Following is a discussion on the Reviewer item titled: Solution to Problem 535 | Economic Sections. Feel free to add your own comments!
What is the rationale behind calling y=124 lbs. 14 ft. from R2 a critical section? You will see this in computing for the maximum moment of the dead load. I just need to remember why?
Benjamin A. Molina III
I am not Mr. Verterra but from the live load diagram, it is the location of zero shear.
The critical section for moment will usually occur at the point of zero shear. From the shear diagram of the applied loads, the point of zero shear is located 14 meters from the right support. The maximum moment will occur at that point.
Considering the effect of the weight of the member (which is significantly small compared to the applied load), the location of maximum moment won't change (if it does, it is only very small - the solution here did not check if it does). Thus, the critical point for bending is still 14 meters from the right support.