Saturday, January 15, 2011

Seismic Design For Fire Sprinkler Systems

How do you know when seismic bracing is required for fire sprinkler piping?  Lets walk through the general procedure to determine if bracing is required.  (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 (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 - Ss and S1 Design Ground Motions
As noted in IBC section 1613.1 above, we are going to use IBC section 1613 to determine our seismic design category.  The first step is determine how strong the forces will be at your site.  There are two types of forces to consider: Short Period (Ss) and 1-Second Period (S1).  These values are typically determined by the USGS and the official definitions are as follows:

  • Ss = mapped MCE, 5 percent damped, spectral response acceleration parameter at short periods
  • S1 = mapped MCE, 5 percent damped, spectral response acceleration parameter at a period of 1 second
  • MCE = Maximum Considered Earthquake effects
Most building codes use 0.2 second and 1.0 second to represent the approximate natural period of vibration of a short and tall building, respectively

The USGS provides and excellent Java based calculation program for looking up these and many other values based on lat/lng and zip code. Or you can just go to our Seismic Calculator App which is based on this same data and provides some addition capabilities.

If I asked which area of the country had the heights forces, I bet you would guess California.  However, the highest seismic forces for the continental United States are:
  • La Center, Kentucky (37.1, -89.0) with a Ss = 3.4079
  • Harrisburg, Arkansas (35.5, -90.6) with a S1 = 1.36927
  • * Based on USGS 2003 Conterminous US Design Ground Motion data on http://earthquake.usgs.gov

STEP TWO - SITE CLASS, Fa, and Fv (ADJUSTMENT FACTORS)
The next step is to determine the Site Class.  Whether the ground is very stiff or very soft greatly affects the way the seismic forces are transferred to the structure.  In general, a stiff ground transfers the energy efficiently and a soft ground just absorbs the energy.  As such, the IBC provides a standard formula to adjust the S1 and Ss values determined above for your soil profile.  We strongly recommend that you consult with the Structural Engineer of Record since he already had to determine this class for his calculations.  That being said, the most typical soil type is 'D' or Still Soil Pile.

Table 1613.5.2 - Site Class Definitions
Site Class
Soil Profile Name
AVERAGE PROPERTIES IN TOP 100 feet, SEE SECTION 1613.5.5
Soil shear wave velocity, Vs (ft/s)
Standard penetration resistance, N
Soil undrained shear strength, Su (psf)
A
Hard Rock
Vs > 5,000
N/A
N/A
B
Rock
2,500 < VS <= 5,000
N/A
N/A
C
Very Dense Soil and Soft Rock
1,200 < VS <= 2,500
N > 50
Sa >= 2,000
D
Stiff Soil Profile
600 <= VS <= 1,200
15 <= N <=50
1,000 <= Sa <= 2,000
E
Stiff Soil Profile
VS < 600
N < 15
Sa < 1,000


You then look up the site coefficients from IBC tables 1613.5.3(1) and 1613.5.3(2) below.  Note that you can apply a straight-line interpolation for intermediate values and this can make a significant difference.

TABLE 1613.5.3(1) VALUES OF SITE COEFFICIENT Fa (a)
Site ClassMapped Spectral Response Accleration at Short Period
Ss <= 0.25Ss = 0.50Ss = 0.75Ss = 1.00Ss >= 1.25
A0.80.80.80.80.8
B1.01.01.01.01.0
C1.21.21.11.01.0
D1.61.41.21.11.0
E2.51.71.20.90.9
FNote bNote bNote bNote bNote b
  • (a) Use straight-line interpolations for intermediate values of mapped spectral response acceleration at short period, Ss.
  • (b) Values shall be determined in accordance with Section 11.4.7 of ASCE 7.


TABLE 1613.5.3(2) VALUES OF SITE COEFFICIENT Fv (a)
Site ClassMapped Spectral Response Acceleration at 1-Second Period
S1 <= 0.1S1 = 0.2S1 = 0.3S1 = 0.4S1 >= 0.5
A0.80.80.80.80.8
B1.01.01.01.01.0
C1.71.61.51.41.3
D2.42.01.81.61.5
E3.53.22.82.42.4
FNote bNote bNote bNote bNote b
  • (a) Use straight-line interpolations for intermediate values of mapped spectral response acceleration at 1-second period, S1.
  • (b) Values shall be determined in accordance with Section 11.4.7 of ASCE 7.

