The Pratt Truss originated from Caleb and Thomas Pratt (father and son) when they applied for a patent in 1844. Interestingly, their patent had less to do with design and more to do with a method for building trusses. After reading their patent application, my understanding is their patent was related to the type of connections of the truss members to the bridge frame as well as the inclusion of iron. Iron was becoming cheaper during that time, and offered improvements over all wood bridges.
Their construction method quickly became very popular and thus their name has been stuck to a particular truss design that we know today as the Pratt truss, although it is different than their patent.
You can read their original patent at the USPTO Website.
You can see that their example drawings do not look like the current “Pratt Truss”. To further complicate matters, the Pratt has many variations, most with their own unique name. For instance, the Baltimore, Pennsylvania, and the Parker are all based off the Pratt. However, today we have a commonly accepted truss design known as the Pratt, and it is very useful when building real and model bridges.
Let’s explore why.
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 a localized load in the center of the bridge. The second shows the load being applied across the entire top of the bridge. In both cases the total load = 100.
Please note that the force diagrams do not change if the load is applied to the bottom of the bridge instead of the top.
These diagrams bring up several interesting things. Notice that there is no difference in force on the two end angle members. 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. In addition, the forces are increased on the top chord when the load is centered.
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 goes for any model bridge design, not just the Pratt Truss.
Is the Pratt Truss any good for model bridges?
The Pratt Truss design as pictured above 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.
What makes it so good, and gives it an advantage over the Howe truss, is how it spreads out the forces when under load. The longer, angled truss members are under tension. The shorter, vertical members are under compression. You’ll even notice a few vertical members have zero load.
Why does this matter?
Wood will eventually buckle (break) when under compression. The longer a piece of wood is, the sooner it will buckle when all other things remain the same.
To avoid buckling, you either have to increase the strength of the member by making it bigger, changing materials, or changing the shape of the member.
Or you can make it shorter. This is what the Pratt truss design does compared to the Howe truss.
The roots of the Pratt truss involved using iron, not wood for the angled members. It used wood for the vertical members. However, with most model bridges, you are just using wood. I think this does give the Pratt an advantage, but read my Best Bridge Design article more for information.
Better yet, build your own Pratt and Howe truss bridges and compare their strength on your own. I have kits and plans just for that purpose in my store.