Special member characteristics
The member parameters, that aren't important for the most of structures, are marked as "Special" in the software. These parameters are mainly the member model choice, excluded stresses and spring end conditions.
Shear effect
Two theoretical models of members can be used during analysis. First model is based on Bernoulli - Navier theory, where the planar cross-section perpendicular to the member axis remains planar and perpendicular to deformed member axis also after the deformation. This model ignores the effect of shear forces on deformations. It is suitable for typical trusses and frame structures, where member lengths are significantly longer than cross-section dimensions. Members with larger cross-sections (massive beams) should be analysed with the help of the second model based on Mindlin theory. According to this theory, the planar cross-section perpendicular to the member axis remains planar after the deformation, however, isn't perpendicular to deformed member axis any more. The stiffness of the member is decreased due to shear impact on deformation.
Warping prevention at member ends
Torsion causes both the deformation of cross-section in its plane and in the perpendicular direction (parallel to the member axis). This behaviour is called warping. If the warping is not prevented in the structure, torsion induces only shear stresses and the cross-section is deformed in both directions. Such behaviour is called St.Venant torsion. If the warping is prevented, the torsion induces shear and axial stresses and such torsion is called warping torsion. Warping does not appear for all cross-sections. Warping is common mainly for steel cross-sections with warping coordinate ω and warping constant Iω greater than 0. Warping parameters can be specified only for these cross-sections.
Warping prevention can be specified with the help of the constant with the interval <0;1>, where 0 means free warping and 1 means warping absolutely prevented. The intermediate values describe combined behaviour.
Three different internal forces induced by torsion may appear on members subjected to warping: St.Venant torsional moment Tt, bimoment B and warping torsional moment To. Moments Tt and To induce shear stresses in cross-section, bimoment B induces axial stress.
Special end conditions
The end conditions can be set for all degrees of freedom at the beginning and end of the structure. Free, fixed and spring connections can be combined.
Excluded tension and compression
The tensile or compressive stresses can be excluded for any member. In these cases, the superposition principle for calculation of internal forces is not valid any more. Internal forces for all combinations have to be calculated by the direct analysis, not as a sum of particular load cases. The calculation is much more time consuming and it can't provide results in real time for more complicated structures with a lot of combinations.
The analysis uses the iteration principle. The members with unallowed stress are excluded from stiffness matrix gradually. Important note, that exclusion is done completely including the self-weight of the member and load applied to this member.