Truss Bracket

Problem:

The truss structure shown above is mounted on the sides of buildings during construc-tion for use as scaffolding for workers. A design team has created a new bracket design(shown below) to use with the truss structure and want to know if the new bracketdesign is safe. The material for the bracket and the connecting pin is steel with a yieldstrength of 35 kpsi. The loads on the pin are know to be 14,825 lb in the minus X direc-tion and 4856 lb in the Y direction. Determine if the new bracket design is safe usingyielding as the failure criteria.

X

Y

Truss BracketOverview

Outcomes1) Explore possibilities with the graphical user interface (GUI)2) Learn how to create and mesh a complex geometries3) Increase efficiency in problem set up and solving speed

Tutorial OverviewThis tutorial is divided into four parts:

1) Tutorial Basics2) Preprocessing3) Solution4) Post Processing

Anticipated time to complete this tutorial: 1 hour

AudienceThis tutorial assumes familiarity of ANSYS 8.0; therefore, it does not go into step by stepdetail.

Prerequisites1) ANSYS 8.0 in house “Structural Tutorial”2) Completion of all Basic Machine Design Tutorials3) Completion of the 3D Knuckle Pin Joint Tutorial

Objectives1) Create a solid model representation of the bracket2) Create contact elements to model the interaction between the pin and the bracket3) Determine if the bracket is a safe design for the given loads.

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Truss BracketTutorial Basics

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In this tutorial:Instructions appear on the left.

Visual aids corresponding to the textappear on the right.

All commands on the toolbars arelabeled. However, only operationsapplicable to the tutorial are explained.

The instructions should be used as follows:

Bold > Text in bold are buttons, options, or selections that the user needs to click on

Example: > Preprocessor > Element Type > Add/Edit/DeleteFile would mean to follow the options as shown to the right to get you to the Element Types window

Italics Text in italics are hints and notes

MB1 Click on the left mouse buttonMB2 Click on the middle mouse

buttonMB3 Click on the right mouse

button

Some basic ANSYS functions are:

To rotate the models use Ctrl and MB3.

To zoom use Ctrl and MB2 and move themouse up and down.

To translate the models use Ctrl and MB1.

Truss BracketProblem Planning

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A log file approach would be an excellent choice for solving this problem. The instruc-tions given in this tutorial follow this approach.

You will begin by creating log file commands to define the material properties, the geom-etry, and the mesh. You will then paste those commands into the ANSYS command lineand build the model up to that point.

Then you will use the contact wizard in the GUI to define contact surfaces between thepin and the hole. You will open the session editor and copy the commands that ANSYSused while creating the contacts and paste it into your log file.

You will finish the tutorial by writing solution and post processing log file commands.

The end result will be a log file useful for solving the entire problem.

As you work through this tutorial, some new ANSYS features will be explained and thecommand lines for the log file will be given to you. Other log file commands you haveused in other tutorials and will not be given to you. The 3D Knuckle Pin Joint uses manyof the commands that you will need to solve this problem as well. It is recommendedthat you complete the 3D Knuckle Pin Joint tutorial before beginning this one. Drawingsof the truss bracket are given below and on the next page. All dimensions are given ininches.

Truss BracketProblem Planning

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Truss BracketLog File

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1) Make the first two lines of the log fileinstructions for ANSYS to clear whateverdatabase it is working on and start a newone.

FINISH/CLEAR,NOSTART

2) Add commands to add a file name andtitle.

/FILNAME,Truss Bracket/TITLE,Truss Bracket Test for Failure

3) One thing nice about a log file is that youcan create prompts to allow the user to enterinformation. This is done through the *ASKcommand. Information on how to use the*ASK command is found in ANSYS help.

For this problem the user will be promptedfor the X and Y loads applied to the pin.Add the two lines of command as shown.The user inputted values will be stored inUFX, and UFY respectively.

*ASK,UFX,'ENTER THE VALUE OF FX (LB),0*ASK,UFY,'ENTER THE VALUE OF FY (LB),0

4) Type the command to enter the pre-processor.

/PREP7

5) Define the beam element for the model.You are going to use a solid95 element forthis model.

ET,1,SOLID95

*ASK, Par, Query, DVALPrompts the user to input a parameter value.

ParAn alphanumeric name used to identify the scalarparameter.

QueryText string to be displayed on the next line as thequery (32 characters maximum). Characters hav-ing special meaning (such as $ ! ,) should not beincluded.

DVALDefault value assigned to the parameter if theuser issues a blank response.

