Static analysis
The SPACE GASS static analysis module is capable of performing linear or non-linear analyses. Furthermore, you can analyse some load cases linearly and others non-linearly in the same model.
For members in a non-linear static analysis you can choose between small, finite or large displacement theory. For plates you can choose between linear "BC" plates or non-linear "DL" plates.
For cable members, SPACE GASS always uses a large displacement theory that has been designed to cope with the highly non-linear behaviour and large deflections that occur within cables (see also Cable members). For structures that contain both cables and non-cable members, it is important to note that while the large local cable deflections are allowed for in the analysis, the non-cable parts of the structure are still analysed using small displacement theory.
Although a SPACE GASS non-linear static analysis includes simple buckling checks on individual members and on the model as a whole, a full buckling analysis is usually required in addition to the static analysis. If the buckling capacity of the model has been exceeded then the static analysis results are invalid and should not used!
If
the static analysis results are to be used for a steel design to AISC-LRFD,
Eurocode 3, AS4100 or NZS3404, the load cases used in the strength design
must be analysed non-linearly unless you know that the second order effects
are negligible.
The non-linear static analysis facility available with SPACE GASS considers geometric non-linearities rather than material non-linearities.
Material non-linearities occur as a result of the non-linear stress-strain relationship of most materials. This effect becomes more significant as the material reaches its yield point and the stress-strain curve flattens out. SPACE GASS does not consider material non-linearities because they are relatively insignificant in comparison with geometric non-linearities and because their effect only becomes noticeable when the material is highly stressed.
There are many types of geometric non-linearities, some of which can be significant and many of which are relatively insignificant. The most important geometric non-linearities are:
P-D effect
P-d effect
Axial shortening effect
Tension/compression-only effect
Catenary cable effect
Gap and fuse members
Variable spring, plastic, friction and one-way restraints
If your model contains gap or fuse members, or variable spring, plastic, friction or one-way restraints, because they are inherently non-linear in the way they behave, you must run a non-linear static analysis to get accurate results. However, if you run a linear static or buckling analysis, or a dynamic frequency analysis then SPACE GASS will allow it, but all gap/fuse members and non-linear restraints will be treated in a simplified linear manner that may not give you the results you expect or want. Gap and fuse members will be treated like normal members with their tension and compression limits ignored. Plastic and friction restraints will be treated as fixed, while variable spring restraints will be treated as springs using a constant spring stiffness that corresponds with a zero deflection from its stiffness vs deflection table. One-way restraints will be treated a bi-directional.
Some sources refer to the additional effects of shear deformations and rigid end gussets as being geometric non-linearities also. While SPACE GASS fully considers these additional effects during the analysis phase, it does not consider them to be non-linearities because they can be solved directly in one analysis and do not require an iterative procedure.
