How to properly resolve a moment on top of a pedestal 1

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I'm having a brain fart...using SPMats for a mat foundation and all of my shear+moment forces need to be resolved into moment forces along each axis.

SPMats loads the foundation from the center of the foundation so shear forces is simply (force x distance of pedestal). When it comes to the applied moment at the top of pedestal I currently have...say a 10 kip*ft force on top of a 6 ft pedestal. How do I properly equate that to the middle of the foundation?

 

[BAretired]

Moment is a vector, so you would have to apply the vector. Maybe a sketch would help.

BA

 

[JLNJ]

You have moment but no shear?

 

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I'm attaching a quick sketch. The SPMat program must be loaded from the center of foundation...all loads on top of pedestals must be broken out.

Shear would be (10k * 6.75ft). My question is with respect to the applied moment at the top of the pedestal and how to properly vector this to the middle of foundation.

 

[Celt83]

When summing moments globally, concentrated moments are included in the sum independent of application point.

If the software does not allow the moment to be applied at it's actual location then bearing pressure computations will be OK (as long as the rigid base assumption holds) but the cross-section designs for the foundation will be incorrect.

I'm making a thing:

http://www.thestructuraltoolbox.com

(It's no Kootware and it will probably break but it's alive!)

 

[ads0221]

That's what I was thinking.


Would you mind expanding on this?

 

[WinelandV]

ads0221,

draw a simply supported beam and load it with ONLY a moment on it. Play around with the location of the moment. You'll see that the reactions don't change, but you will see that the internal shears and moments that the beam experiences do change.

 

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Good point. For awhile I kept looking at it like a fixed cantilever.

Thanks for the replys everyone.

 

[BAretired]

There is a problem with your quick sketch. You are showing an applied horizontal force of 10k and an applied moment of 10'k. The force generates shear of 10k throughout the pedestal and requires a horizontal reaction somewhere in the footing.

The applied moment generates a constant moment throughout the pedestal and an applied moment to the footing, which will require opposing vertical forces acting on the footing to resist it.

Shear would be (10k * 6.75ft).

The above statement is incorrect. The shear from the moment is zero, but shear from the applied 10k load is constant for the height of the pedestal and dissipates into the footing. Moment at the center of the footing is 10*6.75 from the horizontal force plus 10'k from the applied moment, giving a total moment of either 77.5'k or 57.5'k (depending on the direction of the moment) at the middle of the foundation. Check your statics.

BA

 

[JLNJ]

Agree with BAret. That's why I asked about the shear. Rarely do you have moment with no shear in these type of structures.

You have to define all your moments and shears in the plane of the mat. This is because it's essentially a 2D program. RISA Foot is the same except it will do the "statics" for you if you define the pier height.

 

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We're on the same page...my wording was incorrect in the reply previous when I meant to say "how I got the moment" was to take the shear and lever arm.