Working with a simulator lets you virtually test how structures behave so that you can improve the design before you build them. In this case, we are going to use a simulator to test and improve the design of our bridge before we build it.

It's a free online simulator called WPBD (West Point Bridge Designer). You can download it free from the webpage www.bridgecontest.org.
The simulator lets you design a new bridge or adapt a design and improve on it. In either case, the aim is to make the best possible structure – a design that meets the conditions at the lowest cost.

Let's begin the simulation

• Phase 1: choose conditions and features

Screen 1. You can choose to create a new design for your bridge or use a sample design. Try Create a New Bridge Design.

Screen 2. Then describe the features of the land and the bridge. Read and click on Next.

Screen 3. The simulator allows you to take part in a competition. Choose No and continue.

Screen 4. Now you can choose the height of the bridge over the river. Remember that the higher your bridge is, the longer the span will be, which will make your bridge more expensive.

In the same way, with a higher deck your excavation costs will be lower because you won't have to dig or excavate the sides where the bridge will rest.
In this case, your bridge will stand at 20 m, with flat beams (Standard abutments), without supports in the middle (No pier) and no bracing (No Cable Anchorage).

Screen 5. Choose the material for the deck of the bridge and type of load. Choose the default settings.

Screen 6. You can choose a truss design that you know or invent your own (none), which will be more interesting.

Screen 7. Here you can give your project a name and number if you want.

Screen 8. The simulator explains the process that you need to follow:

1. Press Finish to go to the drawing screen.
2. Draw the nodes or joints in the structure (Tool/Joints).
3. Draw the bars between the joints (Tool/Members).
4. Carry out a test to check if your design is stable and resistant (Test/Load test).
5. Add or reinforce elements that failed in the test.
6. Improve the result by changing the position of the bars to reduce costs.

• Phase 2

When you have pressed Finish 1., you will see a screen where you can draw the nodes (the joints between the bars) and the bars that your bridge will have. With these, you can make an initial design 2. and 3.

As you draw the elements of your bridge, you will see a table with numbered rows with the following aspects: material type, cross section, size, length and slenderness (thickness). When you draw a bar, the program assigns a default size.

When you have finished the bridge, test it (Test/Load test) 4.

The program warns you if the design is not stable and if you need to add more triangles.

Once you have corrected the design errors, the program detects any stability problems and shows an animation after checking the design of the structure (Test/Load test) 4.

First, you can see how the bridge holds up under its own weight and, then, under the weight of a passing truck. During the animation, if the stress in any element of the bridge is more than the resistance of the material, you can see that it deforms or collapses.

For example, in this picture you can see that a bar has completely bent in the virtual load test. In this case the bar bearing compression stress was too long.

You can correct the fault by changing the design using shorter bars (putting in joints between them) or by reinforcing the bars.

After making some changes test your design again 4.

Once you have a stable design, go to the next phase.

• Phase 3: optimise the design of the structure

Look at the table with the data for each bar. In the calculation of the stress each one has to bear, they include the traction, the compression and any possible buckling of each element.

Elements bearing compression appear in red and elements bearing traction appear in blue.

The colours will be more intense the closer they are to the tension of the breaking point.

Ideas for improving your design

• You can reduce the traction stress in bars by using thinner solid bars.
• Compressed bars will behave better if you use hollow bars. But they are more expensive so it is best to use them only for thinner bars (slenderness) that might be more prone to buckling.
• For each new bar that you add to the design, the price of the project will go up, so you should always try to use similar types of bars. Each bar should be the same as the one on the other side. Every time you make a change, carry out a new load test of your bridge to see how the stress of the bars is affected.

In this example, you can see the bars that have failed the load test because they are too thin. To change them, use the tool bar. As you get closer to the definitive design of your structure, you will see that the colours are more intense.

As for the budget, you can see how it varies at any time by clicking on this icon in the tool bar.

This simulator also allows you to test default beam models like the Pratt or the Warren. You can also test a model of a suspension bridge: