Pinned or fixed connections? by concrete beam designes

[lolobau]

Hi Structural Team

A student was asking me to explain the difference between pinned and fixed connection by concrete beams.
As I started to explain, I realized that this is actually not easy to explain or define the difference, because we actually never have such cases where the beam reinforcement won't be going through the column reinforcement.
So actually we are always (ok lets say in 95% of the cases) talking about fixed connections by beams, or?
unless we don't specify any top reinforcement at the supports (which is kind of unusual as well)

How would you explain it tot he structural student?

Best Regards

lolobau

 

[BridgeSmith]

In reality, concrete beams are never completely fixed, and are rarely ever truly pinned, unless you actually detail the connection with a hinge. We model them as either fixed or pinned (or often both) in order to approximate the real behavior (or model them both ways to 'envelope' the design and capture the extreme effects regardless of the level of fixity). The level of fixity of a connection is relative to the stiffness of the connected members and the fixity of the other connections in the structure. A particular connection may be stiff enough relative to other parts of the structure to be modeled as fixed with an acceptable margin of error, but it is not truly fixed.

 

[BAretired]

A pinned connection allows all connected members to rotate freely, i.e. moment equals zero.
A fixed connection prevents rotation of connected member(s).
A rigid or continuous connection requires that all connected members rotate equally.
A flexible or semi-rigid connection permits rotation in the joint between members.

So actually we are always (ok lets say in 95% of the cases) talking about fixed connections by beams, or?
unless we don't specify any top reinforcement at the supports (which is kind of unusual as well)

I don't think so. Most beam connections are rigid or continuous connections. Beams are rigidly connected to each other but rotation of the joint is permitted.

BA

 

[elreedyman]

Really, for normal building with beam and slab system the beam and column connection; and column and foundation it is partially fixed. But in design we consider it pinn connection. So after applying the load the straining action will be redistributed and the code accept a cracks between column and foundation to act as a pin connection. In case of high rise builing for frame structure; it is clear that the connection between column and beam is fixed and you can see in construction the steel of beam inserted in column to transfere the moment.

 

[BAretired]

Really, for normal building with beam and slab system the beam and column connection; and column and foundation it is partially fixed. But in design we consider it pinn connection. So after applying the load the straining action will be redistributed and the code accept a cracks between column and foundation to act as a pin connection. In case of high rise builing for frame structure; it is clear that the connection between column and beam is fixed and you can see in construction the steel of beam inserted in column to transfere the moment.

I believe it is important to use the term "fixed" correctly. It is preferable to say that rotation of a column is partially restrained by the foundation rather than partially fixed. If a member is partially fixed, it is also partially hinged. Partially restrained against rotation would be preferable.

In a high rise building frame, a beam may be rigidly connected to a column, but the connection is not fixed; it is a moment connection which is free to rotate as dictated by the loads acting on the structure.

BA

 

[rapt]

Agree with BAretired,

In the old days before computers, Fixed meant completely restrained against rotation. Nearly impossible to achieve.

With frame analysis and FEM software, "fixed" has come to mean "connected". So moment is transferred across the connection. The alternate is to add a pin so no moment is transferred, only shear transfers across the node.

There is a very large difference between the two.

 

[hokie66]

Yes, the use of "F" and "R" in software is very misleading, and we have this type of question frequently by younger engineers who overly depend on software. Instead of "fixed" where joints can rotate, the correct term is "rigid".

 

[name.taken]

As others have said, the true behavior is somewhere between pinned (free to rotate) and fixed (no rotation allowed). A decent software will be able to model the rigidity of end connections based on your inputs and then calculated the end moments which should be a fair approximation. I'd suggest having the young engineer look over old drawings and look at how bar detailing has been done before - this will likely show an industry standard assumption of pinned ends.