Axial Members.

This is a photo I took during my visit to Indore. It's at the Devi Ahilyabai Holkar International Airport, but you could see this type of connections at every airport.

I tried explaining my elder sis why is it so back then, but I might have been wrong. So here it is,  the correct answer.


If you look closely at the connections, you can see that it's not welded or fixed like the smaller ones at the back. The reason is that it allows rotation on that particular connection. One might ask why would you allow rotation in the first place, isn't it dangerous? But wait, I will explain why is it so.

Reactions in our world means how an object would respond to a certain load. If you squeeze a sponge, it gets flattened. So Reactions are basically responses. And if you don't want that particular response, you just allow it to be effected. Alright, it's not a very good explanation. I will try again.
Take an example of catching a ball, the impact on your hand is nullified if you pull your hand back rather than catching it head on. What one does is give it a path to follow so as to reduce the impact. This one might not be the best example too. 
I will try in my term. Moment in a frame is generated when it resists rotation. But if we allow it to rotate, there won't be any Moment Reaction in it at all. Thus pinned connection have zero moment.

And how is it helpful? No moment means we don't have to design for it. Thus we will only Axial and Shear Force. In the kind I have above, Axial dominate Shear and are called Axial Members. Axial Force are Forces that act along the length of a structure. Trusses are usually designed that way.

During my field trip to Saraighat Bridge on my final year, my Prof. Dr. Kaustubh Dasgupta told us. Although Trusses are asummed to be Axial Members, the Bridge was designed for Moment too. I couldn't figure it out back then but now I get what he meant. Not that I can design a Bridge now, that would be stupendous, I just understand what he was trying to tell.

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