5. Structure model
A. Creating and saving a model
In this example we will use the most general mode of creating a 3D model. We start the new project by selecting the command File → New → 3D from the main menu.
To save the newly created project using a name of your choice, select the command File → Save in the main menu or the icon in the toolbar. Enter the name you want to use in the file dialog that will appear. This new file uses the T3D extension by default.
B. Setting up the working plane
The global system of Coordinates is indicated in the workspace and the vertical axis is Y.
We will begin the example model as a single transverse frame in the XY plane. To facilitate the visualization of the frame and the entering of node coordinates, it is useful to set up a 2D working plane.
The XY working plane can be set by using the “2D planes” icon. In the “Working planes” dialog that appears, select the XY plane by clicking on the row header button. At any time we can revert to a 3D display by using the “Switch to 3D” button in this dialog.
C. Definition of Nodes
Before entering nodes, please make sure that the currently selected unit for length is meters by selecting the icon. Also, ensure that the application is in Topology mode by selecting the icon. To define the nodes for the structure frame, select the “New nodes” icon. Nodes can be defined using the mouse, helped by the “Snap to grid” option (), or by entering the node coordinates in the nodes list table. To use the second method, click on the “List” icon. The nodes list dialog is displayed and the X,Y and Z coordinates for a new node can be entered by pressing the down arrow key on the keyboard.
After adding the five nodes as shown above, press the OK button to close the dialog. You can use the “Zoom extents” icon to display a total view of the model. The program workspace should look like the following image.
D. Definition of Beams
Ensure that the application is in Topology mode by selecting the icon. In order to define beams, click on the “New beams” icon, after which you should select the option “Snap/select node” icon. Define each beam by selecting the start node and end node with the cursor. After these actions, the workspace will look like the following image.
To achieve the correct modelling in INSTANT steel the frame rafter and the columns must be divided into beam elements. This arrangement will enable the application of purlins,rails and bracings in the full 3D model.
To divide the rafter into beams, we select and press the “Divide” icon. In the window dialog that will appear, use “By nodes” choice. Select 7 nodes and press OK in order to take place the selection in our 3D model. We apply the same procedure for the other rafter of the frame. Finally, for the two columns we apply the same procedure by selecting 3 nodes on the “Divide” icon.
At the end of the above procedure our workspace in INSTANT steel will be as following:
E. Applying the materials and sections
The INSTANT steel application suite contains libraries of materials and of sections. In addition to the existing items, the user can define arbitrary materials and sections. To apply these attributes to a beam follow these steps:
- Define the material and section if they are not in the standard program libraries.
- Select the material or section that will be applied
- Select the beams on which to apply the material or section.
- Apply the attributes to the selected items.
Before applying materials or sections, ensure that the program is in the Beam attributes mode by pressing the icon. For, the example model only one material is needed: steel. To start the application of a material, press the icon, followed by the icon. In the dialog that appears, select from the table of standard materials the “Steel” item and press the “Copy” button to add it to the list of materials currently in use in the project. You can then close the dialog by pressing OK.
The “Steel” material has to be applied to all the beams in the model. Use the following sequence of commands , and to start the items selection, select all model items and then close the items selection. At the end of this procedure, all the beams and nodes of the model will have been selected. The last step is to apply the “Steel” material to all selected beams using the “Apply” icon.
To unclutter the workspace, the amount of information displayed can be reduced by clicking on the icon and selecting in the dialog that will appear, only the items you want to shown.
Now select the “Sections” icon. For this frame two different sections will be used, HEA 360 for the columns and IPE500 for the rafters. To apply a section on the model beams it must first be selected for use in the current project. To do this, click on the “Initialize” icon which will bring up the Section selection dialog. Select the “Add” button in order to open the sections database of INSTANT steel. Select from the left part of the dialog window HEA360 and press “Select” on the right hand side. Apply the same procedure in order to select the second section that we are going to use IPE500.
