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Base Plate Thickness

Base Plate Thickness

Base Plate Thickness

(OP)
The larger the base plate (area), in general, the thicker the plate must be.

However,

If the bearing stress, on concrete, is acceptable, within an area smaller than the anchor bolt layout perimeter (square), I'm convinced that I do not need to increase the plate thickness just because I must increase the plate area to accommodate the anchor bolt quad.

In other words, whatever plate thickness i have when the bearing stress is acceptable, that is the max thickness my plate needs to be regardless of the anchor bolt layout, and thus, the base plate plan dimensions.

I feel this is elementary principles. But, a program i use requires increased plate thickness whenever the plan dimensions increase. And after all these years, i thought I'd seek a second opinion.

RE: Base Plate Thickness

I agree with your logic.

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.

RE: Base Plate Thickness

I agree with you.

If the bearing pressure is truly uniform under a base plate (which it isn't, by the way), and the base plate is too thin, the base plate will deflect, causing the bearing pressure to concentrate itself under the center of the base plate. Thus, your assumption is justified.

DaveAtkins

RE: Base Plate Thickness

I'll agree too.

To expand a little, you're arguing that for a larger baseplate, the bearing pressure isn't effectively uniform over the entire area, but could be assumed to be uniform over a smaller patch area concentric to the column.

If your assumption happened to be wrong, and the edges of your base plate saw high bearing stresses, they would yield and deflect away, reducing the bearing pressure and causing your premise to be self-fulfilling.

For a ductile steel plate controlled by bending in the weak axis, makes sense to me.

RE: Base Plate Thickness

My slow typing strikes again!

RE: Base Plate Thickness

I've seen some very thin base plates where the phenomenon that we're discussing actually put the anchor bolts in tension. And that would obviously add to the compression burden beneath the base plate. It's not as though we consider that kind of stuff in regular base plate design however. Still KootK approved.

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.

RE: Base Plate Thickness

Is the column loading strictly vertical downward, or is some moment resistance required?

RE: Base Plate Thickness

(OP)
Tmoose, regarding the OP, strictly vertical gravity.

RE: Base Plate Thickness

Another way to look at it--if you had a base plate which was one mile wide by one mile long, and you loaded it in the center, no one in their right mind would think there was bearing pressure at the edges of the plate.

Of course, structural engineers may not be in their right minds...

DaveAtkins

RE: Base Plate Thickness



Quote (BSVBD (Structural))


Tmoose, regarding the OP, strictly vertical gravity

Yeah. Them there oscillating horizontal gravity waves get tricky when you're trying to get the grout under the 1 mile x 1 mile flat baseplates .....8<)

RE: Base Plate Thickness

I have had this argument several times with reviewers who thought the plate (with applied vertical load only) needed to be bigger. Usually the argument arose when the plate was a design build item with a specified minimum plate area. I design the plate thickness for the allowable bearing on concrete and then just increase the plate length and/or width to meet the specified minimum area. As Tmoose pointed out, any moment applied by a column could control the plate size and thickness.

www.PeirceEngineering.com

RE: Base Plate Thickness

I would love somebody to come on here and make the argument that you should provide a layer of compressible material under the area of extra plate. not going to be me.

RE: Base Plate Thickness

If design says a column needs an 18" x 18" x 1" steel bearing plate, the column and/or footing will not fail because you oversized the plate to be 20" x 20" x 1".

www.PeirceEngineering.com

RE: Base Plate Thickness

One of my mentors, very early in my career, said to me, "Have you ever heard of a baseplate failing?"

I can't say that I have.

DaveAtkins

RE: Base Plate Thickness

Quote (DaveAtkins)

One of my mentors, very early in my career, said to me, "Have you ever heard of a baseplate failing?"

Ditto for my mentor. And it was in response to my concern over a column coming down on the corner of an 8" frost wall that shouldn't have remotely worked by the numbers.

Quote (DaveAtkins)

I can't say that I have.

Me neither for gravity only. For base plates with bending, they seem to fail pretty much all the time when tested.

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.

RE: Base Plate Thickness

(OP)
Although this is arguable... IF the base plate did (technically) fail (first), i think it would only be evidenced by the failure of the concrete or masonry... but, let's not prove it!

RE: Base Plate Thickness

The first edition of the Design Guide 1 used to have a design procedure for a "lightly loaded" baseplate. The premise was that only a small area was effective when the load was under a certain amount. I used to use that sizing procedure for steel storage racks. I do not have the version of the guide handy to point you at the pertinent pages.

RE: Base Plate Thickness

@ Klitor, what code does that come from? Doesn't look like Eurocode, or is it?
I recognise the tp ; c and Aeff denominations, but not this figure.

RE: Base Plate Thickness

It is, acctually got the screenshot from SCI P358 "green book".

Seems like a resonable approach to me - I remember ,in books only unfortunately, seing gravity columns of really tall bldgs having huge *stiffened* base plates to distribute gravity loading on mile wide base plate as DaveAtkins would say.

RE: Base Plate Thickness

Thx. Didn't know about this "green book", just googled it and it seems interesting. I always have my EN1993-1-8 at hand though...

RE: Base Plate Thickness

I agree. It's one of the things I try to teach the younger engineers when I see enormous baseplates. If an 8"x8"x1/2" baseplate works, then you add N and B dimensions to it at will.

Enercalc software has an option to design it like this. I call it the "magic button".

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

RE: Base Plate Thickness

(OP)
manstrom - I use Enercalc. Where is this "button" you speak of?

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