Larger footing-larger settlement 1

[haynewp]

I have gotten lower allowable bearing pressure values for mats before which I understand that through activating soils deeper in the ground that are poor would give higher settlements than a smaller footing that affects the upper soils only, assuming they are better quality.

But I was talking with another engineer that suggested you would always get larger settlement with a larger footing. For example, 500 kips on a 10 ft square footing would give less settlement than 500 kips on a 20 ft square mat. Assume the load is distributed over the same area in each case (say 2 square feet at the center).

I don't see how this is possible if the soil that is affected by the pressure bulb is the same quality in each case. I can sort of understand how it could happen if the deeper soils that were affected in the larger footing case were of poorer quality than the upper soils, and assuming a rigid footing. But often I see better blow counts with deeper soils.

Thoughts?

 

[hokie66]

Maybe the "Poke" comes from the bag you gather it in. In Southside Virginia, we called a paper bag a "poke". And a "pig in a poke" was anything you bought sight unseen.

 

[haynewp]

Ron, a girl I know carries hot sauce around in her purse to put on everything she eats (I thought that was awesome).

My family is from Alabama too. My fav is collard greens, black eyed peas, fried chicken and corn bread. I never thought I would be admitting that on THIS site.

 

[hokie66]

You left out the stewed tomatoes.

 

[haynewp]

Yes! My great grandmother had her own recipe using fresh tomatoes we put on everything green. Sweet and hot...

 

[msquared48]

Whatcha guys talkin' 'bout?

Mike McCann, PE, SE (WA)

 

[Ron]

Come on down, Mike....sometimes requires elastic waist though!

haynewp.....and you didn't marry her??!!

 

[oldestguy]

OK you Yankees, here is the straight scoop. My learning about Poke salad was from the guys who made it known that if your means were pretty scarce, you had this as one of the main food course way back then. Notice my spelling of "salad" was not right.

This Google search explains a lot.

https://en.wikipedia.org/wiki/Phytolacca_americana

 

[haynewp]

Already married...

 

[aeoliantexan]

Interesting discussion about salad. In Iowa we ate the leaves of a barnyard weed called lamb's quarter for hot greens. It wasn't very widely known, but it was a very mild green.

As for the original subject, if consolidation is not an issue, the formula for elastic settlement of a square footing is S = 0.95 qb(1-u^2)/E, where q is the ground pressure, b is the footing width, u is Poisson's ratio, and E is Young's modulus. For a constant load, if b is doubled, q will be reduced by a factor of 4, so the settlement is reduced by a factor of 2.

 

[BigH]

When I was in Laos, we were at a picnic and my wife asked about some "greens" - so one of the lads goes over to a bush and does the old sliding the hand up the stem collecting the leaves - then gave them to my wife to eat . . . of course you wouldn't believe what they ate there and what they diligently hunted for . . .

 

[SlideRuleEra]

Be sure to eat Hoppin' John and Collard Greens today! Happy New Year!

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Riggly]

Conclusion, all else being equal, increasing the size of the footing reduces elastic settlement.

 

[BigH]

Riggly - elastic settlement of the footing is proportional to the size of the footing. You said all else being equal - so that would be the bearing pressure, eh? So, if the bearing pressure stays the same, the elastic settlement will increase(or decrease) proportional to the footing size.

 

[hokie66]

BigH,
In the context of this thread, I think "all else being equal" means the force on the footing, not the pressure.

 

[Riggly]

BigH, Hokie66 is correct on the context of the post. A larger footing with the same load as a smaller footing would result in a smaller applied bearing pressure, provided the pressure is evenly distributed, as in the scenario being discussed in this post.