Vmirat, yes read the thread 'Existing open web steel joists' very good comments by PM, ie, your 'lucky if you can find a tag'...try digging around thru ceilings, fireproofing, M&E, etc., to find a joist mrk. no. only. Suppliers/Manufacturers can be helpful but their limited by; time ie project constructed long ago, or their busy with current work ... and protecting their propietory knowledge, and i feel sometimes potential liability. There are alot of curiosities out there that defy analysis and one assumes were proved by testing, and designer/manufacturers that no longer exist .... ??? Original structural design drawings are of questionable value, and shop drawings from time of construction are very difficult to locate. It is strange that such a common occurance in structural engineering ie evaluating an existing owsj system (a major structural component) for a change in loading etc. seems such a difficult task. This problem isn't limited to owsj, we also find it with wood trusses/truss plates .