Column welded to Base Plate - Fixed or Pinned

[cgstrucg]

Hello,

I have a situation in which I have a column channel which is welded (all around) to a base plate. The base plate is bolted to the concrete slab by 4 bolts. For these type of situations, I used to consider the base as pinned and design base plate for shear/axial loads only. I know that's not totally true as there is always some rotation restraint but that was a conservative assumption.

Recently my supervisor considered the same situation as fixed and I am really confused as to why he did that. He actually went ahead and modified the base conditions further. He considered base to be pinned in minor axis bending of the channel and fixed for major axis bending. Can someone explain to me as to how these conditions are fixed and pinned in another direction?

A rough sketch is attached showing the condition.

Thanks

 

[Ron247]

Pinned vs Fixed is generally something in between the 2. You can make a fixed connection more successfully than a true pin connection. The following are some parameters that affect how pinned versus fixed a base plate is.
[ul]
[li]Plate thickness-Really thin plates resist very little rotation whereas thicker plates do aid in resisting rotation. Thin plates flex.[/li]
[li]Anchor Bolt Location-Bolts close to the center of your section tend to act more pinned. Bolts located farther from the center and especially if they are somewhat centered on flanges tend to act more fixed. In your picture, from a bolt location standpoint, it looks more fixed to me in both directions assuming it had a thick enough plate. Look at shear and moment connections for steel to steel and you will see the concept of location of attachment as it relates to pin versus fixed.[/li]
[li]How well the baseplate "mates" to the foundation. A rocky marriage surface makes for a rocky column. That is why we grout heavily loaded fixed base plates but may not grout pinned ones.[/li]
[li]The actual attachment of the column to the plate.[/li]
[/ul]

I am sure others can add to this list.

 

[Bagman2524]

Also, the stiffness of the foundation. If the foundation rotates at all the column connection will act more like a pinned connection.

All I know is P/A and Mc/I

 

[KootK]

I need more clues .

1) can you sketch the base plate and anchor bolt layout?

2) what are the building lateral systems in the directions parallel and perpendicular to the strong axis of the column?

3) Is the column designed to resist any applied lateral load?

In many instances, I'd consider weak axis restraint to be more effective than strong axis restraint at the base.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235