Thursday, January 5, 2012

Seismic Design For Fire Sprinkler Systems - Part 2

Continued from Part 1 of seismic design for fire sprinkler systems.

After you have determined if you need seismic bracing, how do you determine the amount of Horizontal Seismic Force or Fp to apply?  (Hint - You can just go to our Seismic Calculator App and have much of the look up work done for you.)

STEP ONE - APPLICABLE STANDARDS AND CODES
Assuming that you are working in a jurisdiction that has adopted the International Building Code (IBC), you would start with section 1613.1 which states:
1613.1 Scope. Every structure, and portion thereof, including
nonstructural components that are permanently attached to
structures and their supports and attachments, shall be
designed and constructed to resist the effects of earthquake
motions in accordance with ASCE 7, excluding Chapter 14 and
Appendix l1A. The seismic design category for a structure is
permitted to be determined in accordance with Section 1613
 or
ASCE 7.
 Furthermore the International Mechanical Code (IMC) section 301.15 states:
301.15 Seismic resistance. When earthquake loads are applicable in accordance with the International Building Code, mechanical system supports shall be designed and installed for the seismic forces in accordance with the International Building Code.
Looking in the appendix of the 2009 edition of IBC (page 590), we can see that the 2005 edition of ASCE 07 or ASCE 07-05 is adopted.  So what is ASCE 07-05?  The name of the standard is "Minimum Design Loads for Buildings and Other Structures" and is published by the American Society of Civil Engineers (ASCE).


STEP TWO - DETERMINE THE FORCE FACTOR
ASCE 07-05 paragraph 13.3.1 is the applicable section for "nonstructural" components and provides the following formula for determining the horizontal design force (Fp) to be applied:
ASCE Formula
Most of the variables are already defined by NFPA 13 and/or ASCE 07-05 as follows:

Variable
Standard Value
Definition
ap
2.5
Component amplification factor from Table 13.6-1 (Seismic Coefficients for Mechanical and Electrical Components) – “Piping and tubing not in accordance with ASME B31, including in-line components, constructed of high or limited deformability materials, with joints made by threading, bonding, compression couplings, or grooved couplings”.
(Note the 2002 edition of ASCE 7 recommended ap = 1.0)
Rp
4.5
Component response modification factor from Table 13.6.-1 (same item as ap above)
(Note the NFPA 13 2002 TIA 02-1 recommended a Rp = 3.5)
Ip
1.5
Component Importance factor (Ip) per ASCE 7-05 13.1.3 “… The component importance factor, Ip, shall be taken as 1.5 if any of the following conditions apply: 1. The component is required to function for life-safety purposes after an earthquake, including fire protection sprinkler systems…”
(Note the NFPA 13 2002 TIA 02-1 recommended a Ip = 1.5)
z
-
Height in structure of point of attachment of component with respect to the base.  For items at or below the base, z shall be taken as 0.  The value of z/h need not exceed 1.0.
h
-
Average roof height of structure with respect to base.

As you can see the only missing pieces are the Height of Attachment (z), Height of Structure (h), and Five-percent damped design spectral response acceleration at short periods (Sds).

The calculated design force can be reduced by a factor of 1.4 because ASCE/SEI 7 is based on strength design, whereas NFPA 13 uses allowable stress design. Prior to the 2007 edition, all loads in NFPA 13 were at allowable stress levels with the exception of the buckling loads for brace members. In the 2007 edition, tables that contained the allowable loads on braces have been reduced to add a factor of safety appropriate to the use of allowable stress design



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