How do I draw shear diagrams?

Snow is 20lbs per cubic foot and last night it snowed 6in. The roof only holds the snow (ignore the weight of the beam and other loads for this thought-exercise). Horizontal beams, simply supported, 30ft long, 10’ O.C..  What is the maximum vertical shear in the beam?

Click here [https://www.youtube.com/watch?v=sUq2u1dooik&list=PLRqQUel8W0R71cVQvIkcQ529D93rr2ggh&index=3] to watch my video answer. If you are confused on the topic of shear diagrams, you probably won’t be after you watch.

Michael Ermann, Amber Book