Suspended Ceiling Lateral Bracing

[xez]

Hello,

I have a few questions regarding seismic lateral bracing for suspended ceilings. For acoustical lay-in ceiling panels, the building code and ASTM E580 states that four No. 12-gauge wires splayed at 90 degrees from each other at an angle not exceeding 45 degrees from the horizontal plane can be used as horizontal restraint systems. These assemblies are placed 12 ft on center in both direction, which gives us a tributary area of 144 sf.

However, the horizontal loads in Design Categories D, E, & F typically require that the bracing be spaced closer together. That is, the 12 gauge splay wire at 45 degrees with a safety factor of 2 does not meet the load requirement. What happens in this case? It would make sense to just increase the wire to a higher gauge, increase the amount of 12 gauge wires used or place the lateral bracing more frequently across the ceiling. I was not able to find anything in the building code (including California Building Code) or ASTM E580 that references this - only that the assemblies are placed in a 12 ft x 12 ft configuration. What is the reasoning behind this configuration?

Also, I typically design with insulated metal panels which are much stronger and can span farther than 12 ft on center. What about if we want to support the lateral loads in distances greater than 12 ft? Can we increase the strength of the wires used at each location? It would not be uncommon to have tees spaced about 16 ft on center. Hanger rods would be used typically every 5 ft on center. This could give us a 10 ft x 16 ft configuration instead of the 12 ft x 12 ft standard.

Thanks for the help.

 

[thaidavid40]

Every contractor has the 12ga wire. Call for more anchors, and more wired-in points.
Dave

Thaidavid

 

[XR250]

How much horizontal seismic load exists for 144 sq. ft. of ceiling tile?

 

[xez]

One 12 gauge (70 ksi) splay wire in one direction will give 358 lbs. Using a safety factor of two, this gives us a maximum horizontal design force of 126.57 at a 45 degree splay. At a 1.4 Sds and Site Class D, the allowable ceiling operating weight (wp) is 188.35 lbs. I divided this by 4 PSF (minimum ceiling weight) and got 47.09 SF as the maximum area each splay wire can resist. However, the code says 12 ft bracing in any direction which is where I get the 144 SF maximum. It doesn't really say anything about adding more splays or more anchor points. Just trying to get some clarification regarding this. Thanks.

 

[XR250]

Darn, who woulda guessed it was that high.
4 PSF seems pretty conservative for a dropped ceiling. I typically use 2 PSF.

 

[hawkaz]

I believe ASCE7-10 requires you to use 4 psf minimum. I agree with XR250 that this is conservative- but it is required.

I call out 8 gage wire and hold the 12' dimensions.
Tightening up the spacings will require more compression struts.

 

[xez]

Great. This certainly makes perfect sense to me. Just got confused after searching online where some standards don't call for a "minimum" gage or spacing. Thank you.

 

[WAMRE]

Where does the 1.4 Sds come from exactly? I understand that the 4psf minimum is a gravity design load for the main runners and minimum total dead load for the lateral system; isn't the actual horizontal seismic design force less?

 

[Hokie93]

For what is worth, I believe you should be multiplying F[sub]p[/sub] by 0.7 to get the seismic force to a service-load level since the 12 gage wire strength is an allowable load. It doesn't make the issue go away but it helps.

WAMRE; I believe the OP is saying that Sds = 1.4. I was able to verify his numbers using ASCE 7-10 Equation (13.3-1) if I set Sds = 1.4.

 

[xez]

The Sds = 1.4 was an arbitrary value I used for this example.

Hokie93: Thanks for reminding me of the 0.7 service-load factor. Doesn't really help for the example, but can help with future calculations.