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ROBOT Tutorial - Simple Grid-Shell from DXF File  

This very basic tutorial will introduce you to the ROBOT interface along with a few basic model creation and analysis set-up operations. It is not intended to be an exhaustive review by any means and I will even go as far to say it is not important to satisfy ‘real’ structural criteria. Consider this a practice swing…By the end you will have run a structural analysis and will render the output results on the screen.

The tutorial is broken down into the following categories:

1. Import DXF wire-frame geometry
2. Assign structural section and material properties
3. Create supports
4. Apply loading
5. Run analysis and render results

‘Actions’ describe the specific commands you need to do and/or point your attention to something on the screen. In the narrative below, these are associated with a <#> that corresponds to a numbered arrow on the accompanying screen shot.

  A.      Import DXF wire-frame geometry 

To begin, download the sample wire-frame model called: ROBOT Example_Simple Grid-Shell.DXF

Open ROBOT Millennium. The first screen to appear is the ‘project’ screen which gives you the option to select different types of analyses. Don’t worry about what these analyses are. Simply select ‘New’ on the bottom right hand corner of the selection window <1>. Image A_1 

The main consol will appear with a light blue background. For general reference locate the following:  

• The top menu bar contains file functions, model creation and editing functions, etc under the pull down menus <2>
• On the top left you see a consol identifying all of the model components (nodes, bars, plates) by their reference numbers <3>
• On the bottom left there is a consol that effectively describes a component’s properties, i.e. material properties, section properties, length, location, connectivity, etc. If you select an object such as a bar or plate in the model its properties will be displayed here. <4>
• On the far right is a series of tools for defining the structural parameters of the analysis such as support conditions, material properties and defining the loading. <5> You will use this set of tools quite a bit.  

Image: A_2 

Before we import the model geometry, let’s check that the unit of scale is appropriate. <6> Look here to see what unit measure is currently defined. We need to change this to [mm] since the DXF file is in millimetres so simply double click on the [m] to open the ‘units’ dialogue box. Image: A_3 

After double-clicking on the [m] at the bottom right hand corner of the screen the ‘Job Preferences’ dialogue box will open. <7> Here you should modify the top two items and change to [mm] millimetres as seen in the next screen shot. Image A_4 

Now we can open the DXF wire-frame model. Select ‘Open’ from the ‘File’ pull-down menu which will bring up a dialogue box. <8>  Make sure to choose ‘DXF’ under the ‘files of type’ pull-down menu. <9> Select the sample file. Image A_5 

First the ‘Parameters of Loading DXF…’ dialogue box will appear. This allows you to specify how ROBOT should interpret the DXF geometry and is quite powerful. You won’t need to do anything here, just click OK. Image A_6   

Next a ‘Merge Structure’ dialogue box appears. Again, simply click OK. Image A_7   

The DXF wire-frame model has now been imported and appears on the screen like so. If you ‘right click’ the mouse the following menu choices appear. Select ‘Rotate 3D’ and spin around the model <10>. Image A_8   

Lastly, check that the model imported in the correct unit scale, i.e. ROBOT has the correct member lengths. You can pass the mouse over any one of the bar elements and it will ‘light up’. <11> on the bottom of the screen the length of the bar will appear. <12> Image A_9     

B.      Assign Structural Section and Material Properties  

You will notice that if you click on the ‘show extrusion’ icon at the bottom left of the screen nothing happens on the screen <13>. This is because the bar elements on the screen have not been defined as structural sections, i.e. pipe, wide flange, etc. On the right hand ‘tool menu’ select the icon that looks like an I-Section <14>. This will pull up the ‘Sections’ dialogue box. There will likely be a few pre-defined sections listed, but let’s create a new section by clicking the ‘new’ icon <15>. Image B_1   

This will open the ‘New Section’ dialogue box. Select the ‘Parametric’ tab along the top because we want to create a new tube section <16>. Select the icon that looks like a rectangular tube and you will see a picture describing the dimensional parameters you can define <17>. On the right hand side, enter the following parameters: b = 250mm, h = 1000mm & t = 25mm <18>. If you’re not familiar with metric units, this is the same as a rectangular tube that is 3ft deep, 10in wide and 1in thick. You will also notice the material type is most likely pre-selected as ‘Steel’ <19>. If it’s not, change it to steel. Finally, click on ‘Add’ to create the section and ‘Close’ to return to the previous dialogue box <20>. Image B_2   

Before we apply beam properties, make sure the ‘extrusion’ icon is selected <21>. This is not required but will help clarify which bar elements have a section property assigned to them. You will notice a new section in the ‘Sections’ dialogue box, the one we just created <22>. Make sure the section is selected as indicated with a little arrow symbol next to it. At this point, you can simply begin to select bar elements with a click of the mouse. Notice the bar elements turn into beam elements <23>.