Truss BracketLog File

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6) Add the commands to define the materialproperties. The MP command to add amaterial model has been used in other tuto-rials. Additional properties that can beadded for a material will now be shown.For this model we will add the density anddefine data points to map the stress-straincurve for the material. All of the commandsneeded are briefly explained to the right. SeeANSYS help for additional information.

For this model a custom stress strain curvewill be added. This curve dictates that thematerial will start to yield at stresses of 35kpsi.

Once you understand how these commandsare used, add the following to your log file.

MP,EX,1,30e6 MP,PRXY,1,0.3 MP,DENS,1,0.283/386 TB,MISO,1,1,6, TBTEMP,0TBPT,,.0015,45000 TBPT,,.0133,47000 TBPT,,.0266,54000 TBPT,,.05,62000 TBPT,,.1,70000 TBPT,,.15,73000

After you have entered the commandsabove you can run your log file up to thispoint and then view a plot of the materialsstress-strain curve by going to

> Preprocessor > Material Props > Material Models > Material Model Number 1 > Multilinear Isotropic >Then click on the graph button

MP, Lab, MAT, C0, C1, C2, C3, C4Defines a linear material property as a constantor a function of temperature.

LabValid property label. (note only the applicableproperty labels are shown below).

EX -- Elastic moduli (also EY, EZ).

PRXY -- Major Poisson's ratios (also PRYZ, PRXZ).

TB, Lab, MAT, NTEMP, NPTS, TBOPT

LabValid property label. (note only the applicableproperty label is shown below).

MISO -- Multilinear isotropic hardening using von Misesor Hill plasticity

MATMaterial reference number

NTEMPThe number of temperatures for which data willbe provided

NPTSNumber of data points to be specified for agiven temperature.

TBTEMP, TEMP, KMOD

TEMPTemperature value (defaults to 0.0 if KMOD isblank).

TBPT, Oper, X, Y

Oper - Leave blank for this case

X value of the point (strain, or H, or closurevalue).

YThe corresponding Y value of the point (stress,or B, or pressure value).

Truss BracketLog File

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7) Create the geometry for the bracket andthe pin. There are several ways to do this.The challenge of this tutorial will be to buildthe geometry in an efficient and accuratemanner. The geometry can be built usingcommands that have been demonstrated inother tutorials.

The following commands may be helpful:

SET - define scalar parametersBLOCK - create solid blocksCYL4 - create cylindersVSBV - subtract volumesVGLUE - glue volumes together

For example, you could first build the twoplates using the BLOCK command.

Second, you could build a solid cylinder anduse the VSBV command to subtract it fromthe plate to create the hole. Then createanother cylinder in the same place the sizeof the pin. Finally, use the VGLUE com-mand to glue the appropriate geometryparts together. (Don’t glue the pin and theplate together.)

Note: In order to know the appropriate volume num-bers to use in this command, you may have to runyour log file up to this point, plot the volumes, andturn on the volume numbers.

Truss BracketLog File

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8) Set the mesh size for different parts of themodel. Use the LESIZE and AESIZE com-mands to set the number of divisions or ele-ment size for the lines and areas indicated.

Note: In order to know the appropriate line and areanumbers to use in these commands, you may have torun your log file up to this point, plot the line andareas, and turn on the line and area numbers.

The numbers below refer to the number ofdivisions for the lines indicated and the ele-ment size for the areas indicated. These arethe mesh sizes used in the creation of thetutorial. Feel free to mesh the model in adifferent manner; however, note that toocoarse of mesh will not give the correctresult and too dense of a mesh will significantly increase the time the computerneeds to solve the model.

Examples:

LESIZE,60, , ,15, , , , ,1AESIZE,17,0.1, Line, 10

Line, 10

Line, 15

Line, 15

Line, 3

Line, 15

Line, 15

Line, 7

Area, 0.1(outer surface ofhead of pin)

Truss BracketLog File

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9) Mesh the model using the volume sweepcommand.

VSWEEP,ALL

10) Add constraints to the bracket and thepin. This can be accomplished using theASEL,NSLA, and D commands.

ASEL - select areasNSLA - select nodes associated with selectedareasD - constrain degrees of freedom

Constrain the back of the truss in all degreesof freedom and constrain the pin so that itcannot slide out of the bracket.

Example code:

Back of Truss

ASEL,S,AREA,,6NSLA,SD,ALL,UX,0D,ALL,UY,0D,ALL,UZ,0

Pin

ASEL,S,AREA,,17NSLA,SD,ALL,UZ,0

Note: The area numbers given in the code may not bethe same as the ones in your model. Verify that youhave entered the right area number in your code.

Truss BracketLog File

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11) Apply the forces to the ends of the pin.Use the ASEL, NSLA, commands to selectthe two areas that make up the ends of thepin. Divide the force equally among thenodes on ends of the pin.

