WS14b-1 WORKSHOP 14B MODAL ANALYSIS OF A TOWER WITH SOFT GROUND CONNECTION NAS122, Workshop 14b, August 2005 Copyright 2005 MSC.Software Corporation

WS14b-2 NAS122, Workshop 14b, August 2005 Copyright 2005 MSC.Software Corporation

WS14b-3 NAS122, Workshop 14b, August 2005 Copyright 2005 MSC.Software Corporation n Problem Description u In this workshop, the tower structure model is modified to simulate the ground at the base of the tower. A set of 4 CBUSH elements are connected at the bottom of the tower and a spring stiffness of 1E8 and 1E7 are used for the translational spring stiffness and torsional spring stiffness, respectively. u Perform a normal modes analysis of this model and compare the results to those of Workshop 14a to see the difference between a rigid ground and a soft ground connection.

WS14b-4 NAS122, Workshop 14b, August 2005 Copyright 2005 MSC.Software Corporation n Suggested Exercise Steps 1. Import the finite element model of the tower from a Nastran Input File called enf111.bdf. 2. Create 4 nodes offset from the bottom corners of the towers. 3. Modify the RBE 2 so it is connected to the nodes created in Step Create bar elements between the new nodes and the bottom of the tower. 5. Assign CBUSH properties to the bar elements created in Step Submit the model to Nastran for a Normal Modes analysis. 7. Attach the XDB result file after the analysis is completed. 8. Use the Quick Plot feature in the Result Menu to visualize the Eigenvector of the model.

WS14b-5 NAS122, Workshop 14b, August 2005 Copyright 2005 MSC.Software Corporation CREATE NEW DATABASE Create a new database called tower_soft_ground.db. a.File / New. b.Enter tower_soft_ground as the file name. c.Click OK. d.Choose Default Tolerance. e.Select MSC.Nastran as the Analysis Code. f.Select Structural as the Analysis Type. g.Click OK. a b c d e f g

WS14b-6 NAS122, Workshop 14b, August 2005 Copyright 2005 MSC.Software Corporation Step 1. File / Import Import the model from a MSC.Nastran input file called enf111.bdf. a.File / Import. b.Select MSC.Nastran Input as the Source. c.Select enf111. bdf and click Apply. d.Click OK when the Nastran Input File Import Summary appears. b c a c d

WS14b-7 NAS122, Workshop 14b, August 2005 Copyright 2005 MSC.Software Corporation Step 1. File / Import (Cont.)

WS14b-8 NAS122, Workshop 14b, August 2005 Copyright 2005 MSC.Software Corporation Step 2. Elements: Transform / Node / Translate Create 4 nodes at the bottom of the tower. a.First, change the view angle and zoom into the bottom of the tower. b.Elements: Transform / Node / Translate. c.Enter for the Direction Vector. d.Enter Repeat Count of 1. e.Make sure Delete Original Nodes option is unchecked. f.Select the 4 nodes at the bottom of the tower. g.Click Apply. h.Click on Node Size to view the nodes. b c d e g h f

WS14b-9 NAS122, Workshop 14b, August 2005 Copyright 2005 MSC.Software Corporation Step 3. Elements: Modify / MPC Modify the RBE2 to connect to the newly created nodes. a.Elements: Modify / MPC. b.Select the RBE 2 in the model. c.Click on Modify Terms. d.Click on the first cell of the Dependent Terms table. e.Check the Modify option. f.Select the 4 nodes created in the previous step. g.Click Apply. h.Click Cancel. i.Click Apply. b c d e g a f h i

WS14b-10 NAS122, Workshop 14b, August 2005 Copyright 2005 MSC.Software Corporation Step 3. Elements: Modify / MPC (Cont.)

WS14b-11 NAS122, Workshop 14b, August 2005 Copyright 2005 MSC.Software Corporation Step 4. Elements: Create / Element / Edit Create 4 bar elements between the bottom of the tower and the dependent nodes of the RBE2. a.Elements: Create / Element / Edit. b.Change the Shape to Bar and Topology to Bar2. c.Uncheck Use existing midnodes and Auto Execute. d.Select a node from the bottom of the tower. e.Select the node directly below the node selected in Step D. f.Click Apply. g.Repeat the procedures for the remaining three corners. b c d e a f

WS14b-12 NAS122, Workshop 14b, August 2005 Copyright 2005 MSC.Software Corporation Step 5. Properties: Create / 1D / Bush Assign bush properties to the bar elements created in the previous Step. a.Properties: Create / 1D / Bush. b.Enter ground for the Property Set Name. c.Click Input Properties. d.Enter as the Bush Orientation. e.Enter 1e8 for the Spring Constant 1 through 3 and 1e7 for Spring Constant 4 through 6. f.Click OK. g.Click in the Select Members box. h.Use the Selection preference to beam elements. Select the bar elements created in previous step. i.Click Add and Click Apply. a b c d e f g i h i

WS14b-13 NAS122, Workshop 14b, August 2005 Copyright 2005 MSC.Software Corporation Step 6. Analysis: Analyze / Entire Model / Full Run Submit the model for analysis. a.Analysis: Analyze / Entire Model / Full Run. b.Enter tower_nm_ground as the Job Name. c.Click on Solution Type. d.Select NORMAL MODES. e.Click OK. a c d e b

WS14b-14 NAS122, Workshop 14b, August 2005 Copyright 2005 MSC.Software Corporation Step 6. Analysis: Analyze / Entire Model / Full Run (Cont.) Submit the model for analysis (cont.). a.Click on Subcases. b.Select UNTITLED.SC1. c.Click on Subcase Parameters. d.Enter 0.01 for the Lower Frequency Range of Interest. e.Click OK. f.Click Apply. g.Click Cancel to exit the Subcases form. a c d e b f g

WS14b-15 NAS122, Workshop 14b, August 2005 Copyright 2005 MSC.Software Corporation Step 6. Analysis: Analyze / Entire Model / Full Run (Cont.) Submit the model for analysis (cont.). a.Click on Subcase Select… b.Select UNTITLED.SC1. in the Subcases For Solution Sequence and Unselect the Default load case. c.Click OK. d.Click Apply. b a c d

WS14b-16 NAS122, Workshop 14b, August 2005 Copyright 2005 MSC.Software Corporation Step 7. Analysis: Access Results / Attach XDB / Result Entities Attach the XDB result file. a.Analysis: Access Results / Attach XDB / Result Entities. b.Click on Select Results File. c.Select tower_nm_ground.xdb. d.Click OK. e.Click Apply. a b c d e

WS14b-17 NAS122, Workshop 14b, August 2005 Copyright 2005 MSC.Software Corporation Step 8. Results: Create / Quick Plot Create a Deformation and Fringe plot of the normal modes. a.Results: Create / Quick Plot. b.Select the Mode 1. c.Select Eigenvector, Translational in both Result boxes. d.Click Apply. e.Repeat the same procedures for the remaining 9 normal modes. a b c d c

WS14b-18 NAS122, Workshop 14b, August 2005 Copyright 2005 MSC.Software Corporation Summary Summary of Frequencies and Modes for project _______________ ModeFreq (Hz) Description