= 21

Transcription

1 CE 331, Spring 2011 Guide for Using RISA3D to Model a Balsa Structure 1 / 9 0. Example Bridge. An example structure is shown below. Typical results for the RISA model of this structure are shown throughout this guide. All members are 0.50 x 0.50 balsa with material properties specified in Step lb 25 lb = Set the units Click on the units icon, and change the default units to lbs and inches for: Lengths Forces Linear Forces Moments Weight Densities 2. Define the balsa wood material properties. Initially, use assumed material properties for your structure. A good web site for balsa material properties is shown below: An explanation of the material property symbols and typical values for medium density balsa wood are shown below. Symbol Name Typical (medium density) F b Allowable bending stress 2000 psi F t Allowable tensile stress 2000 psi F v Allowable shear stress 450 psi F c Allowable compressive stress 2000 psi E Modulus of elasticity 500,000 psi UW Unit weight 10 lb/in 3

2 CE 331, Spring 2011 Guide for Using RISA3D to Model a Balsa Structure 2 / 9 Follow these steps to define your balsa wood material properties: (a) Select Modify / Custom Wood Species Database (b) Put the cursor on the bottom line and press the Enter key to create a new line. Type in the appropriate information. Use your name in the label to distinguish your material from others. (c) Assign your wood species to a material label by selecting Materials from the Data Entry menu, selecting the Wood tab, creating a new blank line on the bottom of the menu, typing in a label for your wood species, selecting the species you entered in the previous step.

3 CE 331, Spring 2011 Guide for Using RISA3D to Model a Balsa Structure 3 / 9 3. Define Sections Rather than define the section properties of each and every member in your model individually, it s best to group your members into section sets (e.g. bottom chords, diagonals, etc.). Select Section Sets under the Data Entry menu, and select the Wood tab. For wood, you can specify your own custom shape by selecting the red arrow under Shape. For basswood check the Use Full Sawn Size box and enter the dimensions. Note: width is defined as the vertical dimension of your member. Also select the appropriate Material.

4 CE 331, Spring 2011 Guide for Using RISA3D to Model a Balsa Structure 4 / 9 4. Set up your drawing grid by selecting Modify Drawing Grid and typing in the number and spacing for the X and Y axis 5. Layout the members of your structure by selecting Draw New Members Select the Type of Material and Assign a Section Set and select a section set from the list, select Apply and draw your member(s). Repeat to draw other types of members (e.g. bottom chord, verticals, diagonals, etc).

5 CE 331, Spring 2011 Guide for Using RISA3D to Model a Balsa Structure 5 / 9 6. Set the Boundary Conditions by selecting the Boundary Conditions icon 6.1 You will likely be making a 2-D model, so you will need to constrain all joints to displace in the X-Y plane. Do this by setting the following to Fixed and selecting Use : Z Translation X Rotation Y Rotation then select Apply Entries to All Selected Joints and select Apply 6.2 Next,set the support boundary conditions. For the pinned support: set the X Translation to Reaction and select Use set the Y Translation to Reaction and select Use select Apply Entries by Clicking Joints and select Apply select the joint Repeat for the roller support after setting X Translation back to Free.

6 CE 331, Spring 2011 Guide for Using RISA3D to Model a Balsa Structure 6 / Check your boundary conditions by selecting Boundary Conditions from the Data Entry menu. All joints should have Fixed for the Z, X-Rot and Y-Rot 7. Define your Basic Load Cases and Load Combinations by selecting BLC and LC You will only have one Basic Load Case and one Load Combination for your balsa structure. RISA will add the 2 under Point after you have entered the two point loads (next step). 8. Specify the loads by selecting the appropriate loads icon Joint Loads Distributed Loads Point Loads Joint loads are concentrated loads applied at joints; point loads are concentrated loads applied to a member between joints.

7 CE 331, Spring 2011 Guide for Using RISA3D to Model a Balsa Structure 7 / 9 The screen below shows how to specify a -25 lb load (negative because it s downward) 2.75 from the I node (left end). You can review your loads, change them or delete them from the appropriate menu on the Data Entry menu 9. Analyze your model by selecting the appropriate Load Combination, and selecting Solve Current. Or, select the = sign if you only have one load combination.

8 CE 331, Spring 2011 Guide for Using RISA3D to Model a Balsa Structure 8 / Check your model by displaying the deflected shape Select plot options, Load Combination, Include Undeflected Shape and (optional) Animate 11. Check your design by selecting Plot Options, the Members tab, Wire Frame, and Unity (Bending). The goal is to have the unity checks as close to 1.0 without exceeding 1.0. You can reduce the unity check of a member by selecting a larger x-section (Step 3).

9 CE 331, Spring 2011 Guide for Using RISA3D to Model a Balsa Structure 9 / 9 Analysis Results: The maximum unity check in the figure above was 0.37 in the members directly under the load. Since unity check, UC, is defined as stressduetoloads U. C allowablestress The load to cause the unity check to equal 1.0 (and cause failure) for the truss would be Predicted FailureLoad lb lb Under Material Takeoff (on the Results menu), RISA lists 70 inches of members. The predicted weight of the truss is therefore: 1 ft lb Predicted TrussWeight (70")(0.50")(0.50")( )(10 ) = lb in ft 3