material selection on Structural steel

We can really guide you here, but your question does not allow us to make any idea on what you need to know. At this point, I would recommend you to read a good structural mechanics introductory book, like Beer Jonston's one. Or talk to an engineer showing him your problem at first. General guidelines: 1- Define your application geometry and loads. 2- Calculate efforts. 3- Define structural members among commercially available hardware (if possible). 4- Calculate stress in members. 5- Recalculate everything if something does not seem alright varying thickness, choosing from the less expensive material up, trying to keep every little cost down as much as possible. Check standards for minimum requirements and safety margins if applicable. 6- Go for the final drawings, bill of materials, specifications, selection of suppliers, set down your production line. 7- Optimize process, make essays, revise design, listen to your boss complains about dates and costs, listen to your customers complains about defects, cracks, corrosion, pitting, price, delivery times, facility comissioning, solve some lack and incorrect specifications... 8- Complain to yourself about your payment. After all, go home and have a beer, check your email and some CR4 posts just for fun. Another day as an engineer.