The final state should look like the following image:
To apply a section on the model beam, select the section from the “Section selection” dialog table and then press the “Select” button. The dialog closes and the selected section can be applied on a beam either by clicking the cursor on a beam on the workspace screen or by using the grouping commands as following:
→ → Select the columns → →
With the same procedure we apply the section IPE500 to the rafters. After the application of the section on the beams, the frame model should look like the image below.
In order to have a more intuitive view of the frame structure, select the icon. A solid 3D display of the model will appear, as shown in the following image.
Also, a wire display of the frame is available by selecting the icon as shown in the following image.
F. Definition of supports and beam connections
Make sure that the program is in the “Degrees of Freedom” mode by clicking on the icon. To define support conditions for the frame columns select the “Supports” icon and then the “Initialize” icon. In the dialog that appears, the check-boxes represent blocked Degrees of Freedom for translation (Dx, Dy and Dz) and rotation (Rx, Ry and Rz) in the global system (X, Y and Z axis). Unchecking a check-box means that the specified DOF is released. A number field will also become active next to the released DOF and any value other than zero will be used as a stiffness coefficient of a semi-rigid spring support in that DOF.
In this example, the column base is considered to be Fixed in the frame direction and pinned on the other direction (release rotation for global X axis).
After defining the conditions for our supports, activate the item selection mode (), select the two nodes at the base of the frame columns and close the item selection mode by clicking once again on the icon and then apply the support to the selected items by clicking on the “Apply” icon. After this procedure, the model should look like the following image.
G. Generation of the spatial model
After defining a single frame, we can multiply it along an axis to obtain the complete structure model. Make sure that the program is in Topology mode, by selecting the icon. At the lower part of the vertical toolbar you can see three icons that represent the modelling tools available in 3D. Copying , moving and mirroring .
To generate the complete structure model in 3D, we must perform the following steps:
- Multiplication of the existing frame along the Z axis at intervals of 5 meters.
- Definition of structural items that are now missing such as: rails, purlins, vertical windbreaks, roof windbreaks and secondary front columns.
Then we ensure that the currently selected length unit is in meters. Then click on the icon to start copying elements. Select all the structure items using the sequence of commands , and . After the deactivation of the selection tool, the “Vector” dialog will appear and prompt for the direction and magnitude of the copying vector. To affect a copy in the positive direction of Z axis at a distance of 5 meters, enter the vector (0,0,5) as shown below. Also enter the number of copies to be made, in this case 10. After you press the OK button, another dialog appears prompting for confirmation on the copy command and the items it will be applied to. Press OK to accept all.
After the command has completed the workspace will look like the following image:
Using the methods for beam creation, application of sections, materials and beam connections as described in the above chapters, complete the structure with front columns using IPE300, purlins IPE160, rails UPN80, bracings L100x8 and secondary beams for connecting the frames together using IPE300.
We start the procedure by placing the secondary beams, purlins and rails in the first opening.
We alter the orientation of the purlins, by selecting the inclination around their local axis X. From the “Beam Attributes” mode we select the icon . Use the selection tool in order to select the 7 intermediate purlins of the rafter and after we close the selection tool we click “Apply” and then the following window “Beta angle” is active. Make sure that the current selected units for angle is in degrees and then enter 11.8 (Inclination of the rafter). We use the same procedure in order to set the angle of the purlins to 90 degrees.
The connections of purlins, rails and secondary beams are pinned. To model this connection condition, the rotational DOFs in local axes Y and Z will be released at the ends of each beam. To do this enable the “Degrees of Freedom” mode and select the icon. The easiest method to select the various items that must be affected is to use the filter selection. Start the selection tool by clicking the icon and the use the icon to open the beam filter dialog. In the Sections table, hold down the Ctrl key an click on the sections that corresponds to purlins, rails and secondary beams and press OK to apply the filter.
After you finish the selection, click on the icon to bring up the “Connection definition” dialog and make the selections as shown below.
When you press OK the selection will be applied to all selected beam ends.Note: In order to apply a connection release to a particular node (start node or end node), we must click on the beam, near the node that we wish to apply the particular connection.
