How beams work:
Beam action demonstration: Take a Popsicle stick or tongue depressor and place it flat (wide face up) between two supports, e.g. a couple of thick books. You now have a simply supported beam (plank). Put your finger in the middle of the span and push down. You now have a loaded beam (plank), with a concentrated load at mid-span generating bending moment and shear within the beam. The beam will sag, or “deflect”. Push harder and the beam will break. Look at the break. The fibers at the top of the beam will not be severed, but the bottom fibers will be jagged and parted or broken. Now turn the beam over and repeat the exercise. This time the stick will break in two, with the fibers on both sides parted.
In each loading case, the fibers on the bottom were in tension, and the top fibers were in compression. Thus we see that a beam works by internal push in its top flange, and pull in its bottom flange. These are two opposing forces working together in harmony around the neutral axis. Sort of like the law and gospel around grace resisting the load of sin.
Your finger load also created a shearing force at the supports. But because the bending strength of the stick is weak when loaded as a plank, it failed by bending moment rather than shear.
Now, take another stick and repeat the exercise, but this time place the stick with the narrow edge on top. Unless you are as strong as Samson, you will not easily be able to break the stick, because loaded this way it is much stronger in bending. Thus we learn that a beam’s strength depends on the distance between (separation) of the flanges, or the depth of the web. This gives a basic rule of how beams work. “The flanges carry the moment, the web carries the shear”.
Beam action demonstration: Take a Popsicle stick or tongue depressor and place it flat (wide face up) between two supports, e.g. a couple of thick books. You now have a simply supported beam (plank). Put your finger in the middle of the span and push down. You now have a loaded beam (plank), with a concentrated load at mid-span generating bending moment and shear within the beam. The beam will sag, or “deflect”. Push harder and the beam will break. Look at the break. The fibers at the top of the beam will not be severed, but the bottom fibers will be jagged and parted or broken. Now turn the beam over and repeat the exercise. This time the stick will break in two, with the fibers on both sides parted.
In each loading case, the fibers on the bottom were in tension, and the top fibers were in compression. Thus we see that a beam works by internal push in its top flange, and pull in its bottom flange. These are two opposing forces working together in harmony around the neutral axis. Sort of like the law and gospel around grace resisting the load of sin.
Your finger load also created a shearing force at the supports. But because the bending strength of the stick is weak when loaded as a plank, it failed by bending moment rather than shear.
Now, take another stick and repeat the exercise, but this time place the stick with the narrow edge on top. Unless you are as strong as Samson, you will not easily be able to break the stick, because loaded this way it is much stronger in bending. Thus we learn that a beam’s strength depends on the distance between (separation) of the flanges, or the depth of the web. This gives a basic rule of how beams work. “The flanges carry the moment, the web carries the shear”.
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