To understand the answer to this question, you need to understand how the Bridge Designer calculates the cost of your truss. Run the Bridge Designer and load any of the sample bridge designs. Now click the Report Cost Calculations button, located on the Status toolbar. The Bridge Designer will display a table showing exactly how the current cost of the truss has been calculated. You’ll see that the total cost is composed of three components–material cost, connection cost, and product cost. When you reduce the size of a member, you reduce its material cost. However, for each new “product” you add to your design, you are charged an additional $1,000. A “product” is defined as any unique combination of material, cross-section, and size.
This cost algorithm is fairly realistic. If you optimize a structural design by making every member as small as it can possibly be, you’ll end up with a very light but very impractical structure. It will be impractical because it will consist of many different member sizes. When a construction company actually has to build a structure like this, the project will incur a lot of additional cost. It’s harder and more expensive to join two different sized members together than to connect two identically sized members. There is also a cost associated with having to order and manage many different member sizes on a job site. In short, there can be substantial cost saving associated with standardization–using as many of the same sized members as possible in a structure.
This is not to say that using the same member size for all of the members in a truss will produce an optimal design. It won’t! There is a trade-off between light weight and standardization. You need to do some trial and error to find the best balance between the two. Reducing the size of a single member might cause the total cost of a truss to rise; however, simultaneously reducing the cost of five members will almost certainly cause the total cost to drop, because you reduce the material cost for all five members but add only a single additional $1,000 product cost.