DS 2-8 Earthquake Protection for Water-Based Fire ... - FM Global

Earthquake Protection 2-8
FM Global Property Loss Prevention Data Sheets Page 49
December 1998. This data sheet was issued superseding information contained in the FM Global Loss
Prevention Handbook.
September 1998. This data sheet was converted to electronic format.
APPENDIX C SUPPLEMENTAL INFORMATION
Appendix C contains additional commentary and examples related to Section 2.0, Loss Prevention
Recommendations.
C.1 General Concepts of Sway Bracing Design
There are four steps to properly design sway bracing:
• Step 1: Lay out sway bracing locations with respect to the sprinkler piping and to the structural members
to which the bracing will be attached.
• Step 2: Calculate the seismic design load for the sway bracing locations.
• Step 3: Select the proper sway bracing shape, angle of attachment, size and maximum length based on
the horizontal design load.
• Step 4: Select the proper method to attach the sway bracing to the structure and to the piping.
Step 1: Lay Out Sway Bracing Locations (Section 2.2.1.1)
Bracing is needed on all risers, feedmains and crossmains (i.e., regardless of size), and on those branch
lines that are 2- 1 ⁄ 2 in. (65 mm) and larger in diameter. For risers and overhead sprinkler piping, there are two
sway bracing designs: two-way and four-way.
Two-way braces are either lateral or longitudinal, depending on their orientation with the axis of the horizontal
pipe (See Figs. 3 through 6). Lateral and longitudinal braces resist movement perpendicular and parallel,
respectively, to the axis of horizontal pipe. When located close enough to a change in direction of the pipe,
a lateral brace can also act as a longitudinal brace (and vice versa) for an attached perpendicular pipe of
the same or smaller diameter.
Four-way sway bracing resists movement in all horizontal directions, and is typically provided on risers and
drops. When located close enough to a change in direction of the pipe, a four-way brace can also act as
a longitudinal and lateral brace for an attached horizontal pipe of the same or smaller diameter. Four-way
bracing on a horizontal pipe is simply a location where lateral and longitudinal sway bracing coincide. This
four-way bracing may be used to satisfy lateral and longitudinal design requirements for horizontal piping at
changes of direction.
A key concept is that, regardless of the direction the earthquake motion, the combination of lateral and
longitudinal sway bracing that is properly located will result in a sway bracing system that has the best
chance to minimize potential damage to the system. For example, if the lateral sway bracing is aligned in the
north-south axis, and the longitudinal sway bracing is aligned in the east-west axis, an earthquake that
creates movement in the northwest-southeast axis will require proper interaction of the entire sway bracing
system to minimize potential damage. Neither the lateral nor longitudinal sway bracing by itself would be
expected to handle non-axial seismic loading.
Sway bracing layout locations will usually need to coincide with the structural members to which the sway
braces will be attached.
The maximum spacing between sway braces given in Section 2.2.1.1 may need to be reduced depending
upon the actual seismic design load determined for each sway bracing location in Step 2.
Step 2: Calculate the Seismic Design Load for the Sway Bracing Locations (Section 2.2.1.2)
The design load for each sway bracing location is calculated by multiplying the cumulative total weight of
the piping within the zone of influence for that bracing location times the appropriate horizontal acceleration
“G” factor. The zone of influence for a sway bracing location includes all piping to be included in the load
distribution calculation for that particular bracing location, based on the layout of all the bracing on the system.
It is usually helpful to prepare a brace location schedule with the calculated loads to help with Steps 3 and
4. As a practical matter, braces are typically sized based on a few controlling zones of influence; a unique
brace design is not used for every sway bracing location.
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