5. Beam Modeling. They are computationally more efficient than solids and. shells and are heavily used in several industries:

Transcription

1 14. Beam Modeling

2 5. Beam Modeling Beam elements are line elements used to create a onedimensional idealization of a 3-D structure. They are computationally more efficient than solids and shells and are heavily used in several industries: Building construction Bridges and roadways People movers (trams, railcars, buses) Etc.

3 ...Beam Modeling In this chapter, we will present a brief introduction to beam modeling via the following topics: A. Beam Properties B. Beam Meshing C. Loading, Solution, Results D. Workshop

4 A. Beam Properties The first step in beam modeling, as with any analysis, is to create the geometry usually just a framework of keypoints and lines. Then define the following beam properties: Element type Cross section Material

5 ...Beam Properties Element Type Choose one of the following types: BEAM188 3-D, linear (2-node) BEAM189 3-D, quadratic (3-node) ANSYS has many other beam elements, but BEAM188 & 189 are generally recommended. Applicable to most beam structures Support linear as well as nonlinear analyses, including plasticity, large deformation, and nonlinear collapse Easy to use, both in preprocessing and postprocessing phases

6 ...Beam Properties Cross Section To completely define a BEAM188 or 189 element, you also need to specify its cross section properties. The BeamTool provides a convenient way to do this. Preprocessor > Sections > Common Sectns... Select the desired shape, then enter its dimensions. Press the Preview button to view the shape, then OK to accept it. If there are multiple cross sections, specify a different section ID number (and an optional name) for each.

7 ...Beam Properties A sample preview (SECPLOT) of an I-beam cross section is shown below. In addition to the predefined cross-section shapes, ANSYS allows you tocreate your own, user-defined shape by building a 2-D solid model. You can save user-defined sections as well as standard sections with the desired dimensions in a section library for later use. See Chapter 15 of the ANSYS Structural Analysis Guide for more information.

8 ...Beam Properties Material Properties Both linear and nonlinear material properties are allowed. After all beam properties are defined, the next step is to mesh the geometry with beam elements.

9 B. Beam Meshing Meshing the geometry (lines) with beam elements involves three main steps: Assign line attributes Specify line divisions Generate the mesh The MeshTool provides a convenient way to perform all three steps.

10 ...Beam Meshing Step 1: Line Attributes Line attributes for beam meshing consist of: Material number Section ID Orientation keypoint Determines how the cross section is oriented with respect to the beam axis. Must be specified for all cross-section types. A single keypoint can be assigned to multiple lines (i.e, no need to specify a separate keypoint for each line). Each end of a line can have its own orientation keypoint, allowing the cross section to be twisted about the beam axis.

11 ...Beam Meshing Examples of using orientation keypoints:

12 ...Beam Meshing To assign line attributes, use the Element Attributes section of the MeshTool (or select desired lines and use the LATT command). Pick lines Additional attributes for BEAM188 & 189

13 ...Beam Meshing Step 2: Line Divisions For BEAM188 and 189 elements, a single element spanning the entire beam length is not recommended. Use the Size Controls section of the MeshTool (or the LESIZE command) to specify the desired number of line divisions.

14 ...Beam Meshing Step 3: Generate the Mesh First save the database (Toolbar > SAVE_DB or SAVE command). Then press the Mesh button in the MeshTool (or issue LMESH,ALL) to generate the mesh. Pick lines

15 ...Beam Meshing To see the cross-section shape in the element display, activate the element shape key: Utility Menu > PlotCtrls > Style > Size and Shape Or /ESHAPE,1

16 ...Beam Meshing After beam meshing is completed, the next step is to apply loads and solve.

17 C. Loading, Solution, Results Typical loading for beam models consists of: Displacement constraints applied at keypoints or nodes Forces applied at keypoints or nodes Pressures load per unit length applied on element faces Solution > Apply > Pressures > On Beams Or SFBEAM command Gravity or rotational velocity acts on entire structure

18 ...Loading, Solution, Results To obtain the solution: First save the database. Then solve. (Or write the loads to a load step file and solve all load steps later.) Results review is the same as for other stress analyses: View the deformed shape Check reaction forces Plot stresses and strains The main advantage of BEAM188 and 189 is that with the element shape key activated (/ESHAPE,1), stresses can be directly viewed on the elements (similar to solids and shells).

19 ...Loading, Solution, Results Demo: Resume frame.db (contains lines, kp s, loading, element type, material, and two cross sections) Plot the two cross section already defined (SECPLOT,1 & 2) Define a third cross section using the BeamTool: ID=3: Name = peak, Sub-type = box (hollow rectangle), W1=6, W2=6; T1=T2=T3=T4=0.25 Bring up MeshTool, GPLOT, then assign the following line attributes: Sloping lines: mat=1, secnum=3, orientation KP = topmost KP (#100) Left vertical lines: mat=1, secnum=2, orientation KP = #102 Right vertical lines: mat=1, secnum=2, orientation KP = #101 Left & front horizontal lines: mat=1, secnum=1, orientation KP = #1 Right & back horizontal lines: mat=1, secnum=1, orientation KP = #3 Specify size=20 on all lines Save, then LMESH,ALL; then EPLOT with /ESHAPE,1 Solve, then review results: deformed shape (animate), reaction forces, SX stresses (= axial + bending). Select elements with section ID=3 and replot stresses. Repeat for ID=2.

20 D. Workshop This workshop consists of the following problem: W4. Building Frame Please refer to your Workshop Supplement for instructions.