Common column base plate & column angularity tolerance for bearing 1

[GD_P]

Hello community,

I have a doubt, please do share your opinion. In the attached file, right side picture indicate 4 small shop fabricated structures (3m x 3m x 6m high) A,B,C & D each having 4 columns, single storey. These will be shop fabricated, along with the chemical process equipments, piping & accessories and installed on site adjacent to each other in the same manner as depicted in the right side picture. The column bases are modeled as pin type connection and lateral force is resisted by knee bracings.
My question is about the base plate which is common for 4 columns where all modules touch each other i.e., detail ‘P’ which is shown in the left side picture. For all 4 columns of detail P base plate is common & which is modeled as pin type. But as you can see due to practical constraints, it is not possible to weld from one side of every element of column ( flange, web) neither possible to provide anchor bolts in area generally made for single column base plate. There is no uplift at the column base for any load combination. With this scenario, if the available weld & anchors are adequate for required shear, will this be enough to call it as pin type base. I am using Eurocode.
One more point, here the column bases will be bearing against the base plate, so as per the EN 1090-2 tolerance, the angular tolerance for bearing type columns is given as depth/1000 = 120/1000 = 0.12 mm (for HE120A sections). Isn't this too stringent and will have to end mill to achieve this or my interpretation is wrong?

Sorry for such long post.
Thank you.


GD_P

 

[Ron247]

To me, everything looks fixed-base in the detail. Pinned is idealized and somewhat difficult to really achieve unless you raise the column up some and fasten a single bolt through a set of baseplate/foundation mounted clip angles. 2 bolts or flange contact with the baseplate/foundation adds some fixity. There are several things that affect "how pinned you are".

1. The actual connection of the item to the baseplate. For W-sections, one of the main things that creates fixity is restraining the flanges against the connection. I see the column flanges welded to the baseplate, so you have one piece needed for fixity.
2. The connection of the baseplate to the foundation is the next. In a single W-section, we tend to place anchors bolts in between the flanges and straddle the webs for pinned. This type of connection is considered "pinned" but it actually is semi-rigid. If we put anchors straddle the flanges, we have added another item needed for fixity. We have then restrained the flanges to the plate and the flanges to the foundation. Your anchor bolts are well-outside so it appears the baseplate itself wants to be fixed more than it would if your bolts were more "inside". So now, I see a 2nd piece needed for fixity.
3. The thickness of the baseplate. Thin plates flex more than thin ones. Thick plates tend to be more fixed. That is why it is not advised to make baseplates a lot thicker than calculated when you do a pin.
4. The actual foundation. A foundation can provide only so much resistance to rotation based on its dimensions.

Pre-engineered metal buildings are generally pinned base designed. The columns are fully welded to the baseplate, but the baseplate has the anchor bolts inside the flanges. They have built millions of these and they rarely have a problem with the fact the columns are actually semi-rigid.

You could move your bolts more "inside" to get closer to pinned. Any bolt you would have can be fastened before the columns are set because the plate bolts are fully accessible regardless of where they are. Also, you can set AC or BD first and still have access to everything. It is only when the 3rd and 4th piece get set you lose access. I have no idea how to visualize how 4 columns acting differently on one baseplate would be. That would require some knowledge of the loads.

 

[GD_P]

Ron, Thanks for your prompt reply.
Yes it is difficult to imagine how 4 column behave on single base plate. It will become very complex if we will treat it as fixed base plate & I don’t understand how to prove it as fixed both strength as well as rigidity point of view. (In fact I tried to make them fixed connections, ordered the base plate thicker on judgement basis but ended up defining them pinned.)
Hence I only consider the single column on that base plate & design it assuming pin connection (i.e., design for concrete bearing, weld adequacy for shear & anchor adequacy for shear).
As current anchor dimensions are just sufficient for erection tolerances (positioning of columns), ‘moving the anchors inside’ I assume you mean within flanges, but it will result in only 1 anchor per column. Also there exists permanent beam at 300 mm from the ground hence this arrangement will make the erection work (positioning the column as well as tightening the anchor nut) difficult.
As far as columns are considered, they are also designed them for fixed connections.
As you noted that with any erection sequence of A, B, C & D, at least 2 columns can not be accessed for full welding. For such a column, if we have to treat it as pinned connections welding (of adequate size for available weld length) from accessible site only is sufficient or not?
What is your opinion on the straightness or perpendicularity (sorry not angular) tolerance of the column.


GD_P

 

[Ron247]

I thought the plate was to be placed first. Then the columns added. If that is the case, you can put 2 or 4 bolts per column straddle the web. Bolt the plate down. Then add the columns and weld. I assumed this was a 4-module build-deliver-erect project.

 

[GD_P]

Hello Ron,

Probably I have confused you.
I think below statements will clear the intent:
Yes, it is 4 module (skids) built-deliver-erect project, where 2 modules are placed in 1 row & remaining in adjacent 2nd row. all four modules are in contact with each other at centre column base plate (which is shown in 1st post fig).
sequence of erections / installation:
- Preparing foundation & fixing the anchors bolts in foudn
- Base plates will be placed & leveled
- Erect the modules
- Adjust the base plate if required
- Grouting
- Anchor bolt tightening
- Base plate to column welding

"Straddle the web", I could not interpret what you mean to say?



GD_P