1D Elements - Altair University

1 1 1d elements While working with 1d elements sooner or later you will come across the following element types: CROD CBAR CBEAM In brief, CROD Elements support (allow) tension and compression only, whereas CBARS and CBEAMS allow bending as well. A CBAR element is a kind of simplified CBEAM element and should be used whenever the cross-section of the structure and its properties is constant and symmetrical. The CBAR element requires that its shear center and the neutral axis coincide. Due to this requirement CBAR Elements are not useful for modeling structures that may warp, such as open channel-sections. This limitation is not present in CBEAM Elements . Thus, CBEAM Elements are used to model more complicated geometries with varying cross sections.

2 Important to note, in CBEAM Elements the neutral axis and shear center are noncoincident. Overall, this type of element demands a deeper understanding of beam theory. Furthermore, we recommend the beginner to use the predefined 1D element cross-sections available with the OptiStruct library (provided you are using OptiStruct/RADIOSS as your finite element solver). Another benefit of employing the cross-sections available with the OptiStruct library is that the stress recovery points which are needed for postprocessing stress results are predefined already. Of course, in addition to the cross-sections depicted further below (OptiStruct library) you can create your very own and general 1D cross-sections with HyperMesh.

3 CROD Elements In the following the general modeling building process based on CROD Elements is documented. Most of working the steps can also be applied to CBAR and CBEAM Elements , too. CROD Elements This element understands tension and compression loads (axial forces) only. In other words, the nodes of a CROD element only have translation degrees of freedom (still this element has a torsional stiffness). 2 How to model CROD s As we are going to use the FEM program OptiStruct, make sure that the corresponding UserProfile is activated. You may start by building CROD Elements manually (without/without any propertydefintion) by making use of the panel: Pull Down Menus Mesh Create 1d elements Rods Make sure that the element type to be created is set to CROD.

4 In the simplest case a CROD element is created by selecting its bounding nodes. Other more general options to create many CROD s in a single step exist of course. For instance, by making use of the line mesh panel. Pull Down Menus Mesh Create Line Mesh Again make sure and specify the element configuration to be rod . 3 It may happen that the newly created Elements are displayed as CWELD Elements (not what we want). If this is the case, then the element type must be updated: (Pull Down Menus Mesh Assign Element Type) .. set rod= to CROD, select the corresponding Elements and activate update Up to now the element specific information such as cross-section data is missing.

5 The CROD Elements are thus simply displayed as a line 4 In order to define the Elements cross-sectional properties we recommend to use HyperBeam (Pull Down Menus Properties HyperBeam) (also accessible from page 1D HyperBeam) In the HyperBeam panel select standard section (left side): As stated before we will make use of the readily available cross sections in OptiStruct. Therefore make sure that for standard section library OPTISTRUCT is activated. From the list of available cross-sections ( standard section type: ) chose the one of interest, here it is Rod (Box, hat etc. are also valid of course). Note, the cross section is named Rod it doesn t say anything about the 1D element type!

6 Clicking the create button then starts up HyperBeam which allows to interactively changing the radius of the rod cross-section. Note, that all the relevant cross-sectional data such as area, moments of inertia are automatically calculated and listed in the side bar of HyperBeam. This information is stored as a beamsection collector (here named rod ; listed in the side bar on the left). Later the beamsection will be referenced inside the mandatory property collector. Complicated? Not really as you will see .. 5 In order to switch back to the HyperMesh GUI, just activate the icon in the upper left menu bar (beneath Model ). Before we proceed creating a property collector we need to define a material (right mouse click in the Model Browser create Material).

7 In many cases, especially while getting started with FEM, the material card image will be MAT1 (MAT1 describes linear elastic isotropic temperature independent material behavior). The material is named steel using standard values for Youngs Modulus, Poissons ratio and density. Then, in the next step the property collector for the CROD Elements is created (right mouse click in the Model Browser create Property). In the pop-up window of the property collector, the Card Image is chosen as PROD. In here we also reference the previously defined cross-section namely the beamsection rod and the material steel . 6 Note, if you are using CBEAM or CBAR Elements the property CardImage would either be PBEAML or PBARL (L = as we are using the Library) Activate the Material tab next to Property, and assign the previously created material steel to this property collector.

8 Eventually, the property collector is created. Its card image (solver syntax) then reads as with PID = property collector ID, MID = material collector, A area and J torsional constant. In the general analysis work flow (not restricted to 1d elements ) the property collector must now be assigned to the Elements of interest. 7 To visually check the attributes of the 1d elements make sure that Shaded Elements and 3D Element Representation is activated. Then the CROD element will be displayed as: 8 CBAR Elements Simple beams with constant properties (symmetrical and constant cross-section), may be modeled with CBAR or CBEAM Elements . However, what you need to recall is that in CBAR Elements the origin of the element coordinate system is centered at the shear center of the cross-section (shear center and neutral axis coincide).

9 Any offsets between the neutral axis and the shear center is not accounted for. As a consequence, CBAR Elements are not useful for modeling beams that warp as it may be the case with open channel sections. This is because, the cross-sections of CBAR Elements remain plane. In other words, whenever you are modeling open or nonsymmetrical sections be especially cautious with the element type you are going to use. The following cross-sections are available: 9 10 The cross-sectional properties, shear flexibility factors, and stress recovery points (C, D, E, and F) are then computed using the TYPE and DIMi as shown below. As the CBAR and CBEAM Elements also account for bending the orientation of the element and thus of its cross-section becomes important (as it automatically defines the location of the pre-defined stress recovery points; see discussion below).

10 CBAR CBEAM 11 Meshing (building) of CBAR or CBEAM Elements is just a simple as with the creation of CROD Elements ( line mesh panel) This time the element config is set to bar2 (if you miss this step, you still can update the element configuration later). In order to define the cross-section we again make use of HyperBeam and the OptiStruct library. 1. Building (manually) CBAR / CBEAM Elements : Start the meshing process with Pull Down Menus Mesh Create 1d elements Bars The element type set to CBAR 2. Specify cross sectional properties (with HyperBeam) 12 As standard section library OptiStruct is used. Again, the cross section properties can be interactively changed in HyperBeam.