Introducing Multiframe 12 - New Plate Analysis Module

Multiframe Version 12 introduces our greatest ever enhancement to Multiframe - the addition of a new plate element module. In fact the new elements in the Plate module allow you to model all kinds of planar objects, not just plates. This includes shear walls, slabs, stiffened plates and in-plane stress analysis. While adding this capability has added some complexity to the program, we have endeavoured to preserve the same graphical interaction and ease of use to which Multiframe users have become accustomed.

Multiframe building model with plate elements (Enlarge)

As usual our version 12 upgrade also includes a range of performance enhancements and additional functions in all of the Multiframe modules. This includes the newer American AISC code with allowable stress and limit state options as well as support for the European EC/3 steel design code. Management of materials in the sections library has also been improved.

 

The plate element capability and the AISC and EC/3 design codes will all be available to purchase as additional modules in the Multiframe suite. The other enhancements will automatically be delivered to members of our subscription program shortly. These updates will be available for download from our web site.
 

Plate Element Formulation

The plate element formulation included in Multiframe Version 12 is a general purpose 6 degree of freedom per node triangular element. This is compatible with the existing formulation for beam elements used in Multiframe. This type of element can be used to account for both in-plane and out of plane forces and moments.

The plate element we have implemented in Multiframe is known to be stable, including dealing well with the so-called “drilling” degree of freedom at the corners of plate meshes. This is the rotational degree of freedom rotating about an axis perpendicular to the plane of the plate. The element formulation uses a stabilisation matrix to ensure good performance in all situations. It also uses a Mindlin formulation for the plate bending calculations but adapted so that it can deal with both thick and thin plates.

Plate Patches

The key modelling tool for plate elements is the plate patch. A patch is a triangular or quadrilateral region which contains a mesh of plate elements. For triangular patches, a network of triangular elements is generated. For quadrilateral patches, a quadrilateral network is first generated and Multiframe then automatically subdivides each quadrilateral into 4 triangles. This ensures a reasonable mesh density while minimising the amount of subdivision required along patch edges.

Patches can be created in a number of ways. First, there are some new generate commands which will generate structures which include plate patches. One example is the new tool for generating circular tanks. Patches can also be created by interactive drawing. This is done in a similar way to load panels with commands for drawing a rectangular patch from corner to corner, as well as sketching triangular patches and quadrilateral patches. For simple planar structures, you can also automatically create patches from the boundaries of existing members. For example, if you have already created the frame for a building, you can then auto-generate the patches for the floor slabs and shear walls by selecting all of the members a plane at a time, and using the Generate Patch command to generate the patches in that plane.

If you have any existing Multiframe models in which you have used load panels to model planar areas, you can automatically convert those panels into patches. It is also possible to mix and match load panels and plate patches in the same model however you should never have a panel on top of a patch.

New generation tools include circular tank generation (Enlarge)

Patch Meshing

One difference between the plate elements and our existing beam elements, is that while the beam formulation is exact, the plate formulation follows the usual finite element principle of requiring a finer mesh density to achieve a more accurate result. For beam elements, no subdivision of the members is required to obtain an accurate result. For plates however it is necessary to subdivide into a mesh density suitable for the geometry and loads being applied.

In order to maintain nodal compatibility and continuity, it is therefore necessary to subdivide any beam elements which are connected along the sides of a plate mesh. Multiframe facilitates this in version 12 by automatically detecting when this occurs and subdividing the side members to achieve this continuity. As well as automatically subdividing the members which run along the sides of a patch, Multiframe will also detect which other patches share a common edge and adjust their mesh density to match the nodes along the common side(s). In this first release of plate elements, Multiframe supports uniform and non-uniform meshes on triangular and quadrilateral patches. A follow-on release later this year will support n-sided patches and arbitrary meshing with holes and mesh constraints within a patch.

A range of patch meshing options can deal with varying geometry (Enlarge)

In most cases you will want to model your structure using patches and apply properties and loads at the patch level. However it is sometimes necessary to go down to the detailed plate level within a patch and this can be achieved by turning on the option to select individual plates. This allows you to select one or more plates within a patch and override their properties. You can also move the nodes at plate boundaries and even delete individual plates from within a patch.

Finally, all of Multiframe’s commands for modifying and replicating frame geometry can also be applied to patches. This includes linear and cylindrical axis duplication as well as rotation, rescaling and mirroring.

