I'm sorry, but your tone here has certainly changed during the course of the thread. Your opening post was accusatory and hinted that the frame material is designed too weak... The fault seems to lie with Lumia for creating an inferior product.
Until now you are just providing a public warning on the chances of damaging an 830...
I've long known that damage may occur to my phone in any accident, no matter how "slight" the accident might appear on the surface of it. Physics is a complicated and wonderful beast. In your case if your phone struck not too hard (your words, but highly subjective) against a table leg... You have to take into account many factors including (possibly) a corner on a square leg which acts like a blade edge to focus and concentrate force, multiplying it beyond what casual observation would assume the force to be. Also there is square area and force to consider. .25kg/meter/second dissipated over 1 square meter is entirely less damaging than the same forceful acceleration is when concentrated on 1 square millimeter. These things are known. The variables of cell phone accidents are multitudinous. The fact that a phone can get damaged is not a mystery at all.
My philosophy is: "Cell phones are like people, they should never leave the house naked. It is just asking for trouble."
Another useful thought is: "Stuff happens."
Thank you, though, for the public service warning. Truly there are many users who do not think of the consequences until those consequences are visible as damage to their device... This board gets plenty of threads expressing this. So does every tech forum I have ever visited: regardless of device, OS or manufacturer.