Software Option for Plus versions

IMDplus

The IMDplus option extends the Interactive Modal Dynamics (IMD) techniques available in all LUSAS products, and which model a single loading event in a single direction, to allow multiple loading events with more advanced loading conditions to be solved. IMDplus is used for two primary uses: seismic response analysis of 2D and 3D structures subjected to acceleration time histories of support motion, and for the analysis of 3D structures, such as bridges, subjected to constant moving vehicle or train loads.

An IMDplus analysis uses conventional eigenvalue analyses to obtain the undamped modes of vibrations for a structure over the frequency range of interest and, for accuracy, should include eigenvalues outside the frequency range of interest too. The modal response in the form of frequencies, participation factors and eigenvectors are then used by IMDplus to compute the response of each mode of vibration to the input excitation. Superposition methods are used to calculate the total response of the structure from each of the contributing frequencies which can then be viewed. The assumption of linear structural behaviour enables the IMDplus facility to make use of such summation ("linear superposition") techniques. As a direct result of this, IMDplus is of considerable benefit when compared with traditional direct integration time-stepping procedures - in many cases results are obtained in seconds as opposed to several hours. The IMD facility therefore offers large savings in design and analysis time and greatly reduces the computational resources required for lightly damped, linear transient dynamic analysis of this type.

The seismic analysis option analyses the response of a 2D or 3D structure to an acceleration time history input to the support nodes. An acceleration time history is specified for each excitation direction and therefore all support nodes are accelerated with the same time history record for this direction. Different acceleration records can be applied to each global direction and factored based on design code requirements. Up to seven earthquake combinations can be analysed in a single IMDplus analysis. Time histories, peak summaries, Secondary Response Spectra (SRS), and modal combinations can all be output.

With the IMDplus moving load option dynamic response analysis of a bridge for different train speeds is simplified by maintaining a consistent train motion across that bridge and simply adjusting the time interval for the modal calculations. Such moving load calculations can process a range of train speeds on each execution of the IMDplus facility. Results are readily obtained for either a single train speed to give full time history output or for a range of train speeds to give peak response output.

Results can be displayed using the extensive LUSAS graph plotting facilities or using the standard contour, vector or peak value features. Since both the damping and ‘time-step’ are specified within IMDplus there is no need to carry out a series of computationally expensive transient dynamics analyses to assess the effect of these parameters on the structural response.

IMDplus has been successfully used by the LUSAS consultancy department and by numerous consultants throughout the world for the design of structures where both the seismic and moving vehicle dynamic responses are required. Projects where IMDplus has been used to speed up design timescales include:

• Devonport Royal Naval Dockyard, UK
• Liquid Natural Gas Tanks, Worldwide
• Newark Dyke Rail Bridge, East Coast Mainline, UK
• Bridges for West Coast Mainline, UK
• Sg. Tuaran Road Bridge, Malaysia

• Multiple event simulation
• Fast Solution times
• Linear dynamics effects
• Support for displacement/velocity/acceleration response of model nodes for 2D Plane Strain, 2D Plane Stress, 3D continuum, 2D and 3D beams, 2D and 3D joints and 3D thin and thick shell elements
• Support for forces/moments and stresses/strains for 2D Plane Strain, 2D Plane Stress, 3D continuum, 2D and 3D beams, 2D and 3D joints and 3D thin and thick shell elements
• Composite axle definition to allow the analysis of multiple vehicle types using only a single analysis
• Unlimited problem sizes and complexities with typical models solved easily on a machine with 256MB of memory (solution limited solely by the specification of the computer)
• Output of raw results in a number of formats and automated generation of DPlot graph files for quick visualisation of results. A DPlot viewer is available free at www.dplot.com

Back to Top