Bridge Designer Tutorial (Obsolete)

***This tool no longer works***

Instead, check out the new Truss Simulator free design tool.

***Old post below***

This tutorial was originally published in 2005. Since that time, web technologies have changed and the java applet that this program runs in is no longer supported by Chrome, Firefox, and Microsoft Edge web browsers. However, you can use Microsoft Internet Explorer 11.

The Bridge Designer is perhaps the greatest online tool for building model bridges. It allows you to design a virtual truss, put a load on in, and it calculates how the forces are spread out. You can use this as a design tool as well as an evaluative tool. Once you have built and tested a bridge, plug in your design and load when failure occurred and it will show you how much force was on each member.

I was first introduced to the Bridge Designer by a civil engineer, who explained to me the usefulness of the free program. However, it took me, a 6th grader at the time, over a year of playing around with it to finally figure it out. This program has some limitations that you must understand if you are going to use it properly. Watch the video tutorial or read through the steps.

Link to the Bridge Designer

The Bridge Designer allows you to create a virtual truss, put a load on it, and see how the load is spread out. It is very useful to use after you test a bridge to failure, then plug in the bridge design, and see how much force it took to break it.

Here is a text version of the video. When you first load the program, you are presented with this:

To begin, you will need to click the “Add Nodes” button. A “node” is simply a joint. You must add the joints before adding the actual members.


Usually, I start with the bottom left corner of the bridge, and then count over to the right however many squares as my bridge is long. I count one square per inch. If your bridge is really short, you might do two squares per inch. Or if it is really long, 1/2 square per inch.

After I have the bottom length, I add the top nodes. It might look something like this:


Now it is time to add the members. Click the “Add Members” button. Left click on one of the nodes you have added, and drag the mouse over to the next one. You must hold down the left button to do this. It doesn’t matter what order you add the members, but you must connect every node.


After adding all the members, click the “Calculate” button on the bottom. Now look just below the top buttons. In red, you should see something like “There must be one fixed and one roller node”. If you see “Members +3 must equal twice the Nodes”, you need to adjust either the number of nodes or members.


Once everything is okay, click the “Fixed node” button which is one the bottom left. Then click the bottom left node. That node should turn yellow. Make the bottom right node the “Horizontal rolling node”, it should turn red.


Alright, you’re almost done. Click the “Add a Load” button, and choose which node to put it on. You can add more than one load if you want. Once you click that node, Pull your mouse straight down. You should see a number right next to the load increase as you go farther down. Once the load is correct, click once. If you mess up, don’t worry. You easily remove any load, member or node using the buttons.


Now click the “Calculate” button again, and also click anyway on the grid. If everything is set up right, the members should change colors to red, blue, and sometimes green.

You might get the message, “Cannot compute, matrix is singular”. If you do, then somewhere a member might be missing. You need to create triangles for this program to work correctly. If it doesn’t think that your design is “safe”, it won’t work.

Occasionally the program thinks I have an extra node somewhere, and the only fix is to clear everything and start over. Don’t worry, with time you will get the hang of it and create designs in just a few minutes.

14 thoughts on “Bridge Designer Tutorial (Obsolete)”

  1. Warning!!! The bridge designer only suitable for advance users for reference, as it is not suitable for calculating bridges with more than 1 layer of triangles. For those type of bridges top and bottom part has to be separated and calculated separately and added up using cross sectional method.

    The designer supports only point loads, when it comes to distributed load, often there will be different point loads for different number of supports rather than only 1 point load. Without these knowledge the designer will give wrong values.

  2. Thank you KianWah Lee that is some very useful information for us all to use. I myself was having a problem with bd as well and you gave a possible solution.

  3. What exactly do the numbers represent? You would think that if you applied a load of 50, the numbers would show how much of that weight each member was under. For example: in a 7 member bridge, 4 members would say 5, and the other 3 would say 10. It does not do that. What do the numbers mean???

    • Andrew,
      The numbers represent how much force is applied to each member. These numbers do not add up to the total load, they will usually be much more. It is also possible for a single member to have more force than the total load, although that does not often happen in model bridge designs.

  4. This worked fantastically. This website is just perfect for Science Olympiad. I love it so much. Thank you for creating such a helpful public site.

  5. This Bridge designer shouldn’t work anymore on most browsers, it requires java(if you have it), and also the site’s made in HTML 4(I think) and doesn’t have a DOCTYPE declaration thus is expected to be HTML 5 on most browsers as of 2014 September. if you have java, then it should work on MSIE (Microsoft Internet Explorer). but i’m not sure about anything else. reply if you have found otherwise.


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