Base Plate Thickness

[BSVBD]

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.

 

[KootK]

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.

 

[DaveAtkins]

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

 

[Lomarandil]

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.

 

[Lomarandil]

My slow typing strikes again!

 

[KootK]

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.

 

[Tmoose]

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

 

[BSVBD]

Tmoose, regarding the OP, strictly vertical gravity.

 

[DaveAtkins]

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

 

[racookpe1978]

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<)

 

[PEinc]

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.

http://www.PeirceEngineering.com

 

[mijowe]

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.

 

[PEinc]

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".

http://www.PeirceEngineering.com

 

[DaveAtkins]

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

 

[KootK]

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.

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.

 

[BSVBD]

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!

 

[RobertHale]

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.

 

[Klitor]

European code has a formula for effective area. The same goes for moment connestions (T-stub method).
Does your code have something similar?

 

[kingnero]

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

 

[Klitor]

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.