pls pls explain why the C used at question no 3 is 275 I tho

pls pls explain why the C used at question no. 3 is 275? I thought C is supposed to be based on the neutral axis? i hoped that makes sense. thank you!

Timber Design 19. Problem: The beam is constructed from four boards as shown on the figure. If it is subjected to a moment M22 kN.m Determine the stress at A. Determine the stress at B. Determine the resultant force the stress produces on the top board C. 25 mm Solution Stress at A: AI 25(250) (2) 12500 mm2 A2= 400(25)-10000 mm2 A3 = 250(25) = 6250 mm2 A = 28750 mm2 = 125 T2 = 262.50 3 400 25 mm 25 mm 25 mm 28750 y = 12500(125) + 10000(262.50) + 62501400) y = 232.61 mm 25(250)3 12 + 25(250)(107.61)2] 2 +- 1er + 400(25)(2989)2 25250)9 + 25(250,(06739)2 12 / 426981431.2 mm* 250 292.39 Stresses at A 161.39 400 MC 25 262.o29.89 232.61 22 x 10° (292.39) 250 10.61 426981431.20 125 In= 15.07 MPa 2S 50 25 43/60

Solution

If you observe in the question 3 they asked resultant force caused by stress on top board. In this case the stress will be calculated with respect to top board of C not the with respect to centroidal(NA) axis. so we find stress with distance from bottom to top board which is 275.