Force or Pressure
Force or Pressure
(OP)
Load on the supporting structure should be considered as pressure or Force?
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RE: Force or Pressure
RE: Force or Pressure
RE: Force or Pressure
eg the weight of an object can be repreented as a point force at the CG or as a pressure applied to an area about the CG (so that the total force (the integral of pressure over area) has the same line of action (ie through the CG).
clear as mud ??
RE: Force or Pressure
The force will probably due if I'm looking at the stress in the chair legs, but if I'm sitting in a desk chair, the bending stress on the seat will not be seen without the pressure load approach (or some distributed force loading).
RE: Force or Pressure
So using superimposition it makes no difference whetehr you apply a force or a pressure, remote from the loading point.
Cheers
Greg Locock
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RE: Force or Pressure
RE: Force or Pressure
there should be little difference between a pressure load in the upper surface, points loads at a set of upper surface nodes (all of them, some of them), or a single point force at the point of application with RBE3s to the surface of your model (i'd suggest that you don't use this method untill you understand how it works).
RE: Force or Pressure
Pressures on an element face are distributed to the nodes of that face (including mid-side nodes if present) via the element shape functions, as any other form of distribution will not be equivalent to the specified pressure and will create the false very high local stresses that Crisb referred to.
Nastran RBE3 elements cannot correctly emulate a uniform or smoothly varying pressure dustribution either.
If the structure in the vicinity of the load application is of interest then you'd be best to follow Crisb's advice.
RE: Force or Pressure
I believe that a pressure distribution adjusts the loads at each node to provide a balanced loading. What I mean is that if you have an axially loaded bar, and you apply a pressure to the end face, the bar will expand uniformly such that the end face remains plane. Now if you were to apply the pressure as discrete forces, all of equal value, then you would see more deflection around the free edges and less in the middle as the face dishes. If the other end of the bar is a sufficient distance from the applied face, then both methods should produce the same stresses at that end.
jetmaker
RE: Force or Pressure
RE: Force or Pressure
I agree with you, if you want to simulate the pressure correctly, then you do as you have stated. What I was refering to was that if you apply the loads evenly to each node, as some people do, then the effect will not be the same as a pressure applied (except when remote to the applied loading).
jetmaker
RE: Force or Pressure
RE: Force or Pressure
[N/mm^2]=[N]/[mm^2]
RE: Force or Pressure