Structural analysis is usually performed on an idealized, geometrically perfect model. However, nothing is perfect in the real world. The structural geometry, the material behaviour and the position of the loads are all imperfect. Geometrically and materially nonlinear analysis with imperfections included (GMNIA) may be used effectively instead of linear methods for strength and stability verification of structures, structural elements and details. This is especially true in the case of complex structural details and parts where the stability and load-bearing capacity can be hardly verified with simpler methods.
The IMP module of AxisVM allows users to account for such geometric imperfections, which can be created by scaling and then superimposing the buckling-mode shapes. The effect of imperfections can be taken into account via a geometrically nonlinear analysis on a load combination, which includes imperfection-type load cases.
Requirements / recommendations
- NL or PNL (e.g. NL1S, PNL3P) configuration is required to account for geometric imperfections based on buckling mode shapes
The IMP module is design code independent
- scaling and superimposing of buckling mode shapes
- displaying of created imperfect geometries
- including imperfection-type load cases in load combinations
- performing nonlinear analysis on models with geometric imperfections
CREATING EQUIVALENT GEOMETRIC IMPERFECTIONS
The imperfect geometry can be compiled from the mode-shapes obtained from a buckling analysis, even corresponding to multiple load cases. Displacement components and the respective maximum values for each buckling mode shape can be specified. The created geometric imperfections can be included in load combinations.
NONLINEAR ANLYSIS OF MODELS WITH EQUIVALENT GEOMETRIC IMPERFECTIONS
Internal forces may be obtained on models with geometric imperfection via a geometrically nonlinear analysis include large-displacement effects. The loss of stability mode can be determined with a displacement-controlled nonlinear analysis. Finite element simulation-based design is also possible by using nonlinear material models and geometric imperfections together.
Design codes/standards provide guidance for the inclusion of wind pressure factors only for structures with close to regular geometry. In the case of more complex or compound structures, these methods are not applicable, and the pressure coefficients must be determined individually by a fluid dynamics simulation (CFD modeling) or a wind tunnel test. The CFD module allows for importing these results into AxisVM.
The CFD module is a generic interface which enables defining pressures caused by flowing substances over domains or load panels.
Requirements / recommendations
- the module is an option in the basic packages (analysis options), which is recommended when designing structures with loadings from flowing substances
The CFD module is design code independent
- it is recommended for structures with irregular geometry or with loads not covered by design codes/standards
- currently it can only apply static pressure loads (constant functions of time)
- it interpolates pressure values from CFD simulations on the contour of the surface elements of the structure
- it is sufficient to specify the spatial coordinates of the measurements and the corresponding pressure values, from which the program determines the surface load values perpendicular to the surfaces
The SOIL module allows more precise modelling and consideration of the soil-structure interaction. In the so-called direct approach, the soil and the structure are modelled and analysed together. The settlements and the induced internal forces can be calculated, as well as the stresses and strains in the soil beneath and around the structure. The software is able to generate a spatial soil model with layers based on the given borehole samples. A further possibility is that the Winkler stiffnesses for supports can be estimated based on interpolated soil profiles.
The SOIL module is design code independent.
- User defined borehole samples
- Interpolated soil layer profiles
- Calculation of Winkler stiffness for conventional support elements using interpolated profiles
- Generation of soil model using solid finite elements
- Modelling of soil-structure interaction
Definition of borehole samples
The soil model, built from solid elements, is used to more accurately model the interaction between the soil and the building structure. The stresses and strains in the soil induced by the loads can also be calculated. When specifying a soil model, the building structure is not supported by nodal, line or surface supports but by the soil model itself. In this case, the pad, strip and slab footings must be modelled with their actual physical dimensions as domains in contact with the soil. In order to build a soil model, at least one soil modelling domain needs to be specified. This is a planar polygon similar in geometry to conventional structural domains. The soil model is a region of space situated below soil modelling domain and meshed with solid finite elements, where the properties of the solid elements follow the layer profiles defined by the borehole samples.
Solid finite elements
The shape of the generated solid elements can be hexahedron or wedge-shaped or a mixture of both, or they can also be tetrahedron shaped. The volumetric mesh is constructed by vertical projection of the surface mesh. When using triangular or rectangular solid elements, the layer boundaries are handled by element shortening, but in the case of tetrahedron meshing, the size and number of tetrahedra are dynamically adapted to the layer thickness.
The soil model can follow the interaction between the soil and the structure more accurately than a calculation where the soil is modelled with springs. Consequently, the additional internal forces induced by relative settlements can be more accurately calculated. By modelling the soil around the structure with solid elements, there is no need to estimate soil stiffness based on different theories, and the model can track changes in stiffness due to variation in subsoil conditions. The soil model can be effectively used in the case of pad, strip and slab footings, even with complex geometries.