Re-designing Existing Pylon Sign for Wind.

[JohnnnyBoy]

I am re-designing a Pylon sign (30' tall). I say re-designing since the structure has been installed for a while according to my client. No exact date but over 10 years. The reason I am asking is the way I would typically design them is failing the sign.

NBCC 2015 Commentary: Billboards: Fn=Cf*Cn*q*Cg*Ce*h*lw (Figure I-24)

My question is have my assumptions always been to conservative.

Cf=1.3 depending on height/width ratio
Cn= 1.0
q= Always considered 1/50 year. Can this be reduced, to me 1/50 should be used but maybe 1/10?. (Life expectancy roughly 50 years).
Cg = 2.5. I consider it the main structural member but maybe I can reduce this to 2?
Ce = I always consider open terrain for larger signs even within a city (30' tall). Do others tend to agree?
Iw = 1.0 Normal importance.

Client is removing a sign and replacing with a smaller sign thus reducing the wind load (95% of previous wind load). Why I'm hesitating is they are only reducing load and now I am leaning towards telling them the sign overall needs to be reinforced or strengthened. Just wondering how other professionals would handle a situation like this.

 

[azcats]

On the exposure, I generally (almost always) lean towards open terrain on new construction but would be flexible in a case like this if it meets the definition of rough.

Similarly, I also think there's some wiggle room on the importance category for a sign. (but again I'd almost always use normal importance on new CX)

If using one or both of those doesn't make the structure adequate, I'd assume that the sign wasn't previously engineered and go on about reinforcing as required.

 

[phamENG]

I don't design under the NBCC, but ASCE 7 is pretty similar.

I don't know what your Cg is, so I can't speak to that (although if it has something to do with primary/secondary structural members, the pole of a pylon sign is absolutely a primary member).

For exposure, I'm fine with using rough terrain if the site meets it. If this were on some country road with no buildings and no trees, then open terrain would be appropriate. But most wind design data is based on wind at 10m/33ft off the ground and calibrated based on height. For a 30ft sign, I'd use rough if it's in the middle of a developed area. (Under ASCE, a parking lot at a big store can sometimes tip it back to open, so be mindful of that.)

I would stick with normal importance and, if the code gives you the option of selecting a return period I would use the return period for structures in the vicinity. A sign like this, if it fails, can become a very hazardous projectile(s). If it comes down well below the surrounding buildings, it could case significant damage or injury, especially if people are still out and driving around.

As far as telling them it has to be reinforced - so be it. I can go out and build a house that will stand up for years with no shear walls or diaphragms, or even a steel building with all bolted, idealized-as-pin connections with no bracing. They stand up. They don't fall down. They even go through big storms. The question isn't whether or not it will fall down as soon as it's built, is a question of how reliable it is under the code mandated worst case scenarios. Buildings are built, used, and torn down without ever experiencing those worst cases. And then some are built, used, and collapse because they did experience them but weren't designed properly.

 

[dik]

I would use your values... Cg = 2.0 can be used, but for cantilevered signs I usually use 2.5 due to lack of redistribution.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[JohnnnyBoy]

Thanks for the consensus everyone. I think I got a pretty good way to go forward.