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Seismic requirements for Fixture tie off in ceiling grid SDC D

joetheinspector

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Joined
Jan 1, 2010
Messages
152
IBC 2009, Seismic design category D

Question: what are the requirements for tie off of light fixtures (Standard 2ft by 4ft florescent)? I am finding out the weight of the fixture.

Some of the fixtures are tied off to the wall (light gage steel stud) not the roof structure and some of the wires are tied off with two wires on one side of the fixture.

I am used to seeing the fixtures tied off on opposite corners (diagonal).

Question: Is there a code or standard that requires the tie wires to be on diagonal corners (not the same side)?

Also I thought that there was a code or standard that required the fixtures be tied off to the roof structure (not the walls). However I cannot find this section now.

Question: is there a code or standard that requires the tie wires securing the light fixture be secured to the roof structure or something secured to the roof structure? Or can the tie wires be attached to the wall?
 
Assuming you are talking about suspended ceiling the code is as follows

IBC 2012

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 11A. The seismic design category for a structure is permitted to be determined in accordance with Section 1613 or ASCE 7.

ASCE 7 -10 Section 13.5.6.2.2 requires compliance with ASTM E580

ASTM E580 Section 5.3 requires the supplemental wires to the structure above

10 lbs or less = 1 wire

Greater than 10 lbs but less than 56 lbs = 2 #12 wire to the structure above

It does NOT require the wires to be on opposite corners. However on lower ceilings it is a problem with the light fixtures having a "hinge" effect during a seismic event and hitting people on the head. Good industry practice is to place the wires in diagonal opposite corners
 
Technically, 2009 IBC references ASCE 7-05 which references CISCA's Guidelines for Seismic Restraint for Direct-hung Suspended Ceiling Assemblies for Seismic Zones 3 and 4. There is almost know difference between CISCA's guidelines and ASTM E580. Both say the wires are to be connected to the structure above. The California Building Code adds the additional requirement to have the wires at diagonally opposite corners (2013 CBC 1616A.1.20 and 1616.10.16)
 
Yes it is a suspended ceiling. Thanks for the good info.

This is a low ceiling 8'. With the wires attached on one side it will create a hinge effect in a seismic event. This does not meet the intent of the code. What opinions are there out there about requiring the wires to be diagonal to meet the intent of the code?
 
When a piece of equipment is required to be braced it must be restrained against movement in each direction. In the case of a rectangular device such as a rectangular light fixture it needs to also be braced to keep it from rotating in plan.

When wires are used to brace the equipment you will need wires on opposite sides of the device. If you use something that can resist compression forces as well as tension then the brace may be on only one side.

The braces can be tied to walls if the walls have a mechanism to transfer the loads to the floor or roof above.

If your jurisdiction has an engineering plan checker talk with him/her regarding this issue since there can be many ways to provide the necessary bracing.
 
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