Constructing a rectangular beam needs several steps: First, you need to measure and process the needed depth of the beam, next step is detecting the breath of the beam and the third step is to figure out the self weight of the beam. For all three steps you need formulas and knowledge of the math and physics, so the construction would function as it should.  The link bellow will show you more about the needed formulas and the video will explain it all. Take a look!

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