STEP THREE - Sds and Sd1 (ADJUSTED FORCES)
Now that we know the expected forces (S1 and Ss) and adjustment factors (Fv and Fa), we need to crunch some numbers to determine the adjusted force factors per the following simplified IBC formulas:

(IBC equations 16-37 and 16-39 combined)

(IBC equations 16-36 and 16-38 combined)

STEP FOUR - DESIGN CATEGORY AND ACCEPTABLE RISK BASED ON OCCUPANCY
Now that we know the adjusted design forces, we need to determine if these forces are great enough to require preventative actions.  Risk is related to the activity.  A hospital has a much higher risk than a temporary storage facility.  IBC recoginzes this in Tables 1604.56, 1613.5.6(1), and 1613.5.6(2) as follows:


IBC Table 1604.5 - Occupancy Category of Buildings and Other Structures
Occupancy CategoryNature of Occupancy
IBuildings and other structures that represent a low hazard to human life in the event of failure, including but not limited to:
  • Agricultural facilities
  • Certain temporary facilities
  • Minor storage facilities
IIBuildings and other structures except those listed in Occupancy Categories I, III, and IV
IIIBuildings and other structures that represent a substantial hazard to human life in the event of failure, including but no limited to:
  • Covered structures whose primary occupancy is public assembly with an occupant load greater than 300
  • Buildings and other structures with elementary school, secondary school or day care facilities with an occupancy load greater than 250
  • Buildings and other structures with an occupancy load greater than 500 for colleges or adult education facilities.
  • Health care facilities with an occupant load of 50 or more resident patients, but not having surgery or emergency treatment facilities.
  • Jails and detention facilities.
  • Any other occupancy with an occupant load greater than 5,000.
  • Power-generating stations, water treatment for potable water, waste water treatment facilities and other public utility facilities not included in Occupancy Group IV.
  • Buildings and other structures not include in Occupancy Category IV containing sufficient quantities of toxic or explosive substances to be dangerous to the public if released.
IVBuildings and other structures designated as essential facilities, including but not limited to:
  • Hospitals and other health care facilities having surgery or emergency treatment facilities.
  • Fire, rescue and police stations and emergency vehicle garages.
  • Designated earthquake, hurricane or other emergency shelters.
  • Designated emergency preparedness, communication, and operation centers and other facilities required for emergency response.
  • Power-generating stations and other public utility facilities required as backup facilities for Occupancy Category IV structures.
  • Structures containing highly toxic materials as defined by Section 307 where the quantity of material exceeds the maximum allowable quantities of Table 307.1.(2).
  • Aviation control towers, air traffic control centers and emergency aircraft hangers.
  • Buildings and other structures having critical national defense functions.
  • Water treatment facilities required to maintain water pressure for fire suppression.


Table 1613.5.6(1) SEISMIC DESIGN CATEGORY BASED ON SHORT-PERIOD RESPONSE ACCELERATIONS
Value of SdsOCCUPANCY CATEGORY
I or IIIIIIV
Sds < 0.167gAAA
0.167g <= Sds < 0.33gBBC
0.33g <= Sds < 0.50gCCD
0.50g <= Ss1DDD


Table 1613.5.6(2) SEISMIC DESIGN CATEGORY BASED ON 1-SECOND PERIOD RESPONSE ACCELERATIONS
Value of SdsOCCUPANCY CATEGORY
I or IIIIIIV
Sd1 < 0.067gAAA
0.067g <= Sd1 < 0.133gBBC
0.133g <= Sd1 < 0.20gCCD
0.20g <= Sd1DDD

STEP FIVE - IS BRACING REQUIRED?
Finally we have determined a Seismic Design Category based on the forces, adjustment factors, and occupancy of the building.  Based on the worst case seismic design category, we can go back to ASCE 07-05 and look at the following single paragraph:
13.1.4 Exemptions. The following nonstructural components are exempt from the requirements of this section: 
… 2.  Mechanical and electrical components in Seismic Design Category B.
So if Category A or B you are exempt, if C or D you need to provide bracing.

Look for the next post on calculating the forces on individual piping components in accordance with ASCE 07-05 paragraph 13.6.8.3.

2 comments:

  1. Great article. Hope to see part 2 soon

    ReplyDelete
  2. Thanks for such an interesting article here. I was searching for something like that for quite a long time and at last I have found it here.


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