Indeed, think about what the reaction moment would be if AB were only 3 m long. That's not a coincidence, it always holds true. Notice this is also what you get from simply calculating the bending moment caused by D on C (just as you did to calculate the moment reaction at A). Now, something to keep in mind when trying to figure this sort of thing out is the fundamental equation of Euler-Bernouilli Beam Theory: Thankfully you've already done that correctly, so we can move on from there. It is a force which causes something to bend, and depending on how an object is fixed or supported affect the magnitude and shape of the bending moment diagram.So, in such problems, the first thing to do is calculate the support reactions. In practical terms, bending moment occur when a force is applied at a distance away from a reference point (Force x distance). The magnitude of each force lowers as it reaches the centroid of the I-beam, before changing signs (compression to tension). The internal normal forces (see sketch below) indicate compression along the top flange and tension in the bottom flange. The internal forces/reactions to the load are resolved into a resultant couple, and this must be balanced by an internal couple which is called the “bending moment”. When a beam is loaded, internal reactions forces (normal forces – orientated in the beam direction) develop within the cross-section of the beam. If the applied stress is greater than the ultimate tensile strength (or yield stress), failure may occur. When a load is applied to the beam which is large in magnitude, failure of the beam can occur. Beams are a structural element, which are associated with bending moment diagrams and analysis. The bending moment is a reaction in a structural element that is subjected to an external force or moment, causing bending.
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