Materials

Once patches have been defined, you can then specify their thickness and material properties. The user has individual control over the bending and membrane thicknesses of the patch. The new materials options described later in this newsletter are used to specify the material properties. In addition, the user can specify a weight per unit area of the patch which is added to any self weight load cases.

Plate Loading

Multiframe supports both distributed and point loads within patches. Distributed loads can be applied to the entire area of the patch in either a global or local direction. There is also an option for applying a point load to a parametric position on the patch (usually at the middle point). This will automatically find a centre node if it exists or distribute it to the nearest nodes around the load point. When you enter positions of loads in the patch point loading dialogs, you can enter calculation expressions for the position.  For example, if you want a load to be at the centre of the patch you can enter L/2 for the left distance and L/2 for the up distance. This also means you can apply this load to a number of patches with different dimensions simultaneously.

For self weight load cases, Multiframe will automatically use the weight of the patch and any additional weight per unit area.

Patch Loads can be applied easily (Enlarge)

Analysis

The existing Multiframe solver is used for linear analysis of the model. Future releases will support non-linear, buckling and time history analysis. The solver can analyse a model containing 5000 plate elements in less than a minute. Actually the most time consuming part of analysis is not the solver itself but the computation of local patch actions and stresses in preparation for the graphical displays.

Plate Results

Once analysis is complete, you can review the behaviour of the plate elements in the model by way of deflection, action and stress plots in the Plot window. Line or area colour coded contouring is used to display these results. The user has control over the number of contours to be displayed and can turn contour labels on and off.

 

As far as numerical display is concerned, the tables in the Result window display the actions and stresses associated with each node in each patch. As well as the actions and stresses associated with each degree of freedom, Von Mises and Tresca stresses are also displayed. The tables can be sorted by any of these actions making it easy to find maxima within the structure.

 

Von Mises stresses in a 2D plate field (Enlarge)

The addition of plate elements to Multiframe greatly broadens its scope of application. Users involved in architectural work will find it useful for modelling slabs and shear walls. Those involved in civil engineering projects can apply it to rafts, retaining walls and bridge decks, while marine and offshore users will find it useful for more accurately modelling stiffened plate structures. This first release of plate analysis capability will be followed in the near future by additional functions to broaden its application to more complex geometries and modes of analysis.

Deflection contours in a floor slab (Enlarge)

Materials Management

Another enhancement in this release of Multiframe is a revision to the way in which materials are managed. When selecting sections, a default elastic and shear modulus is selected with the section. However there is now a new Member Material command which allows you to select a new material for the member while leaving the geometric properties unchanged. For materials like steel, there is no change in the elastic modulus and so analysis results are unaffected. In this case the new materials will be used to vary the steel grade used for Steel Designer code checking. However for other materials like aluminium, this change in material does change the moduli and will affect the analysis results.

New AISC Steel Code

Version 12 of Multiframe incorporates the design code changes which first appeared in versions 11.5 and 11.6. The first of these was support for the updated AISC American steel design code. This code is unusual in that it merges together the allowable stress and limit state approaches. Steel Designer now includes support for both approaches in this code.

EC/3 Steel Design Code

Continuing on our support for design codes, we have added the European EC/3 design code to the long list of codes supported by Steel Designer. After some years of optional usage, this code is now required to be used in the United Kingdom and other European countries. One interesting aspect is the use of National Annexes for the application of the code within a particular country. While the code itself is generic across Europe, each country is able to release a national annex which can adjust particular capacity factors within the code. Steel Designer supports this by defining a range of national annexes as well as allowing custom overrides to be defined.

Yacht Mast Generation (Enlarge)

Yacht Mast Generation

In this release of Multiframe we have included a new spreadsheet which allows for quick, parametric generation of yacht mast models within Multiframe. The spreadsheet allows you to enter the overall parameters of your mast dimensions, it will then use the Multiframe Automation interface to generate a structural model of the mast.                                                
Data is entered in a format consistent with the approach used in the Germanischer Lloyd Rules 2009, Design and Construction of Large Modern Yacht Rigs. The worksheet then calculates spreader positions in even distances below the top spreader. As well as generating geometry, the worksheet allows you to import geometry from Multiframe, change it and export it back to Multiframe. We anticipate that this spreadsheet will be useful to create a first estimate of a new rig. Detailed changes can then be made in Multiframe itself.