruct water from the ceiling sprinkler distribution and therefore, sprinklers are necessary below non-solid floors to compensate for the reduced effectiveness of the ceiling sprinklers.
When designing/planning a production/storage mezzanine, the choice of whether to use a solid/non-solid floor should take into consideration many factors. From a property loss prevention standpoint, the planned occupancies above and below the mezzanine should take precedence. If storage occupancies will be located above or below production areas, solid floors should be provided to provide a minimum separation of the two separate occupancies. If non-solid floors were used with a combined production/storage mezzanine, heat, smoke, and water damage from storage fires could severely affect production operations. Also, fires in production areas will expose storage to ignition and wetting.
Solid Floor Mezzanines
With solid floor mezzanines, the areas above and below the solid deck are evaluated as separate fire areas. The ceiling sprinkler demand should be based on the upper level occupancy, and the lower level sprinkler demand should be based on the lower level occupancy. The sprinkler and hose stream demands would be totally independent of one another, similar to that of a two storied building.
Heavy-duty mezzanines (poured concrete, steel decking, etc.) and structural steel are often required for floor loading support. However, plywood (½ in. minimum thickness) and sheet metal are both acceptable forms of decking from a fire protection point of view.
Grated or Slatted Mezzanines
When sprinklers are not provided below a grated or slatted mezzanine, the obstruction from the non-solid floor reduces the amount of ceiling sprinkler water reaching the base of the fire, although pre-wetting below the non-solid floor will still occur. Sprinklers provided below the non-solid floor are very effective in reaching the base of the fire because they typically are only 10-12 ft. above the floor. Since heat will escape through the non-solid floor minimizing lateral fire spread, a large number of sprinklers under the non-solid floor are not expected to operate.
When occupancies above and below a non-solid floor are both storage or both production areas, it is acceptable to omit solid decking, providing there is adequate sprinkler protection. NFPA does not provide any special fire protection design criteria for beneath non-solid mezzanines. FM Global (Property Loss Prevention Data Sheet 2-8N) recommends that sprinklers at the ceiling and under the non-solid floor or walkway should be treated as one fire area. Their recommended guidelines for determining the total water demand for multilevel production areas, or multilevel storage areas separated by non-solid floors is very similar to requirements for in-rack automatic sprinkler (IRAS). Some of the guidelines are as follows:
• The ceiling sprinkler demand should be based on the highest hazard occupancy for production areas or the highest hazard commodity and total storage height (floor to top of upper level storage) for storage areas.
• The water demand for sprinklers installed under the non-solid floor should be based on the occupancy on the lower level.
• Sprinklers under non-solid floors should be spaced at a 130 sq. ft. maximum for production areas and 100 sq. ft. maximum for storage areas. When sprinklers are installed in racks on the lower level, spacing of sprinklers under the non-solid floor can be extended to 130 sq. ft. Use ½ or 17/32 in. orifice, 165°F rated rack storage-type sprinklers (with water shields) under non-solid floors and walkways to prevent water discharge from sprinklers above from impinging and delaying operation of sprinklers below. Balance the water demands for each system (i.e., ceiling sprinkler system, lower level system, in-rack system, etc.) protecting the non-solid floor or walkway, at the point where the systems connect.
A minimum 18 in clearance should be maintained between the top of all palletized, shelf and bin box storage and the automatic sprinklers beneath the mezzanine. To accomplish this, the top level of the shelving units on the floor level may need to be removed in some instances. This “loss of space” should be considered when designing and planning a mezzanine storage area. Care should also be taken with regards to the location and position of light fixtures below the mezzanine so that they do not obstruct the sprinkler heads.
When sprinklers are installed in lower level racks, including the water demand for the sprinklers under the non-solid floor with the in-rack and ceiling sprinkler water demands is not necessary since only a few sprinklers are expected to operate under the non-solid floor.
ESFR Systems at Ceiling above Mezzanine
Exceptions to the above include suppression mode sprinklers at the roof level with a mezzanine below. FM Global Data Sheet 2-2, “Installation Rules For Suppression Mode Automatic Sprinklers” provides some guidelines for protection with these types of systems.
In order to prevent simultaneous operation of suppression
mode sprinklers beneath and above solid mezzanines, and to allow the areas above and below the solid deck to be treated as separate fire areas, a draft curtain should be provided around the perimeter of the mezzanine or all storage underneath the mezzanine should be kept inside the sprinklers located nearest the perimeter of the mezzanine.
For mezzanines up to 15 ft. above floor level, install suppression mode sprinklers designed to supply four sprinklers at the pressure required for the type of sprinkler used, or quick response, control mode sprinklers designed for the hazard involved as noted above.
For solid mezzanines more than 15 ft. above floor level, install suppression mode sprinklers using the design criteria required for the storage height and type of sprinkler used.
Control-mode sprinklers designed for the hazard involved may be used below a mezzanine more than 15 ft. high only if a draft curtain is provided at the perimeter.
In order to be considered grated, the grating of a mezzanine must be least 70% open. If the grating is less than 70% open, then make the mezzanine solid (in order to allow sprinklers underneath it to operate promptly) and protect it as a solid mezzanine as noted above.
If storage exists either above or below a grated mezzanine, but not both above and below, no additional sprinklers are needed under the mezzanine.
Additional design guidelines are provided for walkways and aisle spaces above and below the mezzanines when ESFR sprinklers are provided.
If you have questions about this subject or other fire protection or property loss prevention issues, please contact us at Risk Logic Inc.