Simple Bracket Analysis using Strand7
Creating the Model
In this tutorial we will create a bracket, as that below, apply loads to it and analyse the stress levels raised within it. We will also limit the stress levels displayed by the system.
• Extrude Nodes • Create Plates
• Extrude Bottom Edges • Create Bottom Plate • Grade Plates
• Restrain the nodes • Apply node loads • Plate properties
• Solve the model
Extrude nodes
To start open a blank model with the units set to mm, MPa and KN. Create the first node using the keyboard shortcut Ctrl + W.
Set the 1st Node as 0,0 by clicking on the Apply button. We will extrude the next nodes, so click on the Tools > Extrude > by Increments menu's at the top of the screen. Set the X direction to 200, select the Node and click on Apply.
Next extrude by Increments the nodes along the Y axis by 200mm . Remember to set the X direction to 0. Click on the Re Draw Icon and your screen will look like that below.
Create Plates
Use the keyboard shortcut, Ctrl + E and select Quad8 plates as above. Now create the plate as in the Non Standard Sections tutorial by clicking on each node in turn, then finally All. Remember to go in an anti-clockwise direction.
To complete the bracket we will need to extrude the bottom edge of the plate
Extrude Bottom Edge
Select the extrude by increments menu, then select the bottom three nodes. Extrude these nodes by 300mm along the Z axis. The screen now looks like:
Create Plates (2)
Use the Ctrl + E shortcut again, select the Quad8 option. Create the plate and the model will look like:
It doesn't really look like the bottom plate is longer than the back plate at the moment does it? Just to check, change the view angle to Isometric (Right click anywhere and select the Angle menu, then Isometric as per the Beam Analysis Tutorial)
Looks OK! The next stage is to put that hole in the back plate. To do this we 'Grade' the plate.
Grade Plates
Before we can grade the plate, we need to sub divide the plate into four plates. To do this use Tools > Subdivide. Note that the A & B elements have been divided by two in both cases.
Select the plate and click Apply. (Ensure that the Plate Select option is selected.)
To Grade the plates, Tool > Grade Plates and Bricks. Select the Quarter Circle Grade, and then select the inside edges of the plate as per below;
The model is now complete, and we can start restraining it and applying loads!
Restraining Nodes
Select the nodes by dragging a diagonal box around the required nodes (re select the center node to de select it) and restrain against translation in the X, Y and Z directions.
Apply node loads
Apply the node loads via the Attributes > Node > Force menu. Note that the load is applied in the Z direction.
Before we can solve the model we need to delete the beams created during the models construction and to define the plate properties. To delete the beams use the global select tool, then use the Delete key on the keyboard. The selected beams appear as dashed white lines, but once deleted the model will look no different.
Plate properties
To define the plate properties of material and thickness use the Ctrl + Alt + P keyboard shortcut. Set the material to Steel - Structural and the membrane thickness to 2mm.
The next stage is to Solve the model and we know how to do this, from the beam analysis tutorials, but if you need reminding, select Solver > Linear Static. To continue we will go to the next page.
Simple Bracket Analysis using Strand7
Analysing the Model
• Develop the Model
• Solve the model
• Displacement Scale
• Plate Stress
• Member Stress
• Limit Stress Displayed
• Hide High Stress Plates
• Animation
Solve The Model
We now have a restrained and loaded model. Click on the Solver > Linear Static menu to solve the model.
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Displacement Scale
One the model is solved set the displacement scale to 10%.
Plate Stress
The result should be that below.
Member Stress
Go back to the Elements Result Display and change the plane being displayed. You will notice that the stress distribution changes, but also that the stress levels change, quite dramatically in some cases.
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Limit Stress Display
You will note that the strongest Structural steel, grade S350 has a material strength of 350N/mm2, therefor at 750N/mm2 (Mpa) the material will have failed. One quite useful feature of Strand7 is it's facility to limit the maximum and minimum stress levels displayed on the model. To do this open the Elements Result Display window and select the settings button. Select the limits tab and set the Min value to -350 and the Max to 350.
The model will refresh when the window is closed. The two screen grabs below show before and after.
Default Range User Defined Limits
Hide High Stress Plates
Another method of hiding highly stressed area's is to select the highly stressed plates and hide them using the hide selected button .
The finer the mesh, or the more plates the more accurate the model. Unfortunately the Demo version of the software limits the number of beams and plates too much to allow anything much more complex than the model we have created, but this is another method of increasing the accuracy of the analysis / results displayed.
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Animation
One final analytical tool, and in my opinion, one of the most useful / enlightening is the animation section.
Unfortunately I don't know how to include an animation in a web page - but I am working on it. In the mean time the link here is to a Winzip file containing a free copy of the SAF player and the animation file Tut1.saf. Download the file by following the link, or follow the instructions to create your own animation.
The menu is simple enough; Results > Create Animation. I suggest setting the number of frames to 10 as the model is not too complex, and it looks much smoother.
