RESOURCES
CODE & STANDARD REFERENCES TRANSCRIPT
What Structural Information do I Need for Suppression Design?
WHAT IS A CEILING? First let's talk about the definition of a ceiling. NFPA has multiple definitions of a ceiling including flat, horizontal, sloped, and smooth. What this means is that there are several different definitions defining the highest point in which heat will be collected and how it relates to the sprinkler system. FUNDAMENTAL DIFFERENCES If we look at a lay-in ceiling, that would be a flat continuous ceiling in a single plane, which we all are used to seeing. But what happens when you don't have a lay-in or drywall ceiling and you have exposed structure? Well there are two fundamental differences in the types of construction that relate to the type of structure. The fundamental differences are called obstructed and unobstructed construction. Obstructed construction is when you have structural elements that are rigid that prevent the flow of heat across the ceiling, which in this case, would be the roof deck or the bottom of the slab of the floor above. It also refers to the type of construction where the structure impedes the water spray distribution from the individual sprinklers. Different examples of obstructed constructions could be solid steel beams, concrete, or concrete T beams. Conversely, unobstructed construction is when you have horizontal structural members that are not solid or where the openings are at least 70% of the cross sectional area and do not impede the movement of heat, as in the case of open web joists. These different types of construction allow for sprinklers to be located at different depths. OBSTRUCTED & UNOBSTRUCTED Location of sprinklers with obstructed and unobstructed construction can give you different vertical height requirements of the sprinkler deflector in relation to the ceiling. Obstructed construction allows your sprinkler deflector to generally be located at a greater depth than unobstructed construction because it has the ability to hold heat within the structural members preventing the heat from traveling across the ceiling. Unobstructed construction requires sprinklers to be much closer to the ceiling because the heat will distribute for a greater distance. You have to remember that sprinklers operate based on heat transfer and you need to have an area where the heat will stay long enough around the sprinkler to activate the sprinkler so if the heat is collected between structural members, your sprinkler head could in theory be lower from the ceiling, compared to if you don't have any structural members like a large concrete slab where the heat is going to travel so your sprinklers are going to be closer to that heat layer in order to allow the sprinklers to activate. NFPA 13 goes into more detail of the requirements based off of material choice and type of structural member relating to where the sprinkler can be located. CHALLENGES Different challenges exist for each type of structural design beyond being obstructed or unobstructed. You also have considerations for the combustibility of the materials and the additional requirements for sloped ceilings. Different building types have different requirements for combustibility of materials. High-rise construction has different code requirements compared to low rise residential construction. The same type of limiting factors for obstructive versus non obstructed apply. As for slope ceilings you have the same requirement to maintain the sprinkler deflector depth as obstructed or unobstructed construction. But if you think about it looking from a plan-view, the sprinklers covering the area below a sloped ceiling will be closer together based on a vantage point from the ground looking up. Because of this, slope ceilings greater than 2 in 12, which means for every horizontal foot, the ceiling rises more than two inches, require a 30% increase in area protection in your calculations. PIPING SUPPORT Once you determine what type of ceiling you have, what type of beam construction you have, and if there's any slope, then the next thing to think about is how you can support your piping network from the structure. There are a variety of ways to support sprinkler piping and it's going to differ based on the requirements of the building. Primarily in noncombustible construction you're going to see sprinkler piping being supported by threaded rods connected to the structure and the pipe. There are other ways such as trapeze hangers which are more common in pre engineered systems to allow for prefabrication or really tight coordinated ceiling spaces. In other areas you may find a variety of other ways to secure pipe to structural members. Despite the many different ways to hang a pipe what you have to remember is all sprinkler piping will be filled with water at one point or another. Wet pipe sprinkler systems will have water stored in the pipe and dry pipe systems will be dry until water is needed. The weight of the steel, copper, or CPVC piping, including water, will add up over the length of the system. The piping material used can also affect your hanger spacing. For example, CPVC piping will require hangers at much shorter intervals compared to steel because of the amount of deflection that pipe will experience when filled with water. Coordinating the weight of the sprinkler pipe with the structural engineer can also determine how the branch lines and mains are routed throughout the facility. On lightweight construction this may be more critical than a more robust construction type. ENGINEER COORDINATION Coordinating with the structural engineer allows the structural engineer to determine where they want the dead load of the sprinkler system affecting their structural members. The other item that can be coordinated with the structural engineer is the spacing of their joists and beams compared to the hanger requirements for the material you chose. This can also make a difference if you're dealing with post tension or pretension concrete and coordinating where those hanger locations may be, which could also include Unistrut molded into the concrete to allow for pipe hangers at specifically designed locations. Sprinkler piping is installed after the structure has been built and for this reason coordinated as part of your design. in many cases across the world design engineers provide performance specifications outlining the design densities for sprinkler systems. In this case during the contractor’s shop drawing review, the hanger locations in relation to the structure shall be carefully reviewed by the fire sprinkler designer of record and the structural engineer. I'm Ben Brooks, this is MeyerFire University.
0 Comments
Leave a Reply. |
VISIT FULL SITEFULL CATALOGSEARCHFILTER BY:
All
ARCHIVES:
October 2024
INSTRUCTORS: |