Analysis of existing billboards... 3

[mrnico]

I have been asked to analyze two existing billboards which were disassembled. Now, the owner wants to put them back up and wants me to tell him they will be safe. They are to go end-to-end for a total sign area of about 12' x 40'. Each section is to be supported by a single pole, tapering from 21.75" to 7" diameter, with a wall thickness of about 3/16", at 39-8" long. About 12' was buried in the ground, and he wants to do the same to put them back up.

The main structure is two 6" I beams with 6" wide flanges and 1/4" thick, turned upright into an H pattern. These are bolted to the pole about 78" apart at bolt pads made of C channel welded to the pole. Spacing the two beams are vertical C3x1.5x7/32" sections 154" long, with holes drilled through 1 flange at 18" o.c. for the horizontal panels to attach with J bolts. These verticals are welded to 5x5x1/4" plates, which are bolted through the beam flange on the front side.

At some point, someone modified these by adding a back side surface, supported by the same verticals, spaced off the beams to clear the poles with spacers made of the same C sections with the 5x5 plates welded on each end. However, the back side has L2.5x2.5x0.25 clips bolted to the verticals, with horizontal running angles bolted on top of the clips. These supported plywood vertical panels, which rested on the upright angle leg via clips.

So...I figure I can analyze it for wind, etc. based on height and area of the sign panels. Then check the beams for adequacy in bending about the strong axis, then check the verticals, etc. Finally, go to the plates and check for bending, etc. and bolt strength. I will have to assume steel types for the bolts, plates, channel, etc, as it's half site-built by rednecks. Any advice on best or easiest way to proceed? I have done flag poles, but this is new to me. Thanks.

 

[MiketheEngineer]

Sounds like you know what you are doing... Don't foget to check for unbalanced loads, ice maybe, snow maybe.

I worry about signs fluttering - not sure how to figure that.

 

[JedClampett]

There's no magic here. Track the loads (predominately wind) from the little members to the bigger members to the posts to the foundation. If you are concerned about an element (connection, beam, base plate) detail it out and check its pieces. Take a look at the craftmanship and determine what portions (welds, bolting, etc.) you can trust and which ones you can't. There's a lot of rednecks out there on construction sites and some of them are pretty skilled. Others are not.
I would guess, without running any numbers, that all the members will check out OK (maybe close), but the foundation and base plate connection will be the most difficult item to design. But you can do anything you want there.

 

[mrnico]

The bottom 12' of posts will get buried in native soil and compacted (hopefully!). No other foundation to be built. I will check moment at the base and see if it looks like the soil will hold. I don't have a soil report available, so will have to go with the NRCS website soil data and the soil book. It is in the desert, so our snow loading in this area is zero. Maybe 5psf just for good measure...

 

[azcats]

Be sure to check out ASCE 7-05 figure 6-20. The eccentric loading from cases B and C can induce quite a bit of torsion in the column and case C (if applicable) can really spike the loading at the ends of the sign face.

 

[mrnico]

I am looking at ASCE 7-05 and also have 7-10. I still have to get into the new book to see if it is better. They have made some big changes to the wind sections in the new one.

 

[slickdeals]

You can use equations from IBC/UBC to calculate embedment required for poles in soil. You may not need 12' depending on the shears/moments.

 

[Ron]

mrnico...if you use ASCE 7, use the version that is referenced in your applicable code.

 

[azcats]

I'd be surprised if 12' is enough foundation depth for a ~20" pole when utilizing the UBC/IBC pole embed criteria and backfilling with native soil. You can drill a larger shaft and will with concrete and possibly utilize the larger diameter for design.

And now that I understand that you're putting two 12x20 faces next to each other, the case B and C wind loading is going to heavily load one pole.

Also, don't forget to check that walkway for live load. It will be well beyond the snow load. The problems from a failing walkway are much worse than if it blows over in a field.

 

[broekie]

You can also check out the publication: AASHTO Specification for Structural Supports for Highway Signs, Luminaires, and Traffic Signals