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The previous video clears that two of the assumptions are incorrect. In this video, second iteration has done to get the result for pure bending fixing the assumptions that the bottom steel has yielded, middle steel layer has adjusted from compression to tension and steel layer 1 & 2 are considered to be elastic. Very first, the video shows how to get tensile force for steel layer 3 from the product of yield stress and respective area. Next, the video shows the calculation of tensile force for layer 2 & directly take the value of compressive force for concrete and steel layer 1 from previous video.
Moving on, the video equates summation of all compressive forces equals to tensile forces for pure bending; subsequently gives input the required obtained values & finds out the distance to neutral axis from the top side of the given section. Later, the video refers to next video to check whether the assumption made this time is correct.
