I did not ‘misunderstand’.

The original poster never offered any ‘actual’ or ‘meaningful’ dimensions in his original posts.

So either the original poster needs to add information regarding the length of joists and rafters…the number of stories…the location of the girders..

…OR the original poster needs to retract his original statements as meanignless and false for the floor spans, size of girders and joists, or begin again…

So far, the original poster (nor anyone else) has provided enough information to even begin to remotely answer the original poster’s question(s)….