ansys - a simple deflection analysis

8/10/2019 ANSYS - A Simple Deflection Analysis

1/25


2/25

The building to analyse was generated in Rhino, and will need to be imported intoANSYS

The building is shaped like a stadium roof and is assumed to be fixed to supports alongboth the inner and outer edges

The question to answer is, if we imagine that the structure is made of an equal membersize throughout and then turn on the gravity, how far does the building move down?

Step 1: Export geometry

ANSYS really isnt a nice program for creating geometry in. It is much better if you cancreate the geometry in the form of a line drawing in a program you are more comfortablewith. In the image above, I was using Rhino.

For this analysis, it is possibly better to ensure that all the curves are actually straight lines.The easiest way to do this is to use the Rebuild command. Select the entire model, andrebuild with 2 points and 1 degree.

Pgina 2 de 39A simple deflection analysis in ANSYS 13 | James Ramsden

19/10/2013file:///C:/Users/Ignacio/Pictures/A%20simple%20deflection%20analysis%20in%20A...


3/25

There are two format choices for importing from Rhino to ANSYS:

SAT best for surfaces, but cant be used for lines or curves

IGES can handle both curves and surfaces

For this project, well be using IGES.

To export the project to IGES, select the model again. Then head to File > Export Selectedand save as an IGES file. Remember the folder you saved it in, and keep the file name shortand simple. Ive called mine roofmodel.




4/25

The next window presents IGES options. Just allow it to save with a default type, and clickOK.




5/25

Step 2: Load up the model in ANSYS

Load up ANSYS. You should see this splashscreen:

The splashscreen is the prettiest bit of the program youll ever see here is what will

actually appear when the program is loaded:




6/25

There are two windows the main user interface (right) and the DOS control (left). It seemsthat the user interface is built upon this DOS control all commands can be run throughDOS, and a lot of information and warnings will be outputted here. Even though youll be

working mostly in the user interface window, its good to keep the DOS window visible toone side.

And yes, the user interface is as terrible as it looks! The program does have a huge amountof power, but it does take some time to learn to navigate and to get it to do what you wantit to do.

First things first: before opening or working with a model, set up your job name andworking directory first! The working directory is a folder where the program will save allthe files related to your project. The program doesnt remember which you last used so

you need to set it every time you open ANSYS. Head to File > Change Directory and setyour folder here. Id recommend one folder per project.




7/25

Then, set your job name. Mine is called 3-1.

Then, I would recommend saving your model (yes, I know theres nothing in it yet!). Thefirst time you save, you must do a save as jobname.db!




8/25

If you then go back into Windows Explorer and look into your working directory, you

should see this:

Remember that we set the jobname to 3-1. So when we click save as jobname.db, we

actually created a file called 3-1.db.

Now, we can import our IGES model.




9/25

Just hit OK at the next window.

Ignore and close this error message that appears at the top-left. In fact, ANSYS really likesgenerating error messages, and most of them can be safely ignored.




10/25

Another window appears. Hit Browse to find the IGES file.

Then, hit OK to import the file.

We should then see our imported model!




11/25

Step 3: Set up model properties

At the moment, this is just a collection of lines pieces of XYZ data with no real-worldinformation on member sizes and forces. Firstly, well set up the members of the model.

In order to set up the member sizes, we tell ANSYS about the member properties we wouldlike to use, and then we apply these properties to the lines themselves.

Step 3a: Create an element type

In the menu on the left, select Preprocessor > Element Type > Add/Edit/Delete.




12/25

Then, click Add to create an element type.

There are many different kinds of element type, and the help file in ANSYS can help tochoose one. For our model, we want a beam.




13/25

Using 3 node instead of 2 node gives more accurate results in modelling at the expense ofprocessing speed. But the model isnt particularly enormous, so lets go for the 3 node(called Beam 189). Hit OK.

(Some other kinds of elements commonly used are Shell181 for shell structures and Link10for trusses.)

The element types window should contain the beam weve added. Click close to return to




14/25

the main screen.

ANSYS now knows that we have a type of member that we want to use that is a kind of3-node beam. However, we still need to add some real-world properties to this beam sothat it can be analysed.

Step 3b: Section sizes

Now, we need to apply some dimensions to our section sizes. Under preprocessor, this canbe done through either the Real Constants option or the Sections option. TheSections is a little more intuitive, so lets use this.

Head to the beam library on the left:

Under common sections, enter some beam properties. I want a circular section. ClickingCommon sections will bring up the beam tool.




15/25

ID is the ID of the element type we created in step 3a. If you have multiple element types(which will usually be the case in all but simple examples like this) then you should matchthe ID value here with the ID in the element type you want to modify.

(Help provides good information on what each of the fields mean these fields changedepending on the type of section selected.)

Press OK when finished.

Now we need to define what the section is made from useful for determining self-weight

and elastic behaviour of the structure.

Step 3c: Material properties

Head to Material Models. Properties we need for this model are density, elasticity andPoissons ratio. You can add other relevant material properties for other projects in thesame place.




16/25

For elasticity and Poissons ratio, head for Isotropic.

Bearing in mind the units of your model (N for force, kg for weight and whatever units forlength your Rhino model was in mine was in mm), enter the Youngs Modulus(200000N/mm2) and Poissons ratio (0.3 for steel):




17/25

Hit OK. Then find Density:

The density is 7850*10^-9 kg/mm3 (remember to consider the units again!).




18/25

Click OK.

On the previous screen again, you should notice that we now have two material modelsdefined:

This seems correct. Close the window.

Step 4: Create the mesh




19/25

Now we need to create the mesh that ANSYS will use to analyse the model. Select the MeshTool.

In the following window, ensure that Element Attributes is set either to Global (foreverything) or however you need it. Clicking Set should allow you to confirm that ourelement properties will be applied to the global model:




20/25

Click OK to return. Click Mesh.

The following window should appear. We want to mesh the entire model (i.e. we will beanalysing the whole model).

Select Pick All.




21/25

to this:

The model is now meshed.




22/25

Step 5: Set gravity

ANSYS does not assume there is a gravitational force, so we have to tell it about gravity.Head to the global gravity window.

Convention is that gravity acts in the opposite direction of the Z axis (i.e. gravity isnegative), but for some reason we get the correct result if we enter gravity as positive. Enter9.8 (i.e. gravitational force = 9.8m/s2).




23/25

Click OK.

Step 6: Apply boundary conditions

ANSYS treats boundary conditions (i.e. constraints) confusingly as a kind of load. Underthe menus, we find:




24/25

Lines, areas and keypoints are of the geometry we imported from Rhino, while nodes are ofthe mesh. It is better to assign constraints to the geometry, so that if we have to re-mesh, wedont lose our keypoints too. A keypoint in this context would mean a node of thegeometry. I want to restrain each point, so lets select Keypoints.

The following Apply window should appear:

If you need to reposition your model, click the pan zoom rotate button. This brings upanother window.




25/25

Back to the Apply window, pick on the points you want to constrain.


ansys - a simple deflection analysis

Documents