Protection of sprinklers against
damage by earthquakes — Part 3
By Samuel S. Dannaway, PE,
President, S.S. Dannaway Associates, Inc., Honolulu
The general requirements pertaining to seismic bracing were discussed last month. This article will look at the detailed design of sway bracing. For those not involved in the design of sway bracing but responsible for review and approval of sprinkler system designs, I will try to point out things you should be looking for.
Step 1. The first step is to lay out your bracing.
This will likely be an iterative process. In your first pass, it may turn out that you need more or less bracing. Locate lateral and longitudinal bracing such that the bracing falls within maximum distances between braces and maximum distance from the end of mains. For lateral bracing, that would be 40 feet between braces and six feet from the ends of mains. For longitudinal bracing that would be 80 feet between braces and 40 feet from the ends of mains. Also, include four-way braces at the tops of risers.
It might be prudent to space the bracing a little closer. We are finding out that, with the current load requirements pertaining to bracing, designers are required to space bracing much closer than maximums. This will allow some flexibility in the field in the event that a brace needs to be relocated if it cannot be installed exactly at the location shown. In such a case, one may wish to start with lateral brace spacing at maximum 30 feet intervals. Also, as with locating pipe hanger attachments, the designer must know enough about the structure to know where the bracing can actually be installed.
Step 2. Determine the loads to be applied to each brace.
First, establish the zone of influence (ZOI) for each brace. The ZOI will be all the piping that will be assigned to the bracing. The diagram provided in Figure A.9.3.5.6 (e) of NFPA 13 shows the load distributions or ZOIs for bracing graphically by clouding the piping within the zone. Note that the ZOI for lateral bracing includes all mains and branch lines, whereas the ZOI for longitudinal bracing is limited to the mains.
For four-way braces at risers, add the longitudinal and lateral loads within the ZOI of the brace. For the four-way bracing at the top of a riser, Cp times the height of the riser should be assigned to both the lateral and longitudinal loads as they are separately considered. When a single brace has a combined load from both lateral and longitudinal forces, such as a lateral brace at the end of a main that turns 90 degrees, only the lateral should be considered for comparison with the load tables in 9.3.5.3.2.
Determine the weight of the system for each ZOI. This is referred to as Wp, which equals the total weight of water-filled pipe in each ZOI multiplied by 1.15. The added 15% is to account for the additional weight of pipe fittings and sprinklers. In ASCE 7 the term Wp is actually more generic and represents Component Operating Weight, which is used in the calculations of seismic demands for non-structural components. Table A.9.3.5.6 provides water-filled weights in units of lb/ft and kg/m for Schedule 40 and Schedule 10 steel pipe. For copper tubing, you can find this information here. For CPVC piping you can link here. For other piping used in sprinkler systems, you will have to look to manufacturer’s data for this information.
Determine the seismic coefficient, Cp. In order to find this value, you must know the Spectral Response Accelerations for short period (Ss). As discussed in the June issue, this can be done using seismic maps published in the IBC, or it can be calculated using maps or software that can be downloaded from the Java Ground Motion Parameter Calculator found on the U.S. Geological Survey website at http://earthquake.usgs.gov/hazards/designmaps/javacalc.php. With the Ss, one can determine Cp from NFPA 13 Table 9.3.5.6.2. It is acceptable to interpolate the values on this table. Another option is to calculate the Cp from the following ASCE 7 formula for seismic design force Fpw:
Fpw = 0.4apSDSWp(1+2z/h)
(Rp/Ip)
Since: Fpw = Cp SDS
Cp = 0.4apWp(1+2z/h)
(Rp/Ip)
Where ap = component amplification factor, which is 1.0 for sprinkler systems
Rp = Component response modification factor, which is 3.5 for sprinkler systems
Ip = Component importance factor, 1.5 for sprinklers
z = height of bracing attachment point with respect to the base
h = average roof height with respect to the base
It would be conservative to select z/h as equal to 1.0.
If there is not adequate information to determine the Cp and it is acceptable to the AHJ, then a Cp equal to 0.5 may be used.
Calculate Fpw for each ZOI.
Step 3. Select brace shapes and sizes from NFPA 13 Tables 9.3.5.8.7(a), 9.3.5.8.7(b), and 9.3.5.8.7(c).
These tables provide maximum horizontal loads for steel sway braces with maximum brace lengths for slenderness ratios (l/r) of 100, 200 and 300 respectively. The maximum slenderness ratio l/r cannot exceed 300, and the angle from vertical may be no less than 30 degrees. When designing sway bracing it is desirable to choose values for angle from vertical in the range from 30 to 45 degrees. This is a conservative choice and will provide some flexibility if varying field conditions force you to this condition.
Step 4. Check to ensure that Fpw supported by each brace does not exceed the load permitted by NFPA 13 Tables 9.3.5.3.2(a) through (e) for the type and diameter of sprinkler piping to which the brace is attached.
Because these tables are based on some conservative assumptions and typical conditions the designer can exceed the distances indicated but only with a detailed engineering analysis. Some have argued that the seismic bracing calculation software provided by various manufacturers constitutes the detailed engineering required to supersede this table. Sorry, no dice, unless you are a structural engineer or have one available, I recommend that you use this table as an upper limit.
Step 5. If the total expected loads in the ZOI for a given brace is less than those permitted in Tables 9.3.5.8.7(a), (b) or (c) and less than the maximum values permitted in Tables 9.3.5.3.2 (a) through (e), then move on to the next step to check fasteners.
If not, then either add additional braces or select a brace shape and size that can support the load.
Step 6. Lastly, check that fasteners connecting the braces to structural supporting members are sized to adequately support the expected loads on the braces in accordance with Figure 9.3.5.9.1.
If not, again add additional braces or additional means of support. Note that, if powder driven, fasteners must be listed for resisting earthquake loads. Also, it is not permitted to use C-clamps, even with retaining straps, to support bracing.
Now a few items for those checking sway bracing calculations by others. It is likely you will be getting the calculation for review on a form similar to the one shown as Figure A.9.3.5 (b). As you review the calculation, be sure that the values for maximum horizontal load for the brace, maximum load for the fastener and the total load in the ZOI (Fp) is correct. Also, check to make sure that the loads are within the maximum limits of Tables 9.3.5.3.2(a) through (e). It appears that some bracing calculation software offered by the manufacturers does not offer this important check, so you must do it yourself.
This concludes our three-part discussion of protection of sprinkler systems from seismic events.
Samuel S. Dannaway, PE, is a registered fire protection engineer and mechanical engineer and past president and a Fellow of the Society of Fire Protection Engineers. He is president of S. S. Dannaway Associates, Inc. He can be reached via email at SDannaway@ssdafire.com.