How to assemble C-beam into beam and how to weld it? Nine meters span.

You asked the same question twice? Is there a load on the 9 m span? Two ways.

1, C-beams are welded face to face and made into squares or circles;

2, back-to-back welding, made of I-beam, and then inside the C beam with angle iron to burn a wave, up and down to fasten.