Understanding Bending Moments and Second Moment of Area | Calculation Analysis

[Dcoz]

Hi, I just wanted to know if I have answeedthis question correctly, if not where I have gone wrong. Thank you.

[PLAIN]http://img148.imageshack.us/img148/6245/secondmomentofarea.jpg

From what I have calculated, none are acceptable, but my calculations may be wrong

 

[SteamKing]

Your answers are wrong because the bending moments you calculated are incorrect. In order to calculate the moment correctly, set up a proper shear force diagram and construct the moment diagram from it, or use a table which contains the bending moment formulas for a simply supported beam.

 

[Dcoz]

Thank you for the reply. We've been shown how to make a shear force diagram, but not a moment diagram. This was course work, which he told us was correct except that most of us had forgotten to include the weight of the beam itself. I've tried to correct it by factoring the weight into it. Without the weight of the beam I=1.59 cm^4.

 

[SteamKing]

If you know how to construct the shear force diagram, the bending moment diagram is fairly simple to construct. Start at the left end of the shear force diagram and calculate the cumulative area under the shear curve and plot it on the bending moment diagram as you move along the beam to the right support. If the shear is in N and the distance along the beam is in meters, the units of the moment curve will naturally be in N-m. Remember to keep the sign of the areas in the correct reference system, and a properly constructed moment diagram will begin and end at 0 for a simply supported beam.

 

[Dcoz]

Thank you. I watched a few youtube videos showing how to draw one, but I'm not sure how it would help me get the right answer. Also is it not possible to answer ths question without one?

 

[SteamKing]

If you can't calculate the correct bending moment, how can you determine the direct stress due to bending?