guide for using risa3d to model balsa...

CE 331, Summer 2014 Guide for Using RISA3D to Model a Balsa Structure 1 / 9

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.

1. Set the units Click on the “units” icon, and change the default units to lbs and inches for:

Lengths

Forces

Linear Forces

Moments

2. Define the balsa wood material properties. Initially, use assumed material properties for your structure. You can find properties via the internet, or use the properties below:

Symbol Name Typical (medium density)

Fb Allowable bending stress 2000 psi Ft Allowable tensile stress 2000 psi Fv Allowable shear stress 450 psi Fc Allowable compressive

stress 2000 psi

E Modulus of elasticity 500,000 psi UW Unit weight 10 lb/ft3

25lb

4 @ 5.25” = 21”

4”

25lb

2.5” 2.5”


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. 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 typing the size directly into the cell under “Shape” as shown. The first dimension (0.375) is the width (horizontal dimension) and the second dimension is the depth (vertical dimension) of the member. “FS” stands for “full sawn” meaning the actual dimension of the piece. Also select the appropriate Material.

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).

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”.

6.3 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.

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.

10. 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). 11. Analysis Results: Two numbers that you can use to compare your designs are the predicted failure load and the predicted weight. To calculate the predicted failure load, find the maximum unity check (0.37 in the figure above). The unity check, UC, is defined as

37.0.. stress allowable

loads to due stressCU

The load to cause the unity check to equal 1.0 (and cause failure) for the truss would be

lblb

13537.0

50Load Failure Predicted


To calculate the predicted weight of the truss modeled in RISA we will need to set the material density artificially high so that RISA will display the structure weight to more than one significant figure.

1) Select “Materials” from the Data Entry menu, then the “Wood” tab, then the density field corresponding to your balsa wood.

2) Change the unit weight of your balsa wood to a number 1000 times larger (e.g. to 1000 x 10 lb/ft3 = 10,000 lb/ft3). Then, after solving your model, select “Material Takeoff” from the “Results” menu.

3) Solve the model (e.g. by clicking the “=” icon), and select “Material TakeOff” from

the “Results” menu.

The weight of the structure modeled in RISA is calculated by dividing the listed Weight by 1000 (e.g. 101.1 lb / 1000 = 0.101 lbs)

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