baseplate thickness governed by n dimension

[shaneelliss]

I am creating a spreadsheet to design baseplates based on AISC DG1. It all seems pretty straight forward except for two things. The first is I can't conceptually understand why the thickness of a baseplate with large moments would be governed by the n dimension of the baseplate on the compression side of the plate. The n dimension is the distance from the edge of the plate to the tips of the flange (in direction parallel to flange). I can picture in my mind why you would have to check this for a baseplate with small or no moments, but if you have a large moment trying to bend the plate in one direction, why would plate bending in the other direction ever control? The theory seems to assume the plate will bend along the full baseplate length or width either in the direction parallel to the web (at the edge of the flange) or parallel to the flange near the flange line. If the force from a large moment means that only half the plate has a compression force on it, would the full length of the plate actually ever bend in that direction perpendicular to the moment force?

The second thing I notice is that if you have a larger concrete bearing area than your baseplate area, your baseplate will be thicker. Also if you have stronger concrete, your baseplate will have to be thicker. I understand that concrete bearing pressure is allowed to increase due to effects from confinement on the loaded area, but I don't see how this really affects the steel baseplate in real life. The equations use the max bearing strength of the concrete to determine the force on the baseplate under compression, so if there is higher concrete bearing strength, there are higher forces bending the baseplate up away from the concrete. And the max bearing strength is a function of f'c and bearing area. But does this actually happen? This means that if I have a baseplate on a very large pad sitting next to an identical baseplate on a smaller pedestal, that even if both baseplates have the same forces, the baseplate on the large pad will fail in bending prior to the one on the small pedestal. Am I understanding that correctly because it makes sense by the numbers but makes very little sense to me when I look away from the theory and try to think physically how it will behave.

To make the thinnest baseplate possible, I should always assume weak concrete and a small pedestal. Is that a "good practice" way to design baseplates economically?

 

[shaneelliss]

I haven't gotten any responses so I am wondering if my question is unclear. Does anyone have any input?

 

[JLNJ]

It seems to me that the bearing pressure right under the web and flanges of a column is much higher than the load at the edges of the plate, yet we tend to distribute it all out evenly. For bearing-only plates, I believe this leads to much too conservative results, leading to large plates and some non-sensical results. I'd like to see a gravity base plate which has failed in bending.

I had a similar discussion with a young engineer regarding masonry bearing plates. She was trying to tell me that a wider (longer) plate with the same load as a shorter plate would be required to be thicker due to the increased bending moment. This makes no sense, as the fibers in the middle of the plate don't know how wide the plate is. I tried to tell her that the load under the plate is not as uniformly distributed as we assume it to be and the loads really concentrate under the first few inches of plate. She was not buying it and insisted on bumping the 1/2" plate size up to 3/4" when she made the plate larger (longer in plane of the CMU wall).

 

[paddingtongreen]

JLNJ, Does the bearing pressure that you favor, exceed that permitted for the concrete. the bearing pressure under a baseplate depends to a large extent on the elasticity of the concrete.

Your colleague was getting into a self fulfilling prophesy zone, the thicker the plate, the stiffer the plate, the more even the pressure distribution.


Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[JLNJ]

In the case of my colleague, I was not advocating a bearing pressure beyond that allowed for masonry. I have seen cracks and spalls under masonry bearing plates so I definitely believe the allowable loads are reasonable. I do espouse the idea that the plate doesn't necessarily distribute the loads uniformly over its entire length. As you say, the load path goes where there is stiffness. Just because you make the plate wider doesn't mean the load gets distributed (evenly or otherwise) out to the ends of the plate. It is non-sensical to have a design methodology where a narrow plate of a given thickness fails if you make it wider under the same load.

As for base plates on steel columns, if the pier is much larger than the plate, then it seems to me that the load will drive more or less straight through the plate into the pier (based on stiffnes, of course) rather than being distributed out over a large plate area. I have not done or seen research to prove this theory, but I imagine that the crushing/compressing stiffness of concrete is great when compared to the flexural stiffness of a steel plate, even a very thick plate. The bearing stress condition may be more analogous to the tip bearing of a steel pile inside a pile cap (or on rock, for that matter) than the even pressure distribution assumed in, say, a soil bearing spread footing.

It may be that the design methodology of evenly distributing the force under a base plate has its origins back when many foundation elements were unreinforced, stepped masonry footings rather than the relatively strong and stiff concrete in a modern footing, and base plates were quite large.

 

[shaneelliss]

So...are you saying that the analysis in the AISC design guide for baseplate design is based on questionable theory and should be ignored? If so, how are people doing baseplate design, if not by the baseplate design guide procedures?