After the above selections now the model in the workspace looks as in the following picture:
H. Application of loads
In this example we are going to use the following loading static cases:
- Self weight
- Wind in X direction
The first static loading case (Self weight), is calculated automatically from the software. On the other hand, for the other loading cases the user must follow the procedures described in Eurocode 1, depending on the data of his study.
In this example, we will consider an area loading (kg/m2) and we will make the distribution of the area loading to an intermediate purlin and rail, taking into consideration the relative distance between purlins and rails, which for this particular example is 1,5 m (loading area from the left-hand-side of the purlin/rail equals to 0,75 m and from the right-hand-side equals to 0,75 m, so total loading area equals to: 0,75 + 0,75 = 1,5 m).
Then, for simplicity reasons we will apply the loading of the intermediate purlin/rail to the rest of the purlins/rails.
Let us consider that the total weight for the cladding equals to 15kg/m2, the snow loading equals to 80 kg/m2 and the wind loading equals to 100 kg/m2.
In order to define the above loading cases, first make sure that the program is in Static Loads mode by pressing the icon. From the vertical toolbar select and in the “Loading cases” dialog that appears, enter the data for the first loading case. To add a second row in the table for the second loading case, use the key “Down arrow” on the keyboard.
- For the first loading case, Self weight, we enter the following data. Name (self weight) and type of loading (G – permanent load).
- For the second loading case, Cladding, we enter Name (cladding) and type of loading (G – permanent load).
- For the third loading case, Snow, we enter Name (snow) and type of loading (S – Snow).
- For the fourth loading case, Wind in X direction, we enter Name (Wind X) and type of loading case (Wx+ Wind pressure).
The rightmost column shows the Mass factor coefficient ψ2i, which is used for determining the quasi-permanent effect of variable action. It is the coefficient of the specified loading in the accidental loads combinations as well as being used in the automatic calculation of the equivalent mass of the loading in the “Loads to masses” tool. The value chosen by the program automatically from the type of loading can be also edited by the user. This value will only affect the masses calculated automatically with the “Loads to masses” tool and not any masses applied directly by the user.
In order to apply the self weight to the beams of the structure, we select the loading case “Self weight” and then press OK. This action has as a result to activate the selected loading case and any loads that will be applied on beams or on nodes will be considered for the specified loading case.
At this point, we make sure the the currently selected units for length and force are m and kN correspondingly.
Then we select the icon, from the vertical toolbar. Use the commands , and to activate selection mode, select all items and then close selection mode. Finally, click on the “Apply” icon (). After the completion of the above sequence the workspace will have the following look.
The next step is to apply the loading for cladding distributed to the purlin and rail beams. For that reason we have the following:
15 kg/m2 x 1,5 m = 22,5 kg/m = 0,225 kN/m
Select the second loading case, Cladding, and press OK. Then choose the icon from the vertical toolbar. Select “Initialize” . This action will have as a result to open the window dialog “Define distributed load”. Enter the following value -0,225 kN/m in global reference system. Choose the Dy direction and the negative sign of the loading action will result in applying the force from top to bottom.
Activate the selection mode by pressing the icon, select the beams that you wish to enter the specified load and then deactivate the selection mode by pressing the icon again. Finally, press the icon. After, completing the above procedure the model will look as following:
Using the same procedure as described above and after selecting the loading case “Snow” we apply the uniform distributed snow load to the roof beams. The snow load is considered to be -1.20 kN/m (80 kg/m2 x 1.5 = 120 kg/m = 1.20 kN/m), in global reference system. Choose the Dy direction and the negative sign of the loading action will result in applying the force from top to bottom.
Finally, the next step is to define the loading case “Wind X”, which is a uniform distributed load and is calculated as following:
100 kg/m2 x 1,5 m = 150 kg/m = 1,50 kN/m, in global reference system. Choose the Dx direction, select positive sign for the rails perpendicular to the global X direction (left-hand side of the model) and apply the calculated force. Then, apply a uniform distributed load of 1,0 kN/m. In global reference, choose the Dx direction, select positive sign for the rails perpendicular to the global X direction (right-hand side of the model). Finally, apply a uniform distributed load of magnitude 1,0 kN/m at the purlins. Choose Dy direction and select positive sign.
