Base plate thickness

[Kin214]

I am new to the engineering industry. I am designing a 20 feet post. I want to have a 12" square base plate on a 30" square footing. The bending moment that I have is huge compare to the axial load. So eccentricity is outside of the plate about 7 feet away. I know I need to introduce bolts, but I dont know what to do next. I try to follow the "Base plate and Anchor Rod Design" manual, but the large moment base plate example have eccentricity e inside the base plate. Please help. Thanks.

 

[BridgeSmith]

You'll have to design the anchor bolts to transfer the moment as a force couple and then design the base plate as a cantilever for the force in the bolts.

That will get you to the footing, which is where you seem to have a bigger problem, if I reading your post correctly. If your e is 7' from the post, the footing will overturn unless it too is anchored or restrained from rotation in some way. Otherwise your footing will need to be much bigger.

 

[KootK]

If it's something more like sign post, you might find useful information here: Link. It really is just a minor extension as the same theory you've been looking at for lesser eccentricities though.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BAretired]

If the eccentricity is 84", I don't think you can use a footing as small as 30" square because it would overturn.

Apart from that, you need to calculate the tension on the tensile bolts. With a 12" square plate you would be slightly conservative with:

T = P(e-d/2)/d
where P is axial load, T is tension in the tensile bolts and d is the bolt spacing.

So if d = 9" and e = 84" then T = 8.83P

BA

 

[sekwahrovert]

Don't forget to consider that the horizontal load creating the bending moment may be applied in a diagonal direction. If applicable, this load direction will almost certainly control the design of the anchors and base plate and may also control the design of the footing if the footing is limited by soil bearing stress rather than overturning.

Regardless of the direction of the load, the first step would be to calculate the anchor bolt axial forces. This will require that you determine an appropriate location for the center of compression. If the base plate is grouted, that can be a challenge, but it's probably near the edge of the column. If the base plate is held up on leveling nuts, then the compression load travels through the anchors and it's a much simpler problem. As HotRod10 stated, you can design the base plate as a cantilever transferring the bolt tension (and compression if applicable) back to the column. The cantilever length is the shortest distance from the anchor center to the nearest face or corner of the column. The effective width of the cantilever plate element is usually taken as 2 times the cantilever length, but should be truncated if the effective widths of adjacent anchors overlap or if the effective width extends outside of the physical edges of the plate.