My suggestion is to use Q345B for columns and main beams, and Q235B for secondary beams and diagonal bars.
Ladders and steel plates are also made of 235B.
For specifications and dimensions, please refer to the Code for Design of Steel Structures. The general design theory is that columns and main beams are large in size, followed by diagonal bars, steel plate beams and so on.
Your 28m span is divided into two spans, each span14m. It is estimated that 800*800mm H-beam will be used for such a large span.
I don't think so, because the steel main girder is also very large in size.
Can be designed to span 3~4. If 3 spans are 9.3m per span and 4 spans are 7m per span, it is estimated that 600*600mm profiles can be used, and it is not necessary to customize large-size H-beams.