enter image description here For the connection above, I think that once the beam is loaded, their is no room for rotation of the beam.

But as per British Standard this is a pinned connection. So I have been trying to determine how a connection is defined as Pinned or fixed. After reviewing American and British codes, I have come to the conclusion that steel structure designers need to follow their Standard connections for design (for which they have done experiments) or conduct experiments on connections to determine to evaluate whether its a pinned or connections based on certain criteria. Is my understanding correct?


Pinned connection in structural context doesn't mean free rotation/zero moment. Pinned connection means much bigger rotations and much lower strength compared to the connected members. According to Eurocode 3 (EN1993-1-8:2005 section:5.2), if the moment strength is below 0.25 of the connected beam and its stiffeness is below a limit (relative to the beam) then it should be considered nominally pinned, provided adequate rotational capacity is available (for example through ductile critical mechanism of failure).

Normally one should find the moment strength and stiffness of the connection and classify it against the limits. A connection can be classified either as full strength/rigid or pinned or semirigid.

For the connection in question, it is obvious that no transfer of force occurs through the beam flanges. The bending moment finds its way through the flanges mostly. So this connection is ineffective to transfer enough of the beam end moment. Also take into account that the connection is bolted to the web of the column which is quite flexible.

Finally, i want to emphasize that the following statement is not accurate:

once the beam is loaded, their is no room for rotation of the beam.

Don't assume that based on the undeformed geometry!

  • $\begingroup$ Good answer, but it could be improved with an actual quote and clause number rather than just "the Eurocodes". $\endgroup$
    – AndyT
    Jan 20 '17 at 10:57
  • 1
    $\begingroup$ You are right! The Eurocode reference have been added/ $\endgroup$ Jan 20 '17 at 18:02

A fixed connection is a connection that would develop at least full plastic limit of the beam before yielding in rotation. Any connection less than that is considered pin connection.
All the members joining at the connection have to be reinforced by stiffeners and connection details to bear the moments and stresses and transfer them per approved load tested methods. The joint in your diagram has a couple of grave weaknesses: the flanges are not continued into the connection, the connection is into the web of the column which has the least stiffness and will warp immediately allowing rotation of joint in all directions, the bolts will pop under high stress.

Joint design is a very complex process, not only you have to consider service loads you have to mind buckling, torsion, stresses beyond rupture point and delayed critical failure! Many codes provide for predictable ductile mode of failure of the joint in case of a catastrophic event such as explosion or earthquake.
I saw this on youtube, it may be helpful. I do not know them and can not vouch for them.see this clip

  • 1
    $\begingroup$ As I understand it, it would be more accurate to consider connections as springs: heavily "reinforced" connections would be "almost infinitely stiff" (fixed), while simple connections would have "almost zero stiffness" (pinned). Connections such as the one above, which clearly would have some resistance to rotation, would have some rotational stiffness. How to quantify this stiffness is very hard and often empirical - that's why, indeed, all but the stiffest connections are normally assumed pinned. $\endgroup$
    – Wasabi
    Oct 11 '16 at 16:50
  • $\begingroup$ @wasabi So does that mean than a designer cannot design a connection beyond what the 'Codes' specify except if it is empirically proven? $\endgroup$ Oct 11 '16 at 21:12
  • $\begingroup$ @StephenJacob if the Codes specify anything, then you're stuck with that. As far as engineering goes, Codes are God's Commandments. But codes usually don't have an item "almost everything is pinned." That isn't in the codes, it's just a common assumption adopted by most engineers. $\endgroup$
    – Wasabi
    Oct 11 '16 at 22:23
  • $\begingroup$ The engineer is responsible for safety of the building he designs which may last for centuries. Codes are designed to make his job easier and safer and relieve him of the burden of legal liability. The engineering codes are the fruit of work of many scientist, professors and PHD students working in research university labs such as school of engineering at MIT or public science departments such as USGS, testing new ideas and checking failures. Of course one can always suggest a new way but they have to prove it; that is the emergence of new methods and materials like carbon nanotube cables! $\endgroup$
    – kamran
    Oct 11 '16 at 23:59
  • $\begingroup$ @kamran But we are legally liable because the codes absolves itself off responsibility $\endgroup$ Oct 12 '16 at 7:02

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