Optimizing the Design
Let’s return to the Drawing Board and get started on the optimization process. Because our bridge didn’t collapse, it is a successful design. But it is not an optimal design, because it is much more expensive than it really needs to be. Designing an inexpensive bridge is not as important as designing a safe bridge–but cost is still quite important. A bridge might never get built, if it costs more than your state or city can afford.
On the upper toolbar, notice that the cost of our design has already been calculated at $202,560.39.
To see how this number is computed, just click the Report Cost Calculations button (the little calculator) immediately to the right of the cost.
How can we reduce the cost of the bridge without compromising its safety? One way is to reduce the size of members that are stronger than they really need to be. Remember that, in the previous load test, all five bottom chord members were color-coded with a very light blue. The light color indicates that, for each of these members, the tension force was much less than the strength. As a result, all five of these members can be reduced in size without compromising the safety of the structure. In fact, you can reduce their size from 120 mm to 90 mm, and the bridge will still pass the load test.
In making this modification to our structural model, let’s take a little shortcut. Since we have to change the size of five members, let’s use multiple selection to speed up the process. Hold down the Ctrl key on your keyboard, and then click all five bottom chord members. All will be selected simultaneously, as shown at right. Now choose 90 mm from the Member Size list, and all five members will be changed to this size.
Notice that the cost of our design has dropped to $196,929.06–a saving of over $5000. The structure still passes the load test, so the reduction in cost did not adversely affect the safety of the structure.
$5000 is a fairly impressive cost saving, but we can still do a lot more to optimize this design. Many other members can safely be reduced in size. We might also find that we can save some money by using hollow tubes rather than solid bars or by using a different type of steel. Use the other two Member Properties lists to make these changes.
Just by making well-reasoned adjustments to the member properties, you should be able to reduce the cost of this bridge below $175,000. Give it a try!
Remember that size is not the only member property you can change. You can also choose different cross-sections (hollow tubes vs. solid bars) and different materials (three different types of steel).
Optimizing member selections is not the only way to reduce the cost of a design. You can also change the shape of the truss by dragging joints with your mouse, and you can add or delete members. The drawing at right shows one possible configuration. There are literally millions of others. You might also try an entirely different site configuration.
For detailed instructions on all of these optimization strategies–and much more useful information about bridges, engineering design, and the WPBD software–click the Help menu on the upper toolbar. Select How to Design a Bridge from the menu and carefully study each of the steps outlined in the flowchart. Pay particular attention to “Optimize the Member Properties,” “Optimize the Shape of the Truss” and “Find the Optimum Truss Configuration.” These articles contain a lot of helpful hints that will speed your search for a successful, low-cost bridge.
Happy designing!
