Steel column with base plate and anchor rods. Fixed or Pinned support? 1

[rteja]

I'm new to structural design. I've always considered steel column bases / steel beam end connections with anchors as fixed support.
I've noticed that my manager sometimes considers them as pinned type and sometimes fixed type. (for very similar applications)

This confuses me. Can someone shed some light on this?

Thanks!

 

[hokie66]

It is rare that a column base is fully fixed, or fully pinned. Most are considered to be pinned, because fixing column bases is expensive. Fixation requires first that the footing can withstand the bending without rotation, then that the connection is rigid.

 

[jimstructures]

I agree with everything hokie66 says, however I would also include that for many considerations a base plate with all of the anchor rod holes inside the column flanges and toward the center of the column flanges will be primarily pinned and a rigid base plate with many of the anchor rod holes outside the column flanges will be primary fixed if the foundation will support fixed column moments. If you are using a HSS section then consideration of how rigid the base plate is will affect whether or not the base plate is fixed or pinned.

There are methods of analysis that will predict the moment capacity of the base of the column depending on rigidity of the base plate and anchor rod hole spacing and pattern.


Jim

 

[klaus.]

Maybe you should go and ask you manager... he can explain to you

 

[allimuthug]

Hi,
I had this confusion long back but found the answer after a long time.

To see the same base plate can be considered as pinned or fixed based on you assumption.

Case (I) When base plate is considered as pinned.
When the base plate is considered as pinned the moment on the column is distributed to the beams, means the moment connection of the portal frame is capable of carrying the entire moment of moment distribution considering the base as pinned support, Since the base plate is pinned less thickness is provided and will result in small foundation and a big column section.

Case (II) When base plate is considered as fixed.
When the base plate is considered as fixed the moment on the column is partially distributed to the beam and the base plate, means the portal frame is capable of carrying partial moment only, hence the base plate will be thick and foundation will be large and the column size would be small.

So based on your assumption the analysis changes but both are correct since the foundation, column size, Anchor bolt, and foundation changes based on your assumption.
that is why your manager changes based on the situation.
For example if you want small column size you can consider as fixed support, vice versa you want small foundation you can consider as pinned support.
Note for a cantilever support it is always fixed base since that is instable if you consider as pinned.

 

[BridgeSmith]

As hokie66 said, almost all connections fall somewhere between fully fixed and fully pinned. Depending on where the critical loading occurs, the connection is modeled one way or the other.

In order to be sure of capturing the maximum moment transferred to the anchorage and foundation, the connection would be modeled as fixed.

In order to ensure capturing the maximum moments in the frame above, model it as a pinned connection.

Many times judgement, guided by experience, allows the designer to dismiss one model or the other as not being critical. Other times, one or the other may not be reasonable based on the level of fixity of the connection relative to other components; i.e., if the base plate connection is much more rigid than the members and connections of the rest of the frame, then modeling it a fixed may be a reasonable approximation, with the actual behavior only negligibly different from the model.

In bridge design the "15% rule" for concrete cap on multiple column type bridge piers (multi-column bents) was used for many years before automated frame analysis methods became common. It stated that if the stiffness of the column was less than 15% of the total stiffness of the connection node, the connection of the column to cap could be modeled as pinned. We compared results from that model to a frame analysis for a bent where the column stiffness was 22% of the total. The resulting moments were within 5%, with the column moments and cap moments both lower for the frame model.