FE-LTB 7.xx - Dlubal Software

Updated:

08/30/2010

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FE-LTB 7.xx

Lateral Torsional Buckling Design according to the Second-order Analysis (FEA)

The add-on module FE-LTB is used to detach planar sub-structures from spatial RSTAB models to calculate them according to the second-order analysis for torsional buckling considering warping. The program imports geometrical, support and load data from RSTAB automatically. Subsequent to the calculation, FE-LTB displays deformations, internal and support forces as well as stresses. As warping torsion is taken into account by the module's calculations, the program displays also the results for bimoment distribution and the diagrams for primary and secondary torsion.

You do not need to look into technical literature anymore because the required tables, buckling curves and properties of corrugated sheets are already included in the program. For example, the entire methodical approach for determining rotational restraints according to DIN 18800 is implemented in the module.


Program Features Full integration in RSTAB with import of all relevant loads General stress designs with warping torsion according to elastic/elastic method Stability analysis for buckling and lateral torsional buckling of planar continuous members Determination of critical load factor and hence of M_cr or N_cr (the factor can be used in LTB for the el/pl design) Lateral torsional buckling design for any kind of cross-section (also SHAPE-THIN sections) Analysis of members and sets of members with applied torsion (e.g. crane runway girder) Option to determine the factor for ultimate load capacity (critical load factor) Display of torsional modes and eigenmodes on the rendered cross-section Powerful tools for determining shear panels and rotational restraints, e.g. from corrugated sheets, purlins, bracings Comfortable determination of discrete springs like warp springs from end plates or rotational springs from columns Graphical selection of load application points on the cross-section (upper chord, centroid, lower chord or any other point) Free arrangement of eccentric nodal and line supports on the cross-section Determination of value for inclination or precamber according to DIN 18800 by means of eigenvalue analysis Special torsional buckling releases for defining warping conditions on transitions


Input When you have entered the complete structure in RSTAB, you can open the add-on module FE-LTB. To create a design case, define the continuous members that you want to design as well as the load cases and groups for the analysis. The sets of members can be selected graphically by means of the common selection tools. The materials and cross-sections used in RSTAB are already preset in the module but can always be modified and completed. The tables for defining materials and cross-sections offer comprehensive libraries where you can select the data required.


Input Options Comprehensive and easy detail settings in the input tables facilitate the representation of the structural system: Nodal Supports You can edit the support type of each individual node. A warp stiffening can be defined on each node. The resulting warp spring is determined automatically by the input parameters. Elastic Member Foundation For elastic member foundations you can enter the spring constants manually. Alternatively, you can use comprehensive and clear arranged setting options to define the rotational and translational spring from a tension field. Member End Springs FE-LTB calculates the individual spring constants automatically. You can use the dialog boxes and detailed pictures to represent a translational spring by a connecting component, a rotational spring by a connected column, or a warping restraint (types for selection: end plate, channel section, angle, connecting column, cantilevered portion). Member Releases In case member releases for the selected set of members haven't been defined in RSTAB, you can define them directly in the module. Load Data The nodal and member loads for the load cases and load groups selected for calculation are displayed in separate tables. They can be edited, deleted or completed individually. Imperfections Imperfections are applied automatically by scaling the lowest eigenvector.


Design In accordance with DIN 18800, part 2, FE-LTB performs the designs for flexural buckling and lateral torsional buckling separately to simplify the calculation. Generally, the design for flexural buckling in the framework's plane is carried out by a calculation of the planar frame structure according to the second-order analysis, taking into account design loads and pre-deformations. In contrast, the lateral torsional buckling design is carried out on an individual member that is detached from the total structure. FE-LTB takes into account specified boundary conditions and loads according to the elastic-elastic method. In FE-LTB, the decisive failure mechanism is searched for by means of the critical load factor which describes buckling, lateral-torsional buckling or a combination of all types of failure, depending on the structure and applied load. This factor is then recalculated to the required operands. Furthermore, you can decide whether the critical load factor is calculated only due to loss of stability (assumed that the material is defined by infinitely elastic properties) or if the limit stress is taken into account. Moreover, you can modify the value preset for the finite elements in a special input field. In addition, you can adjust the partial safety factor g_M , if necessary. Finally, the iteration parameters are already preset appropriately for all common structures but can be adjusted by user specifications.


Results Subsequent to the calculation, FE-LTB shows the deformations, internal forces, support forces and stresses. As the program takes into account warping torsion, the diagrams of the warping bimoment as well as of the primary and secondary torsional moment are available, too. For stability analyses FE-LTB considers imperfections during the calculation and determines the critical load factors which can be used to determine M_cr and N_cr. The result values in the tables are shown always together with the corresponding cross-section graphic. The various results are highlighted with different colors in the RSTAB analysis model. The colors and values assigned can be modified. The diagrams for result distribution on the set of members allow for a specific evaluation. It is also possible to represent every intermediate value. Finally, you can export all tables to MS Excel or OpenOffice.org Calc or in a CSV file without any problems. All specifications required for the export are defined in a special transfer menu.

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