Compressive Bending of a Base Plate.. How? 6

[MegaStructures]

Question not for a current design, just interested.

I'm going through AISC DG1 - Base Plate and Anchor Design and noticed that the guide recommends designing the base plate for bending when a compression load is applied to the column. How in the world would a base plate bend from an axial compressive force if it is continuously supported by the concrete underneath it? Is this to account for voids in the concrete? I get that the DG treats the bearing force as uniform across the bottom of the base plate and this force profile would cause "upwards" bending of the base plate if it were able to "travel", but the concrete cannot act on the base plate through a distance.

“Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands.”

 

[dauwerda]

If you are going to assume a rigid base plate that evenly distributes the bearing stress you need to ensure that this is a valid assumption. This is validated by checking the bending caused by the uniform force. If the force causes bending (or I should say, if it yields due to bending), you know your initial assumption is no longer valid.

 

[r13]

Because it is assumed that the column is infinite rigid within the rectangular area that encloses both flanges.

 

[MegaStructures]

I think those answers reiterate what the DG says, but don't address how bending is created by a uniform reaction force that cannot cause the plate to bend "upward" in the middle of the section. To bend the middle of the plate up there would have to be some sort of differential movement at the edges of the column and middle of the plate, which can't happen if the base plate is in contact with grout, or concrete.

“Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands.”

 

[dauwerda]

If your plate is undersized (not thick enough) then the rigid assumption is not valid. This means your bearing stress is higher than you just checked/designed for as the plate is not distributing the bearing stress. So, now you may in fact have local concrete crushing due to overstressing the concrete and/or grout and the bending can happen.

 

[MegaStructures]

That makes some sense, but I don't understand why the limit checked is bending, because there is no bending until the concrete does crush, up to the point of concrete crushing there is not bending in the plate, but only compression, no?

“Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands.”

 

[MegaStructures]

Is the below inaccurate? I do somewhat understand that the plate would have the tendency to bend, which is reacted by the concrete more at mid-span, but it's still not quite clicking in my mind strongly

“Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands.”

 

[Celt83]

You are assuming concrete/grout is infinitely rigid which is not the case. While the concrete is significantly rigid it still experiences small strains from the bearing pressure, and once you have any movement at all then you develop curvature in the bearing plate. The rigid plate assumption while in theory not correct has been proven to be safe, in reality the pressure distribution will be more localized around the column profile.

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[MegaStructures]

aha now that makes sense! I made this much more complicated than it is. I can say I learned something today now. Thanks Celt83

“Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands.”

 

[JLNJ]

Intuitively, it seems as if the forces in the concrete pier would be highly concentrated under the column web and flanges. The column would act as a "stamp". This presupposes that the concrete is very stiff.

If the concrete pier were "soft", the base plate would uniformly distribute the forces to the pier under the entire plate. This is how we model it (conservative for the base plate design) but it doesn't seem to be intuitively correct. How many base plates have failed in bending in a gravity column? How many piers have failed by the concrete sintering or crushing right under the column flanges? None?

The truth lies somewhere in the middle, I presume. The pier stress under the column flanges is likely much higher than estimated using the uniform-distribution-under-the-base-plate model and the plate stresses are much lower than we calculate. The pier is OK even though it might be stressed (locally) well in excess of f'c.

Structural engineers feel good about their super-thick base plates because they are following AISC's design methodology and thick base plates look serious (i.e. cool).

 

[r13]

At the end, don't forget this is a conservative practice to ensure the plate have adequate strength to resist incidental bending, as there is no such prefect column exists.

 

[dold]

+1 to r13.
Consider the case where leveling nuts are used, then grout is drypacked (i've seen it done this way, i know, terrible...). I have zero faith that the grout takes any load unless it's flowable and poured into a form.

I think we've had this leveling nut discussion before.

 

[MegaStructures]

These are all good points. Intuitively I wouldn’t imagine a base plate would ever fail in compression bending if constructed correctly, even if grout (or concrete) crushed near the center of the base plate it would be confined by the remaining grout and offer some bearing resistance.

Somewhat off topic, but of interest to me - a base plate connected to a steel beam would be at an even lower risk for compression bending if not 0 risk. It’s not clear in the DG that this check wouldn’t be needed for this type of connection, which is an example of why I have always thought it is absolutely necessary to understand the “why” of the code provisions

“Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands.”

 

[dik]

For lightly loaded columns, I use an offset for the perimeter all around the column... I check this area using the confined concrete bearing strength... if OK then I use the offset as the BPL thickness... but greater than 3/8". If offset requires more area... again, using increased bearing strength then design the BPL as a cantilever on each side of centreline of steel column section... and use plastic section modulus. For heavier loads I use the conventional BPL design using confined bearing strength as applicable and use plastic section modulus... I almost never use stiffeners, I deal with it using BPL thickness. If levelling nuts, then I treat them as levelling nuts and then ignore them...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[r13]

Intuitively I wouldn’t imagine a base plate would ever fail in compression bending if constructed correctly,...

What is your definition of "base plate failure"? If extend your argument, why the need of anchor bolts if ideal conditions exist and the column constructed correctly? For concentrically loaded column, I think the plate thickness can be reduced to much less than that derived from the design method provided by the guide without risking failure, but where is the bottom line? The guide only means to provide a simple efficient method to analyze a rather complicate matter. In order to keep it simple, it must be conservative.

 

[dik]

I generally don't rely on strain hardening, either Just joking...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[MegaStructures]

r13 The conversation was a theoretical one about why base plate bending checks are required, not an argument to remove them from design. Your comment seems to suggest I want to disregard AISC's recommendations as a result of this discussion, which is not true.

But sure I agree with your position.

“Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands.”

 

[Tomfh]

The concrete is a lot softer than the steel, so you are essentially designing the baseplate as a pad footing over the whole area.

 

[r13]

Not meant to criticize you for anything, just trying to remind you to be realistic, and the philosophy of the design guide. Hope you didn't feel it the wrong way.

 

[MegaStructures]

Not at all R13 just making sure my point wasn’t misconstrued. I enjoy the discussion

“Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands.”