Billboard Sign Footing 2

[Gus14]

I am interested in billboard signs design and I got my hands on the structural plans of one. The Billboard sign dimensions are 12 meters x 4 meters with a total height of 40 meter form the top of the footing to the billboard sign center. I added a sketch for reference. The footing dimension are 10 meter x 10 meter with 2 meter depth.

From what it looks like, the structural engineer relied on the weight of the footing to resist overturning. However, for a billboard that tall, do you think a triangular soil stress distribution will cause a visible differential settlement? Would you prefer pile cap footing in this situation ?

 

[azcats]

It's horribly inefficient to design these ballast foundations and try to keep the eccentricity inside the kern. Not uncommon to see it outside - especially in short term loading. If it's a dead load adding moment from a cantilevered type sign structure, I do my best to move the column in the foundation so the static bearing pressure is fairly evenly distributed.

To be honest, the footing above are kinda last resort (in my area anyway) if other options are available. We see many more laterally loaded drilled piers or block footings.

I'm looking at one right now that's going to require a pile cap with (likely) helical anchors. On a site where they don't want a drilled pier due to high water table amongst other considerations, there's several feet of fill so we can't utilize lateral loading for some depth, and the spread foundation would require way too much over-excavation and re-compaction.

Back to your main question, if static loading is balanced, I have no issue with short term loading pushing the resultant outside the kern provided my soil pressures remain within their limits.

 

[Gus14]

Thank you azcats for replying and sharing your experience.

 

[Ron247]

In my area, sign suppliers prefer the foundations as depicted in your sketch for smaller signs such as you show, as long as they can "dig it with a backhoe". After that, they are not happy with whatever you come up with.

The sign you show is one of the simpler layouts. Wait till they send you one that is "fully flagged". The pole would be on one side of the sign (Like a flag). You get wind load bending in one direction with DL bending in the other direction with a healthy dose of torsion on the pole. The more you accommodate what they want, the more creative they get. It has been years since I did any, but one I did was a double stacked, 4m x 16m signs, "V-Configuration", so there were a total of 4 sign faces at 4m x 16m each. At least they did not flag that one. Later, they did a double stacked Back-to-back that was fully flagged. I do not work in a high wind load area.

 

[Gus14]

Thank you Ron for replying. Yeah I agree with you, for a flagged sign, I would probably use a pile cap. Even for the case I sketched ( it was designed by another engineer ) I am not really convinced with the single footing solution. Having to provide minimum reinforcement for such a large footing would be very expensive, while using piles will be more economical and easier to build.