Before, we apply the specified above loads we are going to use the copying elements that exist in the “Design” icon () and more specifically the “Mirror” icon (). Initially, select the three (3) rails, the secondary beam and the seven (7) interior purlins using the “Select nodes or beam” icon .
After, deactivating the “Select nodes or beam” icon, select the “Mirror” icon and the following “Symmetry” window will open. Choose as symmetry plane the YZ axis and set “Distance” 12 m and then press OK. This will have as a result to open the “Apply to…” window that requires confirming the copying elements. Select all copying elements and then press OK.
The workspace now will have the following look:
To the purlins that were created using the “Mirror” icon , we have to give the right “Beta” angle , which is -11,8 degrees, using yet again the “Select nodes or beam” icon in order to select the corresponding purlins.
Then, we apply the loads that were described above for the loading case “Wind X” and the workspace will have the following look:
Then, we apply the “Duplicate” command in the Z axis for all the purlins, rails and secondary beams, exactly as we did initially for copying the main frames.
At this point, we add to the model the vertical and roof wind bracing L100x8. The type of connection for these members is pinned. This is achieved by releasing the rotation in Y and Z axis for the start and end nodes as shown below:
By using the “Mirror” icon and the “Duplicate” icon we have the following result:
We conclude our model by placing the frontal columns IPE300 and the rails UPN80, using all the appropriate commands as already mentioned in this document. For the frontal columns apply a “Beta” angle of 90 degrees. The base support is pinned in both horizontal directions (release rotation in X and Z axis) and the connection of the frontal column to the rafter is set to pinned (release rotation in Y and Z axis).
For the rails we also give a “Beta” angle of 90 degrees and pinned connections for both ends (release rotation for local Y and Z axis). In addition, apply the cladding loading to the rails equal to -0,225 kN/m, in Global reference system for the Dy direction.
Finally, we copy all the specified above elements in Z = 50 m.
The 3D model is now ready. Before attempting a solution it is helpful to perform a verification using the command Data → Information → Model verification from the menu. The “Structure data check” dialog is displayed and lists various errors found in the model. In this case, the model has several beams without material. This error can be easily corrected by using the “Select” button to select all listed beams and then applying the “Steel” material using the following sequence of commands , and selecting the “Steel” entry and using the “Apply” icon.
In the case where the structure geometry has to be verified (node coordinates, beam connectivity, etc.) various information are available at the right-click of the mouse button on an item. The data displayed are presented in the images below. Especially for the nodes, the coordinates can be modified in the dialog fields and changes will be applied when you press OK.
Moreover, by pressing Shift and left click of the mouse button on a support or a connection, we can easily alter the degrees of freedom.
At this point it should be noted that for the new members described above we have not yet applied the self-weight of the specified elements. We apply the self-weight to the whole structure as described in chapter 4.8.
Now, the model is concluded and we are ready to perform analysis and design of the specified elements.
I. Modal analysis
To enter the data related to modal analysis (mass, damping coefficient, number of modes to be calculated, etc.) click the “Modal” icon. Then, in order to apply the modal damping coefficient, click on the “Modal damping” icon and in the dialog that appears leave the preselected values: “Modes” equal to 500 and “ksi” equal to 0.04.
The parameters of the modal analysis can be entered by clicking on the “Free vibration” icon, which opens the “Modes” dialog window.
There, set as the desired number of eigenvalues that will be calculated equal to 40 and select “Automatic readjustment”. Then, select that the masses are “on nodes and beams” and also that the “Density” of the model members is also taken into account. The “solver options” parameters are advised not to be altered in any case and the default values to be taken into account for every example.
By selecting the “Loads to masses” tool (), the following window dialog opens called “Mass creation/deletion from loads”. The automatic transform will take into account the values of quasi-permanent load factor ψ2 entered in the “Loading cases” dialog.
Press from the above window “Create masses” and as a result we have the following picture: