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 CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 43, 2015 

A publication of 

The Italian Association 
of Chemical Engineering 
Online at www.aidic.it/cet 

Chief Editors: Sauro Pierucci, Jiří J. Klemeš 
Copyright © 2015, AIDIC Servizi S.r.l., 
ISBN 978-88-95608-34-1; ISSN 2283-9216                                                                               

 

Seismic Safety Design of Sprinkler Systems.  
Comparison Between FM Global and NTC 2008 

Stefano Grimaz, Fausto Barazza*, Petra Malisan 
SPRINT-Lab - Università degli Studi di Udine, via del Cotonificio, 114 - 33100 Udine, Italy 

 
The recent main earthquakes in Italy (Emilia, 2012, Abruzzo, 2009) highlighted the importance of seismic 
design of plants in particular in the industrial sector. Indeed, in such cases, the indirect costs due to the out of 
services of the plants and the consequent production losses are very high and greater than the direct costs of 
reconstruction/reparation of the plant. For this reason, often, the insurance companies require that the 
principal plants (such as the sprinkler system) should be seismically designed. In case of an earthquake, the 
seismic design of sprinkler systems has to ensure the strength and the stability in order to avoid accidental 
ruptures and onerous out of services. In Italy, the seismic design of the plants became mandatory after July 
2009 with the entering into force of the NTC 2008 standard. Nevertheless, foreign insurance standards, such 
as FM Global, are usually adopted in Italy for the seismic design of sprinklers. This practice raises the 
following question: “Is a sprinkler designed according to the requirements of FM standards compliant with the 
NTC 2008?”.  
This paper illustrates a comparative analysis between FM Global and the NTC 2008, highlighting the 
differences in terms of seismic action and strength. Finally, practical abacuses, for identifying the cases in 
which the FM-Global provides solutions that are in compliance with the NTC 2008 are presented. 

1. Introduction 

Fire risk is one of the main concerns of the safety managers in workplaces. Fire protection systems, and 
sprinklers, in particular, are widely used as effective countermeasure both for controlling the fire propagation 
and for facilitating the evacuation of people. Grimaz et al., 2014a and Grimaz and Tosolini, 2013 illustrate the 
role of these systems in the definition of fire safety and in the evacuation, respectively. For the workplaces 
located in a seismic area, the earthquake effects must be considered also in terms of fire safety. Indeed, the 
seismic damage on industrial facilities could cause both damage on structures and on content (direct damage) 
but also out-of-services of plants and production losses (indirect damage). Grimaz and Maiolo (2010) 
highlighted this aspect after the recent earthquakes in Italy (Abruzzo, 2009 and Emilia, 2012). Moreover, it is 
worth noting that an earthquake could, at the same time, provoke fires and/or compromise the operability of 
the fire protection systems. An out-of-service of the fire protection system could have heavy consequences 
especially if the seismic action provokes a leakage of flammable or explosive substances. This means that a 
fire protection system, and in particular sprinklers, must be designed both to withstand the seismic action and 
to maintain its efficiency after the event, avoiding accidental activation too. 
In order to ensure that a sprinkler system performs correctly after an earthquake, insurance companies require 
the plant to be compliant with specific standards. In the Italian territory, insurance companies often require the 
compliance with NFPA 13 or FM Global (2010); anyway, it is possible to observe that these standards could 
be not always compliant with the specific national seismic code. Following an outline similar to Grimaz et al. 
(2014b), where the comparison between NFPA 13 and NTC 2008 standards for the design of sprinkler 
systems was described, in this paper the Authors illustrate a comparative analysis for the Italian territory 
between FM Global standard and NTC 2008. 
 

                                

 
 

 

 
   

                                                  
DOI: 10.3303/CET1543395 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Please cite this article as: Grimaz S., Barazza F., Malisan P., 2015, Seismic safety design of sprinkler systems. comparison between fm-global 
and ntc 2008, Chemical Engineering Transactions, 43, 2365-2370  DOI: 10.3303/CET1543395

                                

 
 

 

 
   

                                                  
DOI: 10.3303/CET1543395 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Please cite this article as: Grimaz S., Barazza F., Malisan P., 2015, Seismic safety design of sprinkler systems. comparison between fm-global 
and ntc 2008, Chemical Engineering Transactions, 43, 2365-2370  DOI: 10.3303/CET1543395

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2. Seismic action 

2.1 Seismic action according to FM Global standards 
FM Global (in the following FM) 2-8, in the §2.2.1.8 defines the seismic action on a pipe sprinkler system of 
weight Wa by the relationship:  = G ⋅  (1) 
Where 

• Wa is the seismic weight of the considered zone of influence of the pipe system; 
• G is the expected horizontal acceleration; 
• Fp is the horizontal seismic force acting on considered pipe system. 

The expected horizontal acceleration “G” depends on the earthquake zones, as defined in paragraph C.7 of 
FM 1-2 (2010). FM Earthquake Zone Map (FM 1-2, §C.7.3) of Italy differs from the hazard map of NTC 2008 
developed for the Italian territory (Figure 1). FM Earthquake Zone Map provides the mean return period of 
ground motion that could cause “reasonable damage to structures without significant seismic protection” (FM 
1-2, §C.7). FM 2-8 (2010), §2.2.1.2.2 suggests to adopt a minimum G parameter of 0.75 for the “50-year 
earthquake zones”, 0.5 in the “100-year earthquake zones”, and 0.4 in the “250” and “500-year earthquake 
zones”.  

   (a)                  (b) 

Figure 1. (a) FM Earthquake Zone Map of Italy (from Fig. 5E of FM 1-2). Blue zones: G = 0.75. Red zones: 
G=0.50. Orange, green and yellow zones: G=0.40. (b) Seismic hazard map of NTC 2008 for the determination 
of the parameter ag/g. 

The G factors defined by FM are based on the Allowable Stress Design (ASD) analysis method, this implies 
that a coefficient 0.7 has to be used to convert the G parameter values for the application in the Strength 
Design (SD) method. Therefore, the FM seismic action expressed in term of SD becomes: = . ⋅ G ⋅       (SD method) (2) 
2.2 Seismic action according to NTC 2008  
NTC 2008 computes the seismic action on a non-structural element as reported in §7.2.3. The evaluations 
were explained in Grimaz et al., 2014b and are shortly reported in Eq. (3) and (4). For an exhaustive 
description of the meaning of the symbols in the equations, refer to Grimaz et al., 2014b. = ⋅ W = ⋅ ⋅ 5.5 ⋅ C ⋅  (3) 
Where: 

= 15.5 ⋅ 3 1 +1 + 1 − − 12 														with 15.5 ≤ ≤ 1 (4) 
The parameter Cζτ can be interpreted as a reductive factor of the seismic action, depending only on ζ=z/h and 
τ=Ta/T1. Cζτ is maximum in the case of a sprinkler system at the roof level (z=h) and resonance between pipe 
system and structure (Ta=T1).  

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2.3 Comparison between FM Global and NTC 2008 seismic action 
The comparison between FM and NTC 2008 aims at evaluating which is the value of S∙ag/g	in the NTC 2008 
seismic action (Eq. (3)) that produces the same seismic action of FM calculated with the specific G parameter 
(Eq. (2)). Comparing Eq. (2) and Eq. (3) it results: ⋅ = 1.93 ⋅   (5) 
Where {S*ag/g }eq is the NTC 2008 parameter equivalent to FM values.  
If we consider, for example, z=h and Ta=T1 (therefore Cζτ=1), the FM parameters G=0.4, 0.5 and 0.75, 
produces the same seismic action of the NTC 2008 parameters S∙ag/g	=0.21, 0.26 and 0.39. 
3. Comparison between FM Global and NTC 2008 strength 

Sway-brace can be realized by using a rigid (tensile-compressive system) or tension-only system (see Figure 
2). 

 
Figure 2: example of a rigid sway-brace (left) and a tension only sway-brace (right). 

 
As for NFPA 13, FM Global computes the strength of sway-brace referring to the AISC (2005) standard and in 
terms of ASD. This implies that it is necessary to calculate the strength in terms of SD to compare the results 
with NTC 2008 (i.e. divide the ASD value by 0.7). 

3.1 Tensile strength by AISC (Strength Design)  
The tensile strength (expressed in terms of SD) is defined by relationship D2-1 of the AISC: R = 10.7 ⋅ R = 10.7 ⋅ ⋅1.67 = ⋅1.17  (6) 
Where 

• A is the gross section;  
• Fy is the yield stress. 

3.2 Compressive strength for flexural buckling by AISC (Strength Design)  
The compressive strength for flexural buckling (expressed in terms of ASD) is given by the relationship E3-1 of 
the AISC: R = 10.7 ⋅ R = 10.7 ⋅ ⋅1.67 = ⋅1.17  (7) 
Where: 

= 	 ⋅ 0.658 ≤ 4.71 ⋅0.877	 > 4.71 ⋅  
and =  
3.3 Tensile strength by NTC 2008 (Strength Design)  
The tensile strength Nt,Rd has to be computed according to the following equation (NTC 2008, §4.2.4.1.2): 

, = ⋅ = ⋅1.05   (8) 
Where 

•  is the cross section of the sway-brace pipe; 

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•  is the characteristic yield stress of the material; 
•  = 1.05 is the strength safety factor (NTC 2008, Table 4.2.5). 

3.4 Compressive strength for flexural buckling by NTC 2008 (Strength Design) 
The evaluation of the sway-brace compressive strength requires the verification the flexural buckling strength. 
In particular, the buckling strength (NB,Rd) of the brace has to be computed according to Eq. (9) (NTC 2008,  
§4.2.4.1.3.1):  

, = ⋅ ⋅   (9) 
Where: 

• = ≤ 1 is the buckling reduction factor; 
• Φ = 1 + ̅ − + ̅ ; 
•  is the imperfection factor (NTC 2008,§ 4.2.4.1.3). It depends on the section of the brace adopted. 

Typical values of α are 0.21, 0.34 and 0.49, for the instability curves a, b, c, respectively; 

• ̅ = ⋅ = ⋯ =   is the dimensionless slenderness; 
• = ⋅ ⋅   is the elastic critical buckling force; 
• = ⋅   is the slenderness of the pipe; 
•  is the effective length factor (generally  = 1); 
•   is the length of the sway-brace pipe; 
•   is the radius of gyration of the sway-brace pipe; 
•   is the cross section of the sway-brace pipe; 
•  is the characteristic yield stress of the material; 
•  is the Young modulus of the material; 
•  = 1.05 is the buckling safety factor (Table 4.2.5). 

3.5 Comparison between the strength 
For tensile strength (Strength Design), FM Global and NTC 2008 use similar equation. The main difference is 
caused by the adopted safety factors. FM Global strength design is more conservative with respect to NTC 
2008 of a factor of about 10%. As a consequence, for the same seismic action, a steel tension-only sway-
brace designed according to FM is always in compliance with NTC 2008. Note that for tension-only sway-
braces, FM does not permit slenderness greater than 300 (therefore cable systems are not permitted) while 
NTC 2008 does not give any limitation. 
For flexural buckling strength, FM is more conservative than NTC 2008 for low values of the α parameter and 
high slenderness. For example, for a slenderness λ=200 and hot formed brace (instability curve “a”, Figure 3), 
it provides a strength of about 88% of the Italian code. As a consequence, for the same seismic action a class 
“a” rigid sway-brace designed according to FM is always in compliance with NTC 2008. Note that both FM and 
NTC 2008, for rigid sway-brace, limit the maximum slenderness to 200. 

 

Figure 3: Ratio between the buckling strength provided by FM and NTC 2008 for different values of 
slenderness and for the three instability curves “a”, “b” and “c”. 

4. Comparison between FM Global and NTC 2008: design abacus 

In order to simply identify the cases in which FM Global seismic design is compliant with NTC 2008, the 
Authors propose to use the abacus in Figure 4. Figure 4a refers to rigid sway-braces (slenderness λ=200, 

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instability curve “a”) while Figure 4b to tension-only sway-braces. If the G values of the FM design are known, 
and for a given Cζτ (see Eq. (4) or Grimaz et al., 2014b), it is possible to graphically identify the corresponding 
values of S∙ag/g	of NTC 2008. 

(a) (b) 
Figure 4: (a) equivalent S∙ag/g (NTC 2008) parameter for rigid sway-brace with λ=200 and instability curve “a” 
(b) equivalent S∙ag/g (NTC 2008) parameter for tension only sway-brace. The grey filled area represents the 
value of G lower than 0.4, not recommended by FM Global. 

5. Example of comparison  

In order to illustrate the proposed methodology, the {S∙ag/g}eq	parameter is evaluated for some cities in Italy, 
and for a rigid sway-brace originally designed according to FM standard. The results are compared with the S∙ag/g values esteemed on the basis of NTC 2008 requirements considering different soil classes (A, B, C and 
D, as defined by NTC 2008) (Table 1). In the comparison, an installation of the sprinkler system at the roof 
level is assumed (z=h), and the conservative assumption Ta=T1 is adopted, therefore Cζτ=1 is adopted. The 
values of the expected horizontal acceleration (G) for FM come from the FM 1-2 (Figure 1). By using the two 
abacus in Figure 4, it is possible to obtain the maximum S∙ag/g	values ensuring that the FM seismic design is 
compliant with NTC 2008. In Table 1 the values of the S∙ag/g	demand for rigid sway-braces are calculated for 
four levels of importance of structures (Reference Life: VR = 50, 75, 100 and 150 years). The filled cells with 
bold values represent the cases in which the FM standard is not compliant with NTC 2008.  

Table 1. Design compliance FM Global – NTC 2008 for rigid sway-braces (λ=200, curve of instability “a”). 
Values of G, the equivalent {S∙ag/g}eq , and S∙ag/g parameter for A, B, C and D soil Italian classification (NTC 
2008). The filled cells with bold values are the cases in which the FM Global is not compliant with NTC 2008. 

City importance
VR (y) 

G {S∙ag/g}eq S∙ag/g 
(soil A) 

S∙ag/g 
(soil B) 

S∙ag/g 
(soil C) 

S∙ag/g 
(soil D) 

L'Aquila 50 0.75 0.44 0.26 0.30 0.35 0.39 
L'Aquila 75 0.75 0.44 0.30 0.33 0.38 0.40 
Bologna 50 0.5 0.29 0.17 0.20 0.24 0.30 
Bologna 75 0.5 0.29 0.19 0.23 0.27 0.33 
Udine 50 0.5 0.29 0.21 0.25 0.29 0.34 
Udine 75 0.5 0.29 0.24 0.28 0.33 0.37 
Udine 100 0.5 0.29 0.27 0.31 0.35 0.38 
Udine 150 0.5 0.29 0.32 0.35 0.39 0.39 
Tolmezzo 50 0.5 0.29 0.24 0.28 0.32 0.36 
Tolmezzo 75 0.5 0.29 0.28 0.32 0.36 0.39 
Taranto 50 0.75 0.44 0.07 0.09 0.11 0.13 
Taranto 75 0.75 0.44 0.08 0.10 0.12 0.14 

 

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For example, a sprinkler system designed in L’Aquila according to FM Global has G=0.75, and consequently S∙ag/g	of 0.44 for rigid sway-braces; this sprinkler system is always compliant with the NTC 2008 code for the 
considered soil classes. However, a sprinkler system with rigid sway-braces designed in Udine according to 
FM Global (G=0.50) has a correspondent S∙ag/g	 value of 0.29 and then it is compliant with NTC 2008 in case 
of soil A, B and C for small factories (VR = 50 years), soil A and B for schools (VR = 75 years), only soil A for 
civil protection offices (VR = 100 years), and it is never compliant with NTC 2008 in case of hospitals (VR = 150 
years) (Table 1).  

6. Conclusions 

Safety managers of premises located in seismic regions need to evaluate the increase of a fire risk as a 
consequence of an earthquake. Indeed, relevant earthquakes could cause damage on structures and content, 
but also out-of-services and potential losses. For this reason, insurance companies require the design of fire 
protection systems according to specific standards such as FM Global. In Italy, the compliance with these 
standards is not mandatory; on the other hand, the design of new fire protection systems must follow the rules 
of NTC 2008. In order to identify the cases in which FM Global satisfies Italian NTC 2008 and the cases in 
which it is not possible to apply FM standards in the Italian territory, the Authors illustrate the comparison of 
the two approaches for the seismic design of sprinkler system sway-braces. The Authors prove that in several 
cases in Italy the FM standard is not compliant with NTC 2008 and they introduce simple abacuses which can 
be used by designers and Authorities to make easier the compliance verification process.  
 
Acknowledgements 

Part of the work presented in this paper has been developed within a research project funded by ERICO® 

Company and managed by the SPRINT team of the University of Udine (I).  
 
References  

AISC, 2005, American Institute of Steel Construction 360, Specification for Structural Steel Buildings. 
ASCE7, 2010, American Society of Civil Engineers, Minimum Design Loads for Buildings and Other 

Structures. 
FM Global, 2010, Property Loss Prevention Data Sheets, Factory Mutual Insurance Company 
Grimaz S., Maiolo A., 2010, The impact of the 6th April 2009 L’Aquila earthquake (Italy) on the industrial 

facilities and life lines. Considerations in terms of NaTech risk. Chemical Engineering Transactions. 19, 
279-284. 

Grimaz S., Tosolini E., Dolcetti G., 2010, A quick method for emergency evacuation design in workplaces, 
Chemical Engineering Transactions, 19, 433-438. 

Grimaz S., Tosolini E., 2013, Application of rapid method for checking egress system vulnerability, Fire Safety 
Journal, 58, 92-102. 

Grimaz S., Dattilo F., Maiolo A., 2014a, Inspect: a new method for fire safety in existing premises, Chemical 
Engineering Transactions, 36, 61-66. 

Grimaz S., Barazza F., Malisan P., 2014b, Seismic Safety Design of Sprinkler Systems. Comparison Between 
NFPA 13 and Italian NTC 2008. Chemical Engineering Transactions. 36, 307-312. 

NFPA 13, 2013, Standard for the Installation of Sprinkler Systems. Quincy, Massachusetts, USA. 
NTC 2008, 2008, DM 14.1.2008 Italian seismic code (in Italian). 

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