Now that our structural model is complete, we're ready to test our design to determine whether or not it can carry the traffic loads prescribed in the Design Specifications. To start the test, click the Load Test button on the toolbar.

As the load test begins, we see our bridge as it might look after it is built. The structure sags a bit, as it begins carrying its own weight. All that steel and concrete is heavy, but the trusses do their job well. Then along comes a truck--a 360 kilonewton vehicle representing the largest loading that this bridge is expected to experience in its lifetime. The truck moves out onto the bridge and...the structure collapses into the river. Ouch!

Our design has failed the load test--but don't worry. Engineering design is an iterative process. Some trial-and-error is always necessary to achieve a successful product. If at first you don't succeed, then go "back to the drawing board" and try again.

Back on the Drawing Board, you'll notice that the appearance of the structural model has changed. Five of the 18 members are now highlighted in red--indicating that these members are not strong enough to carry the truck loading.

(Recognizing that you would probably need to edit your structural model, the West Point Bridge Designer has already chosen the Select tool for you. It's the arrow button on the Design Tools palette, as shown at right.)

The selected member will turn light blue. Once a member is selected, we can change any of its properties--material, cross-section, or size. The most effective way to strengthen a failed member is to increase its size. To make this change, click the drop-down arrow on the Member Size list (one the of Member Properties lists on the lower toolbar), and choose the next larger size--130 mm. Repeat the process for the remaining four failed members. Select each member, then choose 130 mm from the Member Size list.

This time when we return to the Drawing Board, however, only the two diagonal members on the ends of the bridge are highlighted in red. Increase the size of these two members by first selecting each one, and then choosing 140 mm from the Member Size list.

Before we return to the Drawing Board, take a moment to watch the Load Test Animation. Note that some of the structural members turn red, while others turn blue. This color coding illustrates the type of force in each member. Members that turn blue are experiencing tension--they are being stretched or elongated. Members that turn red are in compression--they are being squashed or shortened. The intensity of color indicates the magnitude of the member force. Thus the dark red diagonal members are being loaded very close to failure, while the light blue bottom chords are much stronger than they really need to be.