Going off my single CIV101 (Civil Engineering) course in University over 10 years ago, so don't take this as perfect. But I didn't see it answered and decided to have a go:
xongsmith writes:
And presumably the pin doesn't allow it to move vertically as well.
Exactly. The intention is to show that the beam is held by "a point" (like a bolt), not glued completely in place. That is, if the beam was longer, the left of the pin would 'swing up' as the right swung down... as opposed to having the left staying flat and causing deflection to the right if it were "glued completely in place."
I still don't have any idea why it was drawn as a Right Triangle facing the way it is instead of an isosceles triangle
Simply general notation (I think).
Does it make a difference if the symbol is drawn facing the other way?
No. It would simply go against standard practice (I think).
So now I would revise my hipshot answer, since only the roller can move away, once the deformation is large enough.
Actually, the roller cannot "move away."
The roller, again, is part of standard notation and doesn't act like an actual, real roller would.
The point of drawing the roller-symbol is to say that the beam will not "drop" at that point, that it is constantly supported vertiaclly, regardless of the amount of deflection. You can imagine the roller "magically" moving inward as the beam deflects, if you'd like.
So, even if the beam would greatly deflect, if something was constructed to represent this diagram... the beam would either be long enough so as to never drop to the inside of the roller, or the roller would be forced to move inward to make sure the beam never dropped due to not having a vertical support at that end. Or it's quite possible that no roller is used at all, perphaps it's some weird upside-down piano-key-like thing where a certain key is pressed up to support the load wherever it moves horizontally... The point is that the load at that end is vertically supported... no matter what happens due to deflection.
The symbols are not physical representations of anything specific. They indicate "I don't care how it's physically done in the real world, but I'm talking about a fixed, rotation-allowed support here, and a verticial-only support where x-axis movement is allowed over here."
Those two symbols are generally used in conjunction with each other. This drawing is generally used to say "hey, we don't care about all the other shit that
could happen in reality, this diagram is to show that we're testing the load force required to break this beam under the simplest of conditions."
In bridges, "rollers" are generally used at one end for purposes of expansion/contraction as well. But that is not the purpose of using that symbol in this diagram.
...from what I can remember, anyway.
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