Substation structure base plates - follow up

[structural-eng]

I'm reposting this thread to this forum as I don't know that it will be seen in the industry forum.

I have some follow up questions to a thread that has been closed for a while as I'm looking at the exact same condition. The thread is thread593-417476: Column Embedment into Base Plate.

The correspondence was between ClarksonEng & transmissiontowers.

I have similar condition/loads to what ClarksonEng was describing (4 kips shear, 9 kips vert down and 55 k-ft overturning). I know how to size the fillet weld around the perimeter of the HSS8x8 column that I have for the overturning and vertical load. I also know how to determine the loads to the anchor bolts. I'm having a hard time seeing how the fillet weld which is sized for the 55 k-ft (plus the vertical load) works for the cantilevered moment in the base plate that is used to determine the base plate thickness (using ASCE 113). I'm trying to find a way to justify having the entire inside of the column open which I know is commonly done.

transmission towers does a good job in the old thread of describing the steps but it left me with questions. Doesn't the weld need to be sized for the cantilever moment from the base plate bending? If so, how do the 2 fillet welds (above the base plate and inside the base plate hole) resist that moment and how can you look at the bending at the portion of the plate beyond the column (due to the opening) and yet resist that bending at the face of the column with the weld. If the weld doesn't need to resist the base plate bending how is this justified? I had someone on a different thread tell me that their understanding was that using the bend line theory (like ASCE 113) was to determine the base plate thickness such that the plate was rigid enough that you just had to size the weld for the loads at the base of the column. While this may be true I can't find anywhere that this reasoning is documented.

Is anyone aware of something that is published that explains this. All the documents we have or I have found online talk about how to design the base plate but never really talk about the welding of the column to the base plate in depth. The only welding information I have found are in ASCE 48-11 which says to full pen weld the joint and a single page (107) in Daniel Horn's Design of Monopole Bases which shows a fillet welded connection similar to what we are trying to justify but it offers no explanation of how that weld resists the base plate bending (or why it doesn't have to). While we do some transmission line projects in our office the poles are typically purchased and we just design the foundations so my question is really for substation structures. Using a full penetration weld between the column and the base plate would eliminate my conerns/questions but we are being tasked with making a fillet welded connection work.

Any information/insight is greatly appreciated.

 

[dhengr]

Structural-eng:
This is an interesting design detail, without an easy answer. History, what has worked for years, is well worth paying attention to even if we can’t pin the stresses down to +/- a singe psi. We really fool ourselves if we think we can always analyze some of these details to the enth degree, in a reasonable amount of time. I’m in St. Paul at rwhaiatcomcastdotnet, and we could talk about this a bit. As much as anything it is important to know that the detail can be easily welded and fabricated; and that your details are not introducing stress raisers in the process. What are the tolerances on the poles, the various range of sizes and thicknesses, sizes of base pls., typical loads, etc? What control do you have over the fabrication, and what can you do to improve that and assure a better joint quality? These are good problems to do some FEA on, just to confirm our common sense engineering thinking, and to improve our understanding of the problem.