Learning Structural Connection Types – Pin vs Fixed (Real World Example)

[rpand4]

Hey everyone,

I’m currently brushing up on structural engineering concepts, especially around connections, and I thought I’d share something I’ve been analysing to get better at spotting connection types in real-world structures.

The attached photo is of an overhead sign structure that I see everyday. The sign is advertising height restriction to a bridge. I’ve been trying to work out what type of connections are used, particularly:

1. The base connection of the post — is that a fully fixed connection?
2. The connection between the vertical post and the horizontal member — pin connection or fully fixed?

From my understanding:

• A fully fixed connection resists moments, shear, and axial forces, keeping angles between members constant under load.
• A pinned connection resists only shear and axial forces but allows rotation, meaning angles can change under load.

Looking at this sign, I’m thinking:

• The base connection likely needs to resist significant moment from wind loads acting on the sign, so I’d suspect it’s designed as a fully fixed connection — but I can’t tell just by looking at the flange and bolts. Am I correct?
• The post-to-beam connection looks like it has stiffening plates, but would that realistically provide full fixity? Or is it designed to allow some rotation, acting more like a pin?

Thanks for your help in advance.

 

[phamENG]

Your understanding of the concepts is generally correct. The best thing to do in these cases is to draw a free body diagram of your frame. What loads do you have acting in the X and Y directions (and Z if you want to get fancy). What reactions do you have in each of those directions? Now, sum your forces and moments. If things aren't balanced, you know you have something messed up somewhere, and go back and reconsider what you assumed as your connections and boundary conditions.

In this case, what do those connections need to do in order to maintain stability for the sign?

 

[Tomfh]

Designed as fixed. In reality, not fully fixed, but close enough.

 

[rpand4]

Designed as fixed. In reality, not fully fixed, but close enough.

What is the tell tale sign that tells you it is a fully fixed - or close enough? Is it the stiffener plates? I am trying to train my eyes to distinguish between connection types.

Thanks for your response

 

[Tomfh]

It would lean if it wasn’t fixed.

In terms of details, bolts positioned outside the section to provide large lever arm to resist bending force within the connection, as well as stiffener plates

 

[rpand4]

It would lean if it wasn’t fixed.

In terms of details, bolts positioned outside the section to provide large lever arm to resist bending force within the connection, as well as stiffener plates

This makes sense. Thank you so much!!!