Geometry and set algebra

Step 1 — Build the parametric geometry

The M01 block remains:

/PREP7
ET,1,SOLID185
MP,EX,1,young
MP,PRXY,1,nu
TYPE,1
MAT,1

BLOCK,0,beam_l,0,beam_h,0,beam_b
ESIZE,mesh_h
VMESH,ALL

A BLOCK isolated creates a volume delimited by six areas, twelve lines and eight keypoints. VMESH adds elements and nodes. Your IDs may change even though the solid represent the exact same beam.

Before reducing any selection, we save the full size of the model:

ALLSEL,ALL
*GET,n_nodes,NODE,0,COUNT
*GET,n_elements,ELEM,0,COUNT

These two values are our baseline. At the end they should reappear exactly.

Step 2 — Think of selections as set algebra

The second command argument xSEL indicates how the new condition is combined with the active set:

That is why a selection sequence must be read from top to bottom. If an operation leaves the set empty, a subsequent reselection cannot retrieve entities that were no longer active.

Step 3 — Control geometric tolerance

A calculated coordinate and a stored coordinate can differ by tiny fractions. MAPDL uses a tolerance on non-integer value selections. Its automatic logic depends of VMIN and VMAX; for example, if both are equal and not null, you can take the 0.5 % requested value

A global selection at X=1.0 m could include a much wider band of the expected when the mesh is very fine. In a reproducible script it is convenient to declare the criterion:

select_tol=MIN(beam_h,beam_b)*1E-5
SELTOL,select_tol

For the base case, select_tol=5E-7 m. It is small compared to the geometry and sufficient to absorb numerical noise. Tolerance remains active until redefined, so in the end we will restore it:

SELTOL,