RISA 2D is a software for finding out the internal forces in members of a truss, in a beam or a frame. It is an easy software, does not require much time to learn. It gives axial force, shear force and bending moment values at different points along the length of a member. It also provides grahical figures of these forces.Here I am going to demonstrate the process of finding the axial forces in members of a simply supported roof truss.
In order to draw a truss in Risa-2D, the following data must be known. Spacing of the truss (s), span of the truss (L), height of the truss (h), so that you can find out the co-ordinates of all the truss joints which are to be input into the software for the calculation of reaction forces and member forces.
The truss I am drawing for finding out internal forces is shown below. It is called a Fink or French Truss.
First of all open the software and click ‘Global’ from the manual bar. The following window appears:
Enter the details of Model Title, Designer and Number of Sections. It provides internal forces (moment, shear and axial forces) at a number of equally spaced sections according to your wish. For a truss design we will choose ‘2’ as the internal force in a truss member is only axial force which is constant throughout a member, so we need value of axial forces at only the both ends of the member. Click ‘ok’.
Now next step is to choose the units in which you want to perform your calculations. Click ‘Units’ from the manual bar. There is an option ‘use consistent units’. It is generally used for hand calculations. Other options are Standard Metric units and Standard Imperial units. I am choosing Standard Imperial units. Click ‘ok’.
Now go to Modify and then Grid. The following window appears.
Here you can change the grid spacing according to your structure. And your grid setting can be saved as well for use in any other structure by clicking ‘save’ at the bottom of this window.
Now you have to enter the data. Go to Spreadsheets and then Data Entry Buttons Toolbars (if it is not visible on screen by default).
The toolbar is shown below:
One by one enter all the data. First is the joint co-ordinates. The following window appears. Enter the co-ordinates. Points will appear at the screen.
Now set the boundary conditions. My truss is simply supported, so at A there is hinge support having two reactions. At point B there is roller support having only one reaction in y-direction.
When you click on X, Y or Rotation a small window appears. I have X fixed and Y fixed at joint A and only Y fixed at joint B.
When you finish entering, supports will be shown.
Define the members with respect to their I and J joints.
In a truss, both ends of a member are pinned. So choose I End and J End ‘Pinned’ in the column ‘I Release, J Release’. Software will automatically calculate lengths of each member. Truss will be formed.
Now apply loads to the truss. As they are acting downwards so their magnitude will be negative. In my truss loads are acting at the joints only, no point loads or distributed loads.
After you have finished entering all the data, click ‘Solve’ from the manual bar. The following small window appears.
One by one click on each of these to get your desired results. Further you can see the ‘Axial Force Diagram’, ‘Shear force Diagram’ as well as ‘Bending Moment Diagram’ by clicking
from the menu.
I have analyzed my truss under 3 types of forces shown below.
Truss subjected to Gravity Load of Unity
Truss subjected to unit Wind Load on Hinge Side
Truss subjected to unit Wind Load on Roller Side
Thats all. I hope you like this tutorial. Also visit other tutorial in my blog like Forces in Members of a Roof Truss. Do provide you feedback regarding my posts. Have a nice day!!!