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What's an authoritative definition of whether the idealized pinned support and pin joint can transfer moments in 3D?

Most sources say both of them cannot transfer moments but are apparently only considering 2D. A real pin joint obviously can transfer moments about all axes normal to the pin's axis. So is an implementation of the ideal pin joint really more like a ball joint than a pin joint?

I assume a pinned support is the same as a pin joint but with one end fixed to ground. But maybe they have different numbers of degrees of rotational freedom?

For example, if this support can carry moments about two perpendicular axes (red arrows), would it be correctly called a pinned support?

Pinned support

In comparison, this one has all 3 rotational degrees of freedom, so is it still a pinned support?

enter image description here

user1318499
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  • Part of the problem may be that people, in general, consider text to be more authoritative than pictures. While in geometrical problems pictures should be held as more authoritative than text. – joojaa May 20 '19 at 06:20
  • some questions with answers on here may help see this for one : https://engineering.stackexchange.com/q/11824/10902 – Solar Mike May 20 '19 at 07:39
  • Solar Mike - I'm asking about idealized joints, not real ones. joojaa - which pictures? – user1318499 May 20 '19 at 07:58
  • @user1318499 those pictures look real - not idealized... – Solar Mike May 20 '19 at 11:07
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    why the semantics is so important to you? – Yaniv Ben David May 20 '19 at 18:18
  • I'm trying to name some things which are an ideal ball joint and a 1-dof pin joint, both possibly fixed to ground. I want to use correct but easily recognizable names so that people will know what they are. – user1318499 May 20 '19 at 23:48
  • Solar Mike - pretend they're not real and the metal parts are infinitely stiff, the sliding parts are frictionless, and there's zero play. – user1318499 May 20 '19 at 23:50
  • @YanivBenDavid engineers focus on precision - otherwise expensive or fatal mistakes happen... Such as using a rubber seal outside the defined temperature range... – Solar Mike May 21 '19 at 10:28
  • @SolarMike - engineers have to formulate the problem and solve it correctly. It really doesn't matter if you name it a dog or a cat as long as you can calculate how fast it may run on ice. The guy here fully understands how many degrees of freedom the different joints allow, so all we have left is semantics. – Yaniv Ben David May 21 '19 at 14:24
  • Jesus, I'm publishing something that will identify these ideal joints by name. I want to use the correct name. Why is that such a problem? If you know what the names mean, then answer the question. If you don't know, don't answer. But don't complain that I don't understand unrelated things. I'm only asking about semantics because that's important for making sure people understand the meaning! – user1318499 May 21 '19 at 21:41

1 Answers1

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Yes it can, and in many designs it should be a pin support along one axis and fixed support along other. In many trusses and bridges that is the case.

The design software when defining a joint have the joint degrees of freedom choices. Among those are the option to define the joint a hinge along one axis but fixed along the others.

You can even have joints or supports that can have, rigid, pin, or spring or even predefined varying stiffness and ductility restriction on rotation, settlement, lateral displacement, moment transfer. This is becoming more advantageous in seismic design. Where one expects different behavior of a structure under different spectra of earthquake.

Between the two extremes of a pin joint and fixed join one can define the semi fixed joint or fixed but after a certain stress behaving like a hinge.

In real construction of a joint there is always joints that are legally considered pin or fixed but they behave a certain mix of both, but you design them depending on what the code defines that type ofjoint.

kamran
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