Base plate design ignores corners of base plate?
Base plate design ignores corners of base plate?
(OP)
The cantilever method for rigid base plate design per AISC seemingly ignores the areas of the base plate that aren't directly perpendicular to the column.
In other words, there is no check for the corners of the steel base plates.
I'm sure this is fine for a small base plate, but what about for a relatively large base plate, where the area of the corners of the base plate is more than the area that is directly perpendicular to the column?
Is there a rationale for ignoring the corners of the base plates?
In other words, there is no check for the corners of the steel base plates.
I'm sure this is fine for a small base plate, but what about for a relatively large base plate, where the area of the corners of the base plate is more than the area that is directly perpendicular to the column?
Is there a rationale for ignoring the corners of the base plates?






RE: Base plate design ignores corners of base plate?
RE: Base plate design ignores corners of base plate?
Also check that your base plate dimensions are not outside the recommended limits (I believe there are some somewhere)
RE: Base plate design ignores corners of base plate?
akastud
RE: Base plate design ignores corners of base plate?
wouldn't you expect the basplate to back into compression on the side of uplieft on the far sde of the anchor bolts? That is where the prying forces come from.
RE: Base plate design ignores corners of base plate?
RE: Base plate design ignores corners of base plate?
The case I'm asking about is for axial compression only. There would be no uplift due to bending / tension in the base plate. The upward force on the base plate would be the allowable bearing pressure, and would be uniform under the base plate.
csd72, I couldn't find any limits on base plate dimensions, either generic, or related to a design theory. My guess is it's because base plates are sort of in between steel design and concrete design, and so neither ACI or AISC like to go into too much detail on them.
RE: Base plate design ignores corners of base plate?
The proposed yield line cuts diagonally across the base plate near the flange corner. The part of the plate outside the yield line is the triangular (or close to triangular part) part near the corner.
If the pressure is uniform over this triangular part, then the resultant force is about 1/3 the way from the yield line (which passes through flange corner, or close to it) to the corner of the plate. This is more favorable than if the resultant was halfway between the yield line and the edge of plate, which is the case for the typical patterns shown in the AISC Manual.
RE: Base plate design ignores corners of base plate?
RE: Base plate design ignores corners of base plate?
PEInc, I'm trying to design a base plate for a temporary steel structure that sits on soil. The base plate is relatively large compared to if it was sitting on concrete. I don't think the traditional rigid plate cantilever method property accounts for large areas of the corners of the plate, but couldn't find a design method which addressed this issue.
Right now, I'm planning on designing based on the cantilever method, and increasing the thickness some nominal amount.
RE: Base plate design ignores corners of base plate?
I see your question. The formula for the thickness of the baseplate does it on a per inch basis (along the length/width of the plate). It basically assumes that every inch of B has a support inch at the critical section line.
I believe Aggieyank's question is how is this possible since the column is providing the support for the cantilever baseplate and the column does not extend all the way along the creitical section line (along either the B or N dimension). Basicaly, should the moment capacity of the plate be determined based on the acutal column dimension and the required moment capacity be determined based on the entire trib area, instead of assuming it on a per inch basis since there is a distance (B-bf) thast has no cantilever support.
Does this represent your question Aggieyank?
RE: Base plate design ignores corners of base plate?
It'd be extremely complicated to analyze, as it would involve two-way bending of the plate in the corners, and I'm sure the analysis would be wrong anyway. A finite element analysis in a computer program would be ballpark, but still wouldn't be perfect, because as soon as the center of the baseplate moves down, a little more load will concentrate there as opposed to the edges.
One solution I've shied away from is adding stiffeners from the edges of columns to the edge of plate, and using the distance from edge of plate to stiffener as my cantilever distance. I'd like to not add the stiffeners as it would involve field welding.
RE: Base plate design ignores corners of base plate?
RE: Base plate design ignores corners of base plate?
RE: Base plate design ignores corners of base plate?
Can anyone comment on this?
RE: Base plate design ignores corners of base plate?
RE: Base plate design ignores corners of base plate?
Given that test results have proven traditional cantilever method for steel on concrete as conservative, I think the best result in my application lies somewhere between the "trapezoid" cantilever method and the traditional cantilever method.
It'd still be nice if there was a reference on this.
RE: Base plate design ignores corners of base plate?
What's the size of this column and base plate? Just trying to get a handle on just HOW far outside of normal it is.
AggieYank and StrlEIT, the answer lies in yield line analysis. The trapezoidal shape is only possible if several other yield lines form. If there's ever a doubt about a particular weird case, it should be checked using YLA, which is really pretty easy.
RE: Base plate design ignores corners of base plate?
"Boy, now that I think about it, wouldn't you have that same issue with a typical spread footing design?"
I think in both cases, the assumption is not that the plate is supported at every point along the bend line, but that if it fails, it will fail by hinging in a line; consequently, one part can't hinge up unless the whole thing does. You may have localized bending stresses that are higher than the average, and that is not prohibited. This approach is implied in ACI-318- 15.4.1 and 15.4.4.1.
RE: Base plate design ignores corners of base plate?