Hanging metal web floor joists Information, help and advice on many architecture, self build and design problems and issues

[Holychihuahua]

With metal web timber floor joists where two 5 meter lengths of 100mm x 50mm are connected to by metal webs giving joists a 250mm depth. Why is the joist only hung on the top 100mm x 50mm length. The ends of the bottom lengths do not hang on anything why?