As an alternative to assigning section properties to each bar individually, you may also select all the bars with the ‘ctrl-a’ keystroke. Notice all of the bars will highlight <24> and a list of selected bars will appear in the ‘Sections’ dialogue box <25>. Simply click ‘Apply’ to assign the selected section property to the highlighted bars, in this case the entire structure. Click ‘Close’ to exit the ‘Sections’ dialogue box. Image B_3   

All of the bar elements in the structure should have assigned section properties and will appear like this on the screen. Image B_4  

C.     Create supports  

Let’s assign supports or, in other words, define where the roof is supported. We also refer to this as defining ‘boundary conditions’. Before we begin, deselect the ‘extrusion’ icon <26> and select the ‘supports’ icon <27>. The structure will return to wire-frame view and now you will be able to visualize the supports as we assign them. Select the ‘Supports’ dialogue box from ‘toolbox’ along the right-hand side of the screen <28>. You will likely see some pre-set supports defined in the dialogue box, i.e. Fixed, Pinned, etc. Make sure that ‘Pinned’ is selected as indicated by an arrow symbol <29>. Image C_1 

For curiosity’s sake, double click on the ‘Pinned’ support definition <30>. This will open the ‘Support Definition’ dialogue box where you can, if needed, create new support types. From here <31> you can name the support type. From here, <32> three degrees of translation are defined, i.e. movement in the X direction (UX), Y direction (UY) and Z direction (UZ). In this instance, all three translation directions are check marked which means they are fixed. In other words, a true ‘pin’ does not allow translation. From here, <33> three degrees of rotation are defined. In this instance, none are selected meaning rotation at the support is not restricted or constrained. Imagine a see-saw that can ‘hinge’ at the center but is held in place. If you modify these properties and select ‘Add’ <34>, a new support type will be created. At this time simply click ‘Close’ to return to the ‘Supports’ dialogue box. Image C_2   

Double check ‘Pinned’ is selected in the ‘Supports’ dialogue box <35>. Now click on the four nodes to apply a pin support <36> <37> <38> <39> and click ‘Close’ to exit the ‘Supports’ dialogue box. Image C_3

D.     Apply Loading

Before we can apply loading we need to define ‘Load Types’ to represent the different ways in which load is applied to a building. For instance, dead load (DL) typically refers to the weight of the structure and finishes. Live Load (LL) typically refers to the occupant’s weight and/or furnishings like desks, filling cabinets, water coolers, etc. Select the ‘Load Types’ dialogue box from the ‘toolbox’ along the right-hand side of the screen <40>. Image D_1

Let’s define the first ‘load type’ as self-weight (SW). This is simply the weight of the structural elements. Enter “SW” in the ‘Label’ field <41>, “SW” in the ‘Name’ field <42> and click ‘New’ <43>. You will see a new load type appear in the ‘List of Defined Cases’ <44>. You will also see this ‘load type’ appear in the top menu bar <45> and all of the bars will have a red double line <46>. With the pull-down menu at the top of the screen <45> you can scroll between ‘load types’ and ‘combinations’ of load types once you have defined these. The reason the bars have turned red is because the first load type that you define automatically includes self weight for all structural elements. You do not need to apply self weight to the elements individually as you did with section properties. Lastly, you will notice the ‘display load’ icon is selected at the bottom right <47>. If you deselect and reselect this icon, the red lines will turn off and on. Image D_2 

Let’s create (2) more load types: Dead Load (DL) and Live Load (LL). Do this by following the same procedure as with the self weight example only this time entering “DL” into the ‘Label’ field <48> and “DL” into the ‘Name’ field <49> and clicking ‘New’ then repeating with “LL”. When complete click ‘Close’ to exit the ‘Load Types’ dialogue box. Image D_3   

Now we need to apply loads to the structure. We want to apply ‘dead loading’ first so start by selecting ‘DL’ from the pull-down menu at the top of the screen <50>. The load values that we define here are not important. They are place holders. For the purposes of this tutorial, it is important simply to learn how to apply loads at this stage. Now, click on the ‘Load Definition’ icon in the ‘toolbox’ at the right-hand side of the screen <51>. Image D_4  

Notice ‘DL’ is selected at the top of the screen <52>. Now click on the far left icon under the ‘Node’ tab in the ‘Load Definition’ dialogue box <53>. The ‘Nodal Force’ dialogue box will open. You will notice a series of fields. On the far left-hand side are X, Y and Z. These are the global coordinate axes. You can see their direction here <54>. The first column ‘F’ describes an applied force, the second column ‘M’ describes an applied moment and the third column allows you to specify an angle of rotation of the applied load relative to the global coordinate system. Let’s apply a 100kN force in the negative or downward ‘Z’ direction. Enter ‘-100’ in this field <55> and click ‘Add’<56>. Image D_5  

The ‘Nodal Force’ dialogue box will close. Click on the nodes indicated in the screen shot below <57>. An arrow pointing in the negative ‘Z’ direction will appear above the nodes you select. If an arrow does not appear, ensure the ‘display load’ icon <58> is selected at the bottom of the screen. If this still doesn’t work, repeat from step <53>. Sometimes ROBOT will terminate a command before you have completed it...Image D_6

