How the forces are spread out
Here are two diagrams showing how the forces are spread out when the Pratt Truss is under a load. The first shows the load being applied across the entire top of the bridge. The second shows a localized load in the center of the bridge. In both cases the total load = 100. Therefore, you can take the numbers as a percentage of the total load.
Pratt Truss With Centered Load Pratt Truss with Spread Load
These diagrams bring up several interesting things. Notice that the two end diagonal members do not change. Also, there is little change on the bottom chord between the two pictures. However, there is drastic changes on the internal truss members. The centered load dramatically increases the amount of force that is applied to the internal members of the bridge. Also, the forces are increased on the top chord of the centered loaded bridge.
This seemingly insignificant change in how the bridge is loaded makes a big difference in how your model bridge will perform. If you have the ability to change and set how your bridge is loaded, I’d shoot for spreading the load across the entire span. This pretty much goes for any model bridge design, not just the Pratt Truss.
Pratt Truss for model bridges
The Pratt Truss is one of my favorites. I have used it often for my model bridges, including balsa, basswood, and popsicle sticks. It is easy to construct, and is a solid choice for a model bridge design.