Cost
The construction of any object has costs that are associated with it. Not all of these costs are measured with money! Things such as the loss of a forested area where people used to hike for relaxation due to the bridge's construction will have a very large negative impact on people's lives, and should be taken into account.
In your bridge-building project you will be tracking cost indirectly by keeping track of the number of Popsicles that you and your group use in your model bridge.
See also aesthetics, function, materials, and safety.