If you want to input A, Iss, etc. directly, you must use a resultant beam formulation, i.e., ELFORM=2. With such a formulation, stresses are not calculated because the shape of the cross-section is unknown. You'll only get forces and moments. ELFORM=2 is compatible with only a few material types. See the material table at the beginning of the *MAT section of the Users Manual.
ELFORM=1 is an integrated beam formulation. With an integrated formulation, the shape of the cross-section is defined in the input and so stresses can be computed at the beam integration points. The parameter CST on Card 1 of *SECTION_BEAM indicates whether the section is circular or rectangular. You must give cross-section dimensions on Card 2 of *SECTION_BEAM.
For circular sections, you give outside and inside diameters at the two ends. (For a solid circular section, the inside diameter is zero.) For solid rectangular sections, you give the cross-section width and breadth at the two ends.
For hollow rectangular tubes, it's a bit trickier as you must also use *INTEGRATION_BEAM which is referenced by setting QR/IRID in *SECTION_BEAM to -IRID, where IRID is given in *INTEGRATION_BEAM. In *INTEGRATION_BEAM, you can leave NIP and RA blank and set ICST to 5. On Card 2 of *INTEGRATION_BEAM, give the 4 values W, TF, D, and TW which are shown in the figure in the Users Manual. If QR/IRID in *SECTION_BEAM is negative, it follows that *INTEGRATION_BEAM defines the location of the integration points. If QR/IRID is positive and CST is zero (rectangular section), refer to the Figure 5.3 on p. 5.11 of the 2006 Theory Manual. If the integration rule is 2x2, 3x3, or 4x4 Gaussian, the locations of the integration points shown in Figure 5.3 are in accordance with the columns labeled as 2 point, 3 point, and 4 point, resp., in the table under *SECTION_SHELL in the Users Manual. Integration points for a circular cross-section are positioned sequentially in the circumferential direction of the cross-section, all at the same distance from the cross-section center. For example, for 3x3 Gauss quadrature, the nine integration points in the cross-section are 40 degrees apart with the first integration point on a ray 20 degrees off the local s-axis (toward the t-axis). The radial position of the integration points for a solid circular cross section is at r = 0.707 * the outside radius.
You can get axial strain at beam integration points by setting BEAMIP (*DATABASE_EXTENT_BINARY) to the number of beam integration points in your LS-DYNA input deck. Then, after running the model, read D3PLOT into LS-PrePost and click History > Int.Pt. > Etype: Beams > (click on any beam element) > Axial Strain > Plot.
Also, plastic strain at beam integration points is written to elout (see *DATABASE_HISTORY_BEAM and *DATABASE_ELOUT).