SHEAR FORCE
Take a few books. Place them next to each other contiguously. Now lift them by pressing the books on both the extreme ends as shown in picture below.
What is preventing the books in between from falling off the stack? Obviously there has to be some force that is acting upward which is preventing the fall. Also, how is the weight of the books transferred between each of the books and finally to your hand?
The answer is obviously friction which is a vertical force generated at the interface between books, induced by the horizontal force applied by your hand. I've separated the books and shown the forces acting on them (free body diagram) in the figure below. Now, obviously, the total downward force should cancel out the total upward force, otherwise the books would be bouncing up and down out of your hands. I have hidden the horizontal force (which is the main cause for friction) for clarity purposes, only vertical frictional forces are shown. For simplicity, weight of the books are considered to be point load acting at the center.
Assume four books each weighing 1 kg, so the total downward force (due to gravity) = 4 kg (not using 'Newton' for simplicity.) To resist this downward force, the frictional upward force generated by both your hands = 4 kg i.e. upward frictional force generated at each hand is 4/2 = 2 kg. Let's proceed from left toward right to plot the vertical forces acting on the books.
Now, if I were to make a diagram of the vertical forces and call it a Frictional Force Diagram (FFD) it would look something like this.
A simplified frictional force diagram.
Now the concept of Shear Force is "kind of" similar to the frictional force explained above. Consider a simple beam as shown in the figure below. Now, instead of books, imagine an infinite number sections throughout the length of the beam. Whenever the beam is loaded, each section tries to slip against its adjacent section, resulting in deformation of the section. The deformation produces a resisting force which prevents the slipping of the section, this force is called the shear force. Sections near supports are trying to slip further from each other compared to the sections near the center. Hence evidently more shear force is developed at the supports (see the force diagram above) than at the center. This is how the loads are transferred onto the supports.
The loads on the first floor of your house viz., weights of people, furniture, etc., are transferred to the beams through shear force developed in the slab, the beams in turn transfer them to columns again through shear force, the column then transfers the load to the footing by simple axial force and the footing spreads the loads safely over a very large area on the ground.
A diagram depicting the shear forces at all the sections is called shear force diagram which is similar to the one plotted above. Please note that I have presented a simple loading condition. Structures in real-life are subject to more complex loading patterns and shear force diagrams will be very different.
A shear failure happens at a section when the shear stress (= shear force divided by sectional area) developed at that section is greater than the shear strength of the material i.e. the maximum permissible shear stress. Civil engineers calculate these shear stresses and design the buildings so that the stresses are well within the materials' shear capacity. Mechanical engineers ensure the same in machines. That photo frame hanging on your wall has not fallen down yet because the shear stresses induced by the weight of the photo frame is less than the shear strength of the screw/nail. Some shear failure pictures are shown below.
Phew! I hope the explanation is clear to a layman. I'm a bit tired now. Will add the bending moment part later, hope you don't mind.
