Seismic Design For Fire Sprinkler Systems – Part 2c: Clearance and Sway Bracing
Part 2: The Fundamentals of Seismic Design and the Design Features Involved.
The third design element involved with seismic restraint is clearance. This feature includes provisions for piping that penetrates specifically concrete and/or masonry floor/ceiling and wall assemblies. Do not confuse this with penetrations through rated assemblies that are framed with wood or steel studs with gypsum board. This section has nothing to do with assembly ratings or the requirements for sleeves or fire caulking. Those are usually a function of other specification requirements and should not be in this section of your specification or drawings notes.
Like separation, this feature is simple but very expensive. This section requires a specific nominal annular space to be provided around the pipe penetrating the assembly. A 1-inch annular space is required around 1-3- inch pipe. A 2-inch space is required around pipes that are 4 inches and larger. Core drilling a 10-inch-diameter hole for a 6-inch pipe is not something most fire protection contractors are very eager to do. This process can be quite involved, and the cost of core drilling is tied directly to the size of the hole.
However, there is a less expensive way to accomplish this penetration. You will recall that I previously mentioned that flexible couplings also could be used as a solution for clearance requirements. This is where couplings prove their worth. In lieu of large clearances, the standard allows for a flexible coupling to be installed on either side of the assembly within 12 inches of the face of the penetration. By providing these couplings, standard hole diameters may be used. My experience is that contractors prefer this method to providing the larger holes.
This section applies to all pipe sizes, so, like the separation requirements, consideration of the piping configuration is important. It is usually better to penetrate once into a concrete- or masonry-assembly room with main piping and then create a smaller tree-type system than it is to penetrate
several smaller holes into the space simply to maintain uniformity. A prudent plumbing designer would discuss these types of design features with the architect during the design development phase to try to minimize the amount and/or configuration of these assemblies as well as the overall sprinkler system cost. Doing so also may help you gain a level of favor with the installing contractor.
The fourth and most commonly referenced seismic restraint design feature is sway bracing. Unlike in other plumbing systems, the water and pipe that comprise fire protection systems are lifesaving features. While the majority will never activate, fire sprinkler systems must perform when needed or people and property will suffer. With that in mind, it becomes obvious why the bracing of fire sprinkler systems has its own rules for spacing, location, and force factor criterion.
The process for laying out sway bracing starts much like that for laying out sprinkler heads. There are three types of braces: lateral, longitudinal, and 4-way. Lateral bracing is required to be spaced at a maximum of 40 feet between braces. We also are required to install a brace within 20 feet of each end of the run of main, which is half the allowable distance between braces. Finally, we must have a brace on the first piece of pipe on each end of the main. Figure 3 depicts an example of lateral bracing.
When applying the rules to each run of main piping, you’ll want to try to maximize the distance between braces as much as possible. However, remember to leave room for the braces to be moved in either direction in case actual field conditions inhibit the fitter’s ability to install the brace at the location shown on the drawing. Also, as the distance between braces grows, so does the total weight that each brace will be required to resist. If you are in a high seismic category or if the site soil or building importance dictates a high force factor, maximizing the spacing may not be cost effective.
Once the lateral braces are located, you lay out the longitudinal braces. The maximum spacing for these braces is 80 feet. As with lateral braces, you are required to install a longitudinal brace within half the allowable distance between braces, meaning you must have one brace within 40 feet of each end of the run of main. Normally there will be fewer longitudinal braces than lateral.
The final bracing that is required is referred to as 4-way bracing. Industry terminology for this feature has been diluted, so for the purpose of clarification, 4-way bracing is not where both a lateral and longitudinal brace are located. Rather it is a bracing assembly that is used to restrict the movement of pipe that is installed in a vertical position such as the riser piping at the fire service entry into the building. As you can see in Figure 4, this bracing usually is installed in the horizontal position and has specific attachments that are designed to meet the intended installation configurations. The brace must be located within 24 inches of the top of the riser.
Like many of the requirements of this standard, nuances and exceptions can be applied. Both lateral and longitudinal braces can serve each other’s purpose if located within 24 inches of the end of the run of main (see Figure 5). Notice that the 4-way brace can be considered as the longitudinal brace as well. As a matter of design, I usually first lay out the bracing for each run of main independently, and then go back and consider the relocation of the braces at each end of the mains as a whole to apply these alternatives. Some designers have been taught to simply install a 4-way brace at every change of direction if sway bracing is required. Not only is this wrong, it is very expensive and does not accomplish the goal of seismic design. Bracing layout needs to be done with consideration of total weight and the ability of the fitter to actually have ceiling space to install the brace.
For example, in ceiling areas with an excessive amount of ductwork above the piping, it will be very difficult to run the sway brace up to the top chord of the structural member. If you have maximized the spacing, little can be done. Whereas if you have allowed for this condition ahead of time, the fitter can relocate the brace further down the main in one direction or the other without compromising the ability of the hanger to carry the weight that it was designed to resist. While it is not cheap, adding a brace to cut down the spacing is much less expensive than having field personnel trying to figure out how to make it work.
It is my hope that you see the importance of the “how” of the process of seismic design of fire sprinkler systems. As with any engineered system, especially life safety systems, understanding the overall goal and applying the standards by which we are intended to meet these goals is very important. Remember: Vince Lombardi said, “Excellence is achieved by mastering the fundamentals.”