# Boundary conditions in a four point bend test

I am looking into the four-point bend test, such as one in this YouTube video.

Sample screenshot from the video illustrating the problem: I am a little confused as to how the loads are prescribed numerically as boundary conditions. My intuition tells me that the object has to have a Dirichlet BC somewhere. But it appears that the 4 supports (2 on top and 2 on bottom) exert the vertical loads, hence they should be all Neumann BCs.

• The forces you show at the bottom of the beam are modeled as Dirichlet BCs; y-displacement equals zero at both points and x-displacement equal zero at one of the two points. – Bill Greene May 22 '18 at 11:20
• Oh hmm. Why aren't both x-displacements at the bottom zero? If only one is zero, then it seems we would lose symmetry? – user27504 May 22 '18 at 12:53
• The x-displacement at one point is required to restrain rigid body motion in that direction. But, it is permissible to set x-displacement equal zero at both points. – Bill Greene May 22 '18 at 13:27
• Ah I see. Would you expect the results from a simulation to differ significantly when setting both points' x-displacement to zero or just one one of them? – user27504 May 22 '18 at 13:40 