ASEL,S,AREA,,17NSLA,SASEL,A,AREA,,21NSLA,A

Note: The area numbers given in the code may not bethe same as the ones in your model. Verify that youhave entered the right area number in your code.

Run your model up to this point. When youuse the ASEL, and NSLA commands ANSYSwill count how many nodes you have select-ed and display that number in the outputwindow. Look in the output window to seehow many nodes were selected. If you can’tsee that number copy and paste the abovecommands into the command prompt in theGUI and press enter. Check the output win-dow again. Make sure that you have theright area numbers.

Based on the mesh used in the tutorial, therewere 204 nodes; 102 nodes on each side ofthe pin. Apply the force evenly over theseselected nodes. Remember that UFX, andUFY are variables that contain the userinputted values for the FX and FYforces on the pin.

F,ALL,FX,UFX/204F,ALL,FY,UFY/204

12) Select all the nodes again.

ALLSEL,ALL

Truss BracketLog File

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13) You will now switch to the GUI and usethe contact wizard to generate contact ele-ments between the bracket and the pin.Once you have created the contacts, use thesession editor to paste the command linesinto your log file. The steps shown in the 3DKnuckle Pin Joint explain the process forcreating the contacts. Use the followinginformation as you create the contacts usingthe contact wizard.

Use inner surface of the hole in the bracketas the target area.

Target Surface = AreasTarget Type = Flexible

Use the outer surface of the hole (that makescontact with the bracket) as the contact sur-face

Contact Surface = AreasContact Element Type = Surface-to-

Surface

Unselect the option to include initial pene-tration.

Select the option to create a symmetric pair.Enter a coefficient of friction of 0.2.Leave the other setting as the defaults.

Once you have created the contact pair,open the session editor in the ANSYS mainmenu and copy all the commands used tocreate the contact pair. Paste them into yourlog file.

14) Add a command to reselect all the nodes.

ALLSEL,ALL

Truss BracketLog File

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15) When the contact pair was created, coin-cident nodes were created at the interface ofthe pin and the bracket. Some unneededdegrees of freedom in these nodes can beremoved by defining coupled degrees offreedom between these coincident nodes(within a tolerance), which will decrease thesolution time. Add the following command.

CPINTF,ALL,0.0001

16) Add commands to setup the solutionand then solve the model.

FINISH

/SOLANTYPE,0NLGEOM,1NSUBST,10,1000,1AUTOTS,1SOLVEFINISH

FINISHExits normally from a processor.

/SOLEnters the solution processor.

ANTYPE, AntypeSpecifies the analysis type and restart status.

STATIC or 0 -- Perform a static analysis. Valid for all degrees offreedom

NLGEOM, KeyIncludes large-deflection effects in a static or fulltransient analysis.

ON -- 1Large-deflection effects are to be included

NSUBST, NSBSTP, NSBMX, NSBMNSpecifies the number of substeps to be taken thisload step

NSBSTPNumber of substeps to be used for this load step

NSBMXMaximum number of substeps to be taken

NSBMNMinimum number of substeps to be taken

AUTOTS, KeySpecifies whether to use automatic time steppingor load stepping

ON -- 1Use automatic time stepping

SOLVEStarts a solution

Truss BracketLog File

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17) Add commands to enter the post proces-sor and to view the stresses. Review thecommand explanations to the right and thenadd the following code:

/POST1AVPRIN,0, , PLNSOL,S,EQV,0,1/REPLOT

18) Run your log file and view the results.When you run the log file you should gettwo user prompts asking for the FX and FYloads applied to the pin. Enter the followingvalues in these prompts:

For FX enter -14825 For FY enter 4856

Note depending on the speed of your com-puter it may take ten minutes or more forthe solution to solve.

/POST1Enters the database results postprocessor.

AVPRIN, KEY, EFFNUSpecifies how principal and vector sums are to becalculated

KEYAveraging key:

0 -- Average the component values from the elementsat a common node

PLNSOL, Item, Comp, KUND, FactDisplays results as continuous contours

Item - SComponent stress

Comp - EQVEquivalent stress

KUND - 0 Do not overlay undeformed structure display

Fact - 1 (default)Scale factor for 2-D display for contact items.

/REPLOT, LabelAutomatically reissues the last display commandfor convenience

Truss BracketPost Processing

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19) If everything was done correctly youshould see a stress plot of the bracket andthe pin similar to the one shown. Look atthe stresses around the interface of the pinand the bracket.

Under these loads thestress in the bracketexceeds the yieldstrength (35kpsi) andthus failure can occur.The bracket is not asafe design for the loads given.

Try running the solution under differentloads.