If you click on the ‘display load value’ icon at the bottom of the screen <59> the value of the applied dead load will appear next to the arrows <60>. Image D_7  

Let’s add a ‘Live Load’ to the structure. Start by selecting “LL” from the pull down menu at the top of the screen <61>. Next select the ‘Nodal Load’ icon under the ‘Node’ tab of the ‘Load Definition’ dialogue box <62>. Let’s add 250kN in the downward ‘Z’ direction. Enter “-250” here <63> and click ‘Add’ <64>. Image D_8  

The ‘Nodal Force’ dialogue box will close. Click on the nodes indicated in the screen shot below <65>. An arrow pointing in the negative ‘Z’ direction will appear above the nodes you select. If an arrow does not appear, ensure the ‘display load’ icon <66> is selected at the bottom of the screen. If this still doesn’t work, as before, repeat from step <61>. Image D_9 

Now let’s define ‘Combinations’ of loads. This is useful when we want to combine dead load with live load before evaluating structural performance. First, click ‘Close’ on the ‘Load Definition’ dialogue box. Now, under the ‘Load’ pull down menu at the top of the screen <67>, select ‘Combinations’. Image D_10  

The ‘Combination Definition…’ dialogue box will appear. Enter “DL” in the ‘Combination Name’ field <68> and click ‘OK’. Image D_11    

The ‘Combinations’ dialogue box will appear. Notice the name of the combination is ‘DL’ <69>. You can use this pull-down menu to switch between combinations in order to preview or edit them. Notice the ‘load types’ that we previously defined appear in the ‘Case List’ field <70> and an arrow appears next to ‘SW’ indicating this case is selected. Enter a value of ‘1’ in the ‘Factor’ field <71> and click the single arrow <72> to move this ‘load type’ into the ‘List of cases in combination’ field. Image D_12  

Next, select ‘DL’ in the ‘Case List’ and repeat steps <71> and <72>. You should now see both ‘SW’ and ‘DL’ load cases in the ‘List of cases in combination’ field <73>. Click ‘Apply’ to save your changes. Image D_13 

The previously created combination was simply combining the Dead Loads. Now let’s create a DL+LL combination. <74> click ‘New’ on the ‘Combinations’ dialogue box to open the ‘Combination Definition…’ dialogue box. <75> enter “DL+LL” in the ‘Combination name’ field and click ‘OK’ Image D_14    

Notice the new combination ‘DL+LL’ appears in the ‘Combination’ field <76>. For load cases ‘SW’, ‘DL’ and ‘LL’ repeat steps <71> and <72> such that they appear in the ‘List of cases in combination’ field per the screen shot below <77>. IMPORTANT: Notice load case no. 4 was not selected for this combination <78>. That is because load case no. 4 is actually the ‘DL’ COMBINATION we previously created. On the contrary, load cases no. 1, no. 2 and no. 3 are LOAD TYPES representing independent load definitions. Make sure to pay close attention to the case numbers to avoid doubling up loading. Click ‘Close’ on the ‘Combinations’ dialogue box to exit. Image D_15   

E.      Run analysis and Render Results  

If you have not saved the model do so now <79>. To run the analysis, click the ‘Calculate’ icon at the top on the screen <80>. After a few seconds the analysis will complete and ‘Results (FEM): available’ will be displayed at the top of the screen <81>. We are now ready to view the analysis results. Image E_1  

Under the ‘Results’ menu at the top of the screen select ‘Diagrams for Bars’<81>. This will open the ‘Diagrams’ dialogue box. Image E_2  

To begin with, select the ‘Parameters’ tab at the top of the dialogue box <82>. This menu allows you to define how the information will be displayed on the screen. Check the ‘Differentiated’ button and the ‘Filled’ button <83>. These are good settings for displaying axial and moment force diagrams. Image E_3 

Now click the ‘NTM’ tab at the top of the ‘Diagrams’ dialogue box <84>. Here you can select the type of force or moment diagram that you would like to display. Select ‘My Moment’ <85> and then click ‘Apply’. You will notice the moment diagram appears on the structure <86>. At the bottom right-hand corner of the screen you will see a description of the maximum and minimum ‘My Moments’ within the structure <87> and a measure of scale of the graphical diagrams. You will also see the specific ‘Load Case(s)’ that are represented by the structural diagram <88>. if you switch between load cases in the pull-down menu at the top of the screen <89>, the structural diagrams will update to reflect the ‘Load Type’ or ‘Combination’ you are interested in. Image E_4   

Deselect the ‘My Moment’ button <90> and select the ‘Fx Force’ button <91>. Click ‘Apply’. The axial force diagram will appear on the structure. Deselect the ‘Fx Force’ button <91> and click ‘Apply’ before preceding to the next step. Image E_5  

The last thing we will look at is the deflected shape. Select the ‘Deformation’ tab at the top of the ‘Diagrams’ dialogue box <92>. Select ‘Exact Deformation’ <93> and click ‘Apply’ to show the deflected shape. At the bottom right-hand corner of the screen the maximum displacement within the structure is given <94>. Image E_6