NFPA 13 Simplified Method vs. Dynamic in ASCE 7?

7/12/2024

I am designing the sprinklers for a building with unusual seismic criteria (it's designed to be seismically resilient and "floats" on seismic isolators).

The structural drawings note that design should be in accordance with ASCE 7-16 equation 13.3-4:
Fp = ai ap Wp Ax / (Rp / Ip)

I see from NFPA 13 (2022) appendix E that the simplified method is based on ASCE 7-16 equation 13.3-1.

Do you think the NFPA 13 simplified bracing method can still be applied/adapted to this alternate equation (without hiring a structural engineer)?

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3 Comments

Dan Wilder

7/12/2024 08:43:44 am

I see a lot of our community taking on roles that should be by others (for various reasons good or bad)....seismic is one. While definitely not a seismic guy, my opinion would be:

The structural engineers should be doing these calculations and provide to us the results (Risk Category, Ss, S1, Site Class, SDS, SD1, Seismic Design Category to name the big ones). Our side only needs some of the results (Cp that is based on the Ss value).

A RFI could always be provided to confirm the use of one formula over another but as NFPA has used the simplified formula throughout, it would require review of all the NFPA formulas for the revised outputs. This also means that any software that could be used would also need to be reviewed for the potential formula adjustment.

Brian Gerdwagen FPE

7/12/2024 09:09:23 am

ASCE 7 allows seismic design complying with NFPA 13 as an alternate to presciptive compliance.

Jack G

7/12/2024 09:28:07 am

First of all, it would depend on your projects engineer that specified that.
Most of the time, the architect hires a seismic engineer specifies that each trade is required to use them , and he performs the calculation for each trade and each trade pays him for their systems.
Having said that ASCE 7 Is the IBC s referenced standard for eq protection. Tables 1613.2.5(1) and (2) are weighted by the risk and category.
A& B zones are wholly ecempt C&D sre not.
For seismic category C-D, components weighing 20 lbs or less or distributive systems weighing 5 lbs/ft do not require seismic protection.
The exemption clause is the same- 2.5 inch conduit or larger must be protected.
NFPA 13 and 72
Chpt 17 in nfpa 13 requires seismic protection requirements. It is important to note that complisnce to Both asce 7 and 13 is not required
In asce 7 paragraph 13.6.7.2, sprinkler sxstems designed and constructed per nfpa 13 are recognized as meeting the requirememts of asce 7.
13 enforces a simplified versiom of the requirement found in asce 7 based on a set of technical assumptions. This is found in Annex E of 13:
Development of design approaches to conform with asce 7.
Nfpa 13 section 18.6 requires branch sprinklers to be restrained ( even 5 /lbs/ft ) but not engineered to a seismic load but restrained so they dont whip around.
After saying all of that, it is all subject to the AHJ, the architect and the Engineer. It would be their call which to use.
I would also ask if there is a project seismic engineer to use on this project.
Opimion.

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NFPA 13 Simplified Method vs. Dynamic in ASCE 7?

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