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How much iron should be in a U-shaped shell (lintel) in the garage?

I am currently planning my garage. Material: aerated concrete blocks: 24 cm Garage door width: 3.50 m Wall length: 5 m.

Above the garage door will be an open wall 1m long. Behind it is a flat roof. Therefore, the supports for the beams are the two side walls, not the lintel. Now my question is: What iron mesh should be placed in the U-shaped shells? I know this has to be calculated by a structural engineer. But perhaps there is someone here who has already built something like this. No guarantees should be made here. I just wanted to get some experience. It's not going to be a skyscraper. 

• garage