Hello, my name is Damian Fediuk and my question is the following: why the bottom plate material is not specified in the tank report? thanks in advance.

Earlier editions of the API-650 Code didn't need much information on the bottom, only the thickness, and then only if you needed to perform a Seismic analysis.

As the Code has evolved over the years, an additional piece of information related to the bottom was needed every now and then. So as needed, these items were added one by one.

Looking at things now, you're right. The software should have another input tab for the bottom where you define the material (and associated properites) as well as the thickness. (Maybe if the Code stops all the massive changes I'll have time to do just that.)