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Do I need a geotech? 1

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DETstru

DETstru

Structural
Nov 4, 2009
395
US
I'm wondering if the sign structure below is worthy of getting a geotechnical report or if presumptive values in the code (US) are good enough.

I guess it's more of a "what will the plan reviewer say" type question, because from a design standpoint I can just make it work with the low bearing/lateral capacities with bigger footings.

sign_odjd7w.png
 
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For us it is usually a question of how big are our footings getting relative to using minimal code/assumed soil parameters - vs. the cost savings (possibly) of getting a geotech to take a boring and give you a report.



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faq731-376
 
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Around here, central prairies north of the 49, we would get crucified if we asked for a geotech for this.
 
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I'd do it with a site inspection prior to pouring the concrete and no geotech report. Presumptive soil bearing capacities are permissible for most structures by my understanding.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
 
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Even with this condition? :)

Sign_with_Baby_r5ubzt.jpg




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This old guy has been around a long time and has done reports for even thousands of structures in the areas where there are plenty of signs. Never been involved with any of these signs other than providing limited above ground structural construction info. Not even those that have blown over. Have been regularly used by contractors of many types for many things, but no signs for support. Just follow the codes.
 
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The AASHTO Standard Specifications for Highway Signs etc. has a very useful design nomograph for lightly loaded signs. This might be useful for your sign.

I think there are older free versions of this specification online. Section 13 is the foundation section.

AASHTO_Nomograph_xtg6sl.jpg
 
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According to JoelTXCive's nomograph - assuming the worst possible soil type, 2,300 lbs. per post (90 mph Exp C) at 15 ft. above grade, and a 24" diameter pier, you get a 9.5 ft. deep pier requirement.
That doesn't seem too bad for such a large sign.
Sign_nomograph_uimgje.jpg


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JAE: ha! Definitely need to design for that.

Thanks everyone. I don't do much work this small and am basically doing this as a favor for a good client.
Thanks for confirming for me.
 
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