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SIGN DESIGN MOMENT ARM 1

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PT999

PT999

Structural
Joined
Oct 3, 2002
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150
Location
US
I am designing a sign foundation with a standard buried rectangular concrete foundation.

Moment Arm (Overturning)
The wind load moment arm is applied from the center of the sign to where..... the top of the concrete foundation or the bottom of the foundation, or is it the center of the foundation? I just can't remember.

The moment resisting. I am calculating concrete weight x
1/2 the width of the concrete foundation, and looking for 2 to 1 factor of safety.

This assumes the soil plays no part in resisting overturning. Is there a way to consider soil also?

 
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Resisting moment is correct (1/2 the footing width in the direction of the load). You can take advantage of the weight of the soil above the footing or you can pour a deep pad (2'-5' deep) with the top of pad 1' below grade (or some other convenient dimension).
 
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Go for the bottom of the foundation for the summation of moments for sizing the foundation for soil pressure. Summing at the top of the foundation gives the moment in the pole or column. If you have the weight of the soil and the location of its c.g. you can account for it. I try to be cautious with that since soil is an indefinite thing in the field. You might not have the assumed depth of soil by the time you get to the design load sometime in the future.
 
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PT, bottom of foundation...it is pretty basic statics. Soil pressure against side of footing doesnt resist. Only soil above footing contributes to dead load resisting moment. I have done lot of pole foundation signs also. There's a formula involved with that.
 
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Ok thanks, the basic point your telling me is that soil does not resist against the side of a foundation. But somehow it seems that is should.

Maybe the point is that it is useless when it is saturated.
 
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PT, the soil doesn't resist against the sides because there isn't enough footing rotational movement to mobilize passive pressure. Only pole footings include this lateral resisting pressure.
 
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How deep is the concrete foundation?
 
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Dimensions are 4' x 4' square x 12 ft deep
 
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Then yeah, I would include soil resistance on the sides to resist the moment. Look in UBC for foundation embedment equations that use passive resistance of the soil. Also, Enercalc has an embedment module that can get you started. Talk to your geotech about what passive resistance you can get on your foundation. I think everybody was assuming you just had a spread footing.
 
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Also keep in mind that it's much easier to construct a sign foundation using sonotubes in a drilled hole as opposed to a formed footing. For two legs use two sonotubes.
 
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Regarding Enercalc's module for pole footing design, for those who don't have that software, you can request a free Excell spreadsheet from me. It doesn't give the iterative solution automatically, like Enercalc does, but it is easy enough and accurate.
Since this forum doesn't want to attract spammers, I can be "elect-mailed" : advjb (not case-sensitive) at yahoo dot com (sneaky way to avoid auto detection of address, LOL)
 
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