Once your has no , your design is successful. However, your design is not
optimum until you minimize its cost. The first step in optimizing your
design is to minimize the cost of your current truss configuration by
optimizing the --material, and . At this stage in the design process, you should not change the shape or
configuration of your current structural model.
To optimize the member properties:
Ensure that you understand how the West Point Bridge Designer
2010 calculates the cost of your design. In
particular, understand the trade-off between material cost and product cost.
Minimize the material cost. There are several approaches you might
use to minimize material cost; the following procedure is recommended for inexperienced
designers:
Start by using the lowest cost (but weakest) available material--carbon steel--for all
members.
Select an appropriate for each member. It is usually best to use solid bars for members and hollow tubes for members. For more information, see Solid Bar or
Hollow Tube?
Now use a systematic trial and error procedure to determine the smallest possible member
size for every member in the structural model. Starting with a successful design, decrease the size of every member to the next smaller
available size. (See Change the Properties of a Member
for more information.) Then run the again. If any member fails, its size is too small; change it back to its previous
(larger) size. For each member that is , decrease its size again, and run the load test. Keep reducing its size and
running the load test until the member fails, then increase its size by one. If you use
this process systematically for every member in the structural model, you will ensure that
every member is as small (and inexpensive) as it can possibly be without failing.
Finally, go back and check if using either of the other two materials--high-strength steel
or quenched and tempered steel--will reduce the overall cost of the design. Both of these
steels have significantly higher than carbon steel, so using them will allow you to reduce the size of members
without reducing their strength. But both high-strength steel and quenched and tempered
steel are more expensive (in dollars per kilogram) than carbon steel. You will need to use
trial and error to determine if the benefit of increased strength is sufficient to offset
greater cost of the high-strength steels. It is permissible to use two or three different
materials in the same design.
Once you have adjusted the materials, cross-sections, and member sizes to minimize the
material cost of your design, be sure to run the load test once more to ensure that all
members are safe.
Optimize, based on product cost. When you minimized the material cost (above), you probably introduced a large number of different
products into your design. Thus, even though your material cost is low, your product cost is probably quite high, and your total cost is almost certainly not
optimum. Use the following procedure to find the best balance between these two competing
cost factors:
Check the to see how many products are currently included in your design. In particular,
identify any products that are used for only a few members in your structural model.
Change the properties of these particular members to match the next larger (or next
stronger) available product in your current design. For example, suppose your design
includes two 40 mm solid carbon steel bars and four 60 mm solid carbon steel bars. Change
the two 40 mm bars to 60 mm bars. This modification will increase the material cost
somewhat, but will reduce the number of products by one. This modification will probably
not reduce the safety of the structure, since you are making the two
40 mm members stronger. If the reduction in product cost exceeds the
increase in material cost, the change is a good one. If not, reject the change by clicking
the .
Continue this trial-and-error process of selectively increasing member sizes (or using
stronger materials) to reduce the total number of products in the design. Generally, you
will find that reducing the number of products creates substantial cost savings at first;
however, as the degree of standardization increases, the cost savings get progressively
less. Ultimately, too much standardization will cause the total cost of the design to rise.
The design that minimizes total cost is the optimum.
Before moving on to the next step in the design process, be sure to run the load test one
more time, even if all of your modifications involved making members
larger. Increasing the size of a member makes that member both stronger and heavier.
When member weights increase, the total weight of the truss increases. As a result of this
increase in load, member will also increase, and some members which were previously safe might become
unsafe.
Notes and Tips
You can optimize member properties very efficiently by taking full advantage of the sorting
and multiple selection capabilities of the Member List.
How to design a bridge
Go back one step
Go forward one step