Now that the 2016 edition of NFPA 13, Standard for the Installation of Sprinkler Systems, has been published and shipped, sprinkler contractors, layout technicians, and Authorities Having Jurisdiction (AHJs) are reviewing the newly added Tables 18.104.22.168.2 (a) through (f ) with respect to the load capacity for post-installed wedge-expansion and undercut anchors in concrete. These revisions to the values in the 2013 edition are the result of several issues which needed to be addressed in order for NFPA 13 to maintain its standing as an equivalent reference to ASCE 7, Minimum Design Loads for Buildings and Other Structures, for seismic protection of fire sprinkler systems. Please note that what follows is only applicable to post-installed anchors in concrete.
One of the major items given consideration was the effect of prying on the anchor capacity. This issue had been addressed in previous editions of the standard in the annex, meaning it was advisory in nature. It is now mandatory to consider this when determining the capacity of an anchor in concrete. Each of the hardware manufacturers will have to provide the prying factor (PR) for their devices in order for them to be properly evaluated, from the tables or from approved software as allowed in newly added Section 22.214.171.124.8.3.
The effect of prying, similar to that of a crowbar or nail puller being used to remove a nail from wood, can reduce the maximum load (FPW) that can be applied to an anchor when all the factors have been taken into account. In order to verify that the impact of this action does not result in a load on the anchor which exceeds the acceptable limits, the prescriptive tables show the maximum loads for a range of prying values for the hardware in all nine NFPA 13 categories.
Additionally, it was learned that in previous editions of the standard, the values for the anchor capacity taken from the manufacturer were converted to “allowable stress design” (ASD) using overly lenient equations. Recent information from the ASCE 7 committee provided more conservative guidance on reaching an acceptable conclusion for the final value of the anchor capacity. A careful review of the new material in Annex E.7 will provide a more detailed description of the processes which were used to populate the new anchor capacity tables.
To develop the values in the new tables, certain assumptions had to be made in order to simplify the process for layout technicians and AHJs alike. Among those assumptions was a “starting point” for prying factors from currently available hardware throughout all the various angle categories, and representative capacities for anchors in concrete. The range of values for the available brace hardware, and the load resistance factor design (LRFD) values for the anchors used to achieve the resulting maximum are shown in Tables E.7(a) and (b), respectively.
With all of the above information taken into account, it isn’t necessary for a contractor, layout technician, or AHJ to seek the services of a structural engineer to select the proper anchor for use in attaching seismic bracing for sprinkler systems to concrete structural members. The values in Tables 126.96.36.199.2 (a)-(f ) provide a workable prescriptive table which can easily be applied.
Among the information necessary to use a post-installed anchor in concrete includes:
- The design strength of the concrete, expressed in pounds of compressive strength, and whether that concrete is to be light or normal weight or over a metal deck.
- The thickness of the concrete into which the anchors will be installed. Manufacturers have limits on the depth of the hole to be used, and the minimum thickness of the concrete. Selecting a longer anchor may not be appropriate as its use may be outside the parameters established by the manufacturer.
- The prying factor from the hardware manufacturer for the intended brace angle, and angle configuration (ABC, DEF, or GHI) per NFPA 13, Figure 188.8.131.52.1
- Verification that the post-installed anchor has been pre-qualified for such use in accordance with the procedures in American Concrete Institute (ACI) 318, Chapter 17. This information will most likely be included on catalog data sheets for the anchor, however it may be necessary to review a document similar to an ES report from the ICC Evaluation Service to verify such a qualification.
Once those items have been clarified the use of the tables is fairly simple.
A couple of quick notes – you’ll notice in some anchor manufacturer’s data sheets the anchor capacity can be shown in both “cracked” and “uncracked” concrete. For the purposes of attaching seismic bracing of sprinkler systems it is considered a more conservative position to regard all concrete as having developed cracking. If the data sheet shows a modification factor for cracked concrete this should be used.
Also, it is worth noting that other capacities than those shown in the new tables are allowed if the calculations are performed by “approved” software. A quick review of the definition of “approved” in Section 3.2.1 of NFPA 13 shows that this designation only requires the acceptance by the AHJ to be utilized. When using such software to establish a capacity for the anchor-hardware combination other than the prescriptive values shown in the new tables it is imperative to use the correct variables (anchor size/length, concrete strength, brace angle, etc.) in order for the results to be valid.
Further information of a much more detailed nature can be found in Annex E.7 of the 2016 edition, and in a new supplemental article in the handbook edition, for those who need or desire such supporting data.
Ken Wagoner, C.F.P.E., C.F.P.S., S.E.T. is a 1977 graduate of Bethany College (Kansas), and owner of Parsley Consulting, located in Escondido, California. He has 35 years of experience in automatic sprinkler system and fire alarm system design, plan review, and construction management. He is an AFSA-designated alternate representative to the NFPA 1031 committee, principal member of the Hanging and Bracing Committee of NFPA 13, and chairman of the NFPA 24 committee. Wagoner has published numerous articles on seismic bracing, hydraulic calculations, system design and the plan review process; has presented online virtual seminars, and teaches classes sponsored by AFSA and the San Diego Fire Protection Association. He is an active member of NFPA, AFSA, and SDFPA.