billboard design

[MetroRAFB]

Hello all, I've got a client that's interested in having me design and seal a simple billboard. I live in central FL, and as best I can tell, the FL Building Code (nor the particular jurisdiction involved) doesn't have much specific information regarding billboards. I'd like to describe the logic I've used so far to come up with a design and I'd welcome as many opinions as possible.

The customer would like a simple two sided billboard made from round wooden "power" poles, with whatever frame would be required to attach plywood to both faces. They want the sign itself to be 30' wide x 10' tall, with the top of the sign being 25' above the ground. So far, here's what I've come up with, and if I'm full of **** or completely off base I'd love to hear it and how come.

I started by looking in ASCE 7-02 for zone 5 of a residential building and got the components and cladding pressures listed there for 120mph wind. (the applicable wind speed for the jurisdiction involved) I then calculated the total force generated for the 300sqft sign, in lbs. I then calculated the moment that would be created above grade at the vertical midline of the sign (20ft above grade). By my calculations, I'd need 5 poles sunk 10' below grade each in order to resist this moment so that the next time a hurricane rolls through here the damn thing doesn't blow over.

This is obviously my first billboard design from scratch and I'd love to hear what you structural gurus have to say about it. I have a thick skin and won't get offended if you tell me I'm a ******.

 

[bcse]

I think you would want to use Section 6.5 of ASCE7, the analytical procedure since it is not a building. If you take a look at Fig 6-20(Cf) Fig 6-4 (Kd) you will see that there are coeff just for signs. I'm not familiar with FL bldg codes so I 'm not sure if there is anything special about billboards and wind. Also see Section 1805.7 of the IBC for information about embedment depth for posts/poles as columns embedded in earth (if that wasn't where you were looking already). I would be nervous designing a sign like that without a calc check or advice from someone who had done it before. I am picturing all the billboards along I-10 where Katrina came through, though I think most of those looked like they just had the framing part that supported the actual sign collapse. Hope this is helpful...

 

[MA_PE]

I agree that you need to be thorough about billboards in FL, because fo the hurricane wind potential. As a minimum I'd consult the latest ASCE 7-05 and take it from there. Also you need to be sure of the soil conditions at the site, or make real conservative assumptions on soil properties, because there's a lot of crap soils in FL.

Good luck.

 

[kevo_55]

BSCE is right. Just use Fig 6-20 for the main member wind load design.

As for the wind speed values, I wouldn't worry about it too much. Off the top of my head, Miami-Dade counties specify a 146 mph exposure C.

Since this is an "other" structure, I do not believe that components & cladding would apply. But for kicks, it might be conservitive to just leave it in.

As an FYI, the 2004 FBC (with 2005 ammendments) uses ASCE 7-02. The new FBC which will become active January 1st, 2008 will use ASCE 7-05.

 

[MetroRAFB]

Thanks for all of your suggestions. I did some more digging in ASCE 7-02 and used the analytical procedure for "open or other structures" and, surprisingly, my wind force calculation was almost identical to zone 5 of a residence. That made me feel good about that aspect of the design, but I've gotta be honest and say that I don't have a warm fuzzy about the soil assumptions I made. I'll check 1805 of the IBC tomorrow and see if I can gain a comfort level. Thanks again, any more comments would be greatly appreciated!