LTB 7.xx - Dlubal Software

Updated:

07/08/2010

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LTB 7.xx

Lateral Torsional Buckling Analysis according to Equivalent Member Method

LTB is not a stand-alone program but an add-on module that is integrated in the graphical user interface of the main program RSTAB. Due to the integration, all structure-specific input data and internal forces are automatically available in the module.
In return, you can evaluate the LTB results in the RSTAB work window graphically and include them in the global printout report. LTB performs the design for safety against lateral torsional buckling for members and sets of members subjected to actions due to uniaxial and biaxial bending with or without axial force. The design is based on the equivalent member method according to

DIN 18800-2:1990-11.

Take advantage of LTB to design your structural systems efficiently. The program determines and applies stiffness-increasing affects conforming to standards, such as rotational restraints, shear panels, warp stiffenings, plastic cross-section resources or specific support conditions. The determination of the moment coefficient z and thus of the ideal elastic critical moment M_cr follows different approaches.


Features Full integration in RSTAB with import of all relevant internal forces Smart presettings of design parameters specific for lateral torsional buckling Determining the distribution of internal forces automatically including classification according to DIN 18800 part 2 Consideration of rotational restraints and shear panels by means of integrated tools for determining the corresponding factors conforming to standards Integrated libraries with corrugated sheets provided by major manufacturers Determination of moment coefficient z and thus of the ideal elastic critical moment M_cr Different support types considering warp springs for different stiffening and connection situations Cantilever design Option for calculation according to Vogel/Heil Consideration of lateral restraint by determining M_cr and N_cr according to Wittemann Output of fastener forces due to rotational restraint for trapezoidal cross-sections and purlins Optimization of cross-sections Parts list of members and continuous members Option to export all results directly to MS Excel or OpenOffice.org Calc


Input When RSTAB has finished the calculation of the structure, you can open the LTB add-on module. First, you create at least one design case in the module for which you define the members and sets of members that you want to design. In addition, you determine the load cases or load combinations that are required for the design. Take advantage of the RSTAB graphic tools to select the members and sets of members easily. The materials and cross-sections used in RSTAB are already preset in the module but can always be modified and completed. Furthermore, LTB provides comprehensive libraries for the selection of materials and cross-sections. Moreover, LTB allows you to optimize cross-sections.


Design Parameters The specifications for the lateral torsional buckling design are defined separately for members and sets of members. The following parameters can be set: Support Type / Buckling Load You can select pinned supports, pinned supports that are built-in and cantilevers. Special supports can be specified by the degree of restraint b_z and the degree of warping restraint b₀. By defining geometrical dimensions, the degree of warping restraint can also take into an elastic warping support for an end plate, a channel, an angle, a column connection or a cantilevered beam. Alternatively, it is possible to enter the lateral buckling load N_cr or the effective length s_cr. Shear Panel The shear panel can be defined by a trapezoidal sheeting, a truss or by a combination of both. Alternatively, it is possible to enter the shear panel stiffness S_prov directly. Rotational Restraint You can select either a continuous or a discrete rotational restraint. Load Application Point You can define the z-coordinate of the load application point freely in a specific cross-section graphic. (upper flange, lower flange, centroid) Alternatively, it is possible to specify the value by graphical selection or manual data input. Beam Type The following beam types are available for common cross-sections: rolled beam, welded beam, castellated beam, notched beam or tapered beam (web or flange welded). For special cross-sections, it is possible to define the beam coefficient n, the reduced beam coefficient n or the reduction factor k_M directly.


Design Generally, LTB performs the analysis according to the equivalent member method described in DIN 18800, part 2. A separate dialog box allows you to define detailed settings for the design: Calculation according to Vogel/Heil You can apply this method to determine the required shear panel stiffness S_req critical lateral buckling force N_cr critical bending moment M_cr. This plastic-plastic method of calculation is only applicable for pinned supports with uniaxial bending together with a load introduction on the upper flange. If the requirements are not fulfilled, for example in case of biaxial bending, LTB displays an error message. Furthermore, it is possible to set the reduction factor k_M for the bending moments M_y to 1.0 by conservative assumption if a rotational restraint is available. Non-designable Internal Forces It is possible to neglect non-designable internal forces to exclude them from the analysis if the quotient of design force and plastic capacity falls below a certain value. In this way, you can ignore for example a low moment about the minor axis to avoid applying the method for biaxial bending. Application Rule according to DIN 18800 part 2, element (320) and element (323) Automatic Determination of z If you want to determine the factor used for the determination of the ideal elastic critical moment M_cr automatically, you can select one of the following options: Numerical solution of elastic potential Comparison of moment distribution diagrams Australian code AS 4100-1990 U.S. standard AISC LRFD When comparing the moment distribution diagrams, you can take advantage of the library with more than 600 diagrams listed in tables.


Results Subsequent to the design, LTB displays the results sorted by cross-sections, members, sets of members or x-locations in clearly arranged results tables. The result values in the tables are shown always together with the corresponding cross-section graphic. In the RSTAB analysis model, the values are highlighted by different colors. In this way, you can easily detect difficult or oversized structural components. The colors and values assigned can be modified. Diagrams showing the result distribution on the member or set of members allow for a specific evaluation. It is also possible to represent every intermediate value. The masses determined during the design are displayed by member as well as by set of member in the form of parts lists. 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.


Result Diagram on Member	3.1 Parts List by Member	Export settings for MS Excel

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