Trapeze Seismic Bracing

[ReverenceEng]

I am working on a job at an airport and part of the scope includes these trapeze hanging directory signs.

These signs are going to be of the trapeze variety, BUT, in contrast to a standard unistrut trapeze to support mech./elec. that includes beam clamps, threaded rod with stiffener as applicable, a unistrut trapeze, and seismic bracing, the system we are working with is essentially a double trapeze.

What we have are beam clamps that support a unistrut trapeze as normal via threaded rod with stiffeners. This unistrut trapeze is just above the T-grid, and we are bracing it back to the deck above with a typical seismic bracing system. All fine. However, we have another trapeze of threaded rod that hangs from that unistrut, goes through the T-grid, and down anywhere between 3'-7' to support a sign directory (you know, things that say gates XX-XX to the right, baggage to the left, etc.).

My question is, how do we resist the moment generated by a seismic event or do we just let these swing wildly below the braced upper trapeze. Obviously, we can't do that, so I am trying to figure out what others may have done in the past to conquer this. For some of sites and signs, the seismic criteria, moment arm, and the weight is low enough that I can get something like GR 105 threaded rod of various thickness to resist the moment, but, certainly not deflection.

Has anyone some across this? There are signs like this all across the country, just I suppose not all in seismic areas.

 

[dreber]

Drawings could help. Never came across that problem specifically. Any signs like that that I have worked with resisted the moment with an HSS.

 

[JGard1985]

Sounds like you're going to have threaded rod in bending(tough to qualify) Why not have pinned connections and let the sign swing like a pendulum. Its going to be so flimsy that its natural frequency won't be near your structure (I'd expect a large deflection). Why not demonstrate that the swing of sign will not adversely impact people or structures (no adverse seismic interactions). Used to check seismic swing of cable trays using similar method when bracing was not available.

Jeff
Pipe Stress Analysis
Finite Element Analysis

http://www.xceed-eng.com

 

[bones206]

Instead of the lower tier hung by rod, could you make the lower tier using rigid strut members?

 

[nackra]

Related to bones 206 suggestion, I have seen this product used:

https://unistrut.us/products/seismic-support

 

[wannabeSE]

Bones206 is suggesting a detail similar to the elevated brace detail on the next to last page of this submittal:

https://www.google.com/url?sa=t&rct...le/Document&usg=AOvVaw3ELN7m9reE6HoRy16allX7.

Note: I just found the link by googling "ISAT elevated brace detail" because it seems like ISAT includes this or similar detail on every submittal.

 

[bones206]

I was suggesting the idea of using the braced horizontal strut at the ceiling level as a fixed base from which to cantilever rigid support members below the ceiling. Alternatively, you could just have rigid members attached directly to the structural roof and eliminate the trapeze part altogether (similar to image below, but with additional bracing as needed installed above the dropped ceiling).

Below is an image I found on google from the airport in Bali after an earthquake. The signs appear to be attached to rigid members, but the weak point seems to have been the connection to the sign itself, based on the missing sign on the right side.

 

[bones206]

Vancouver Airport has some elegant exposed bracing on their signage: Link. Although it's not clear if they are braced in the other direction.