by jdunks » Thu Mar 08, 2018 5:15 pm
Here's how it was explained to me. Total the shear force, 1k+2k+2k=5k. Distribute the 5k evenly among the bays. In this case each bay gets 2.5 kips. Each leg of the frame then gets 1/2 of the bay's load, in this case 1/2 x 2.5k = 1.25k per leg. This ends up giving middle legs 1.25k twice, once for their role in the left bay and once for their role in the right bay. That's why the middle leg is higher than the other two. You'll find that no matter how many bays are in the moment frame, it ends up being that middle or inner legs get twice as much as outermost legs. This works for moment frames (as Coach points out, due to trib load), but shear forces are distributed differently in shear walls and braced frames. And of course post and beam systems don't distribute shear forces, they just wrack and fail.
Credit for this goes to Thaddeus. Can't recommend him enough - I think his course is well worth the price. He and this forum are real life savers.