# Static calculation with one solid part

Hello,

i have one single part with solid elements and i move them
with all nodes on one surface.
Why i need one more boundary condition?

here displacement with one boundary condition:
*BOUNDARY
Nfix,3,3,5

here displacement with two boundary condition:
*BOUNDARY
Nfix,1,1,0
Nfix,3,3,5

wbr & thnx for your help

I had a revealing debate on a similar topic some time ago but with *Boundary applied on beam nodes. Ccx , unlike other structural calculation programs, does not understand.

*BOUNDARY
Nfix,3,3,5

as a placement of the NFix points a distance of 5 in direction 3.

It reads that BC as an offset of the plane containing the point (Nfix points in this case) a specific distance in the normal direction.
If the orthogonal directions (1 & 2) are not properly constrained and there is still some Rigid body mode available , the whole resulting plane is still a valid solution, and the result may be unexpected.
In your case, ¿The most probable is that the solid has rotated around the normal (3) to the plane containing the set of points.

i have these example with GAPUNI,
if you use instead GAPUNI truss element it looks like these:

but usually it should work with only one boundary condition?

wbr

Could you post some gif to understand better what is going on there?

Yep, I would say it is rotating around 3 as anticipated. Your model seems to work properly, just need some extra BC to remove that rotational rigid body mode.

Add rotational restraint around axis at both end nodes n a complex 3D portal frames may lead to unexpected results. Probably , beam element with end rotation released by equation will perform better and represent properly. At least one node along the member required to be restrained in rotation around longitudinal axis. Compression or Tension only material could be use which still keep the structural system stable i.e no rigid body modes occurs.

P.s Alternatively one equation of end nodes in rotational around axis is transmitted.