Stress in a rod with a hole for pin-joint

[Ali Baig]

If a rod is in compression, will the minimum area on rod ( the section where there is a hole for pin) stress free? What if the rod is in tension? The rod BC is in compression and the boom AB is in tension?

http://imgur.com/a/6asfB

 

[PhanthomJay]

I don't understand what you mean by stress free, but you need to look at the cross section net area of the flat ends at the holes, as well as rod stress in main body and a bunch of other stuff. Please explain why you think the rod is in compression and the boom in tension. Is this a homework or study problem?

 

[Ali Baig]

It was a solved example in book Mechanics of Materials 5th Ed. by Beer. I am sorry for including it as part of question. My main question is about a statement given in the book that says

..the sections of minimum area at A and B are not under stress, since the boom is in compression, and, therefore, pushes on the pins (instead of pulling on the pins as rod BC does).​

I want to ask when a given rod or boom is in compression, why we don't consider stress at minimum area (area where there is a hole for pin) but we do consider stress at the minimum area if the rod/boom is in tension? I am not able to understand this statement.

 

[PhanthomJay]

Oh ok , the boom is in compression and the rod is in tension. For the tension case, the pin pulls on the rod from its outer edge at the contact point, causing tensile stress throughout the rod including at the cross section of the hole which is not stress free. For the compression case, the pin pushes on the boom from its inner edge at the contact point, thus there is no stress in the boom from that inner edge to the end of the boom.