Truss Series: Howe Truss

The Howe Truss was designed by William Howe in 1840. It used mostly wood in construction and was suitable for longer spans than the Pratt truss. Therefore, it became very popular and was considered one of the best designs for railroad bridges back in the day. Many Howe truss bridges exist in the North West United States, where wood is plentiful.

How the forces are spread out

Here are two diagrams showing how the forces are spread out when the Howe 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.

Similar to all the major truss designs (Pratt, Warren, K Truss, and Howe), when the load is centered on the bridge the forces are much greater on the internal truss members than if the load is spread out along the top of the bridge. The same principle applies if the load was coming from the bottom of the bridge. I use diagrams showing the load applied to the top of the bridge, because this is how I most often test my bridges. I load my bridges from the top.

When you are designing your bridge, I recommend that you use the Bridge Designer program from JHU and plug-in your design. Load the design in the same way your bridge will be loaded as specific in the rules and guidelines you were given to build your bridge.

Howe Truss in model bridges

One thing that you have to keep in mind when thinking about the common truss designs, including the Howe, is that they were designed a long time ago. They were designed when bridges needed to fill a specific role, and for the particular resources that people had available. For instance, the Howe truss design used a lot of wood as opposed to the Pratt which used more iron. This made the Howe popular earlier on when iron was expensive to produce.

The Howe truss used wooden beams for the diagonal members, which were in compression. It used iron (and later steel) for the vertical members, which were in tension. The Pratt truss was the opposite. Thus, because the diagonal members are longer, the Howe truss used less of the more expensive iron material. It made good use of the cheap wood which was readily available.

For model bridges, we typically only use wood. Our compression and tension members are both made out of wood. If you wanted to be fancy, you could use string or metal wire for the tension members. Nonetheless, in reality, the reasons why the Howe design became popular are not applicable to model builders. It remains a solid engineering model design, but I think I would prefer the Pratt truss over the Howe.

Additional Resources

Pictures of Real Howe Truss Bridges
History of Howe Truss

44 thoughts on “Truss Series: Howe Truss”

  1. The pattern of diagonals is asymmetric for an odd number of bays, as shown in view at top of post with green members. It would be worthwhile to ask a bridge engineer if they would design a Howe Truss with odd number of bays. As an engineer myself I would set the size of each bay in order to produce an even number of bays and therefore the number of diagonal right and diagonal left members will be symmetric around the center point of the bridge.

    I noticed this because my daughter’s class purchased the 21″ Howe Truss Kit, and most of the color photos in the instruction package have some diagonal members incorrectly shown — the scaled plan views are correct, with 4 bays diagonal left and 4 bays diagonal right. However the photos in instruction package show 5 bays diagonal right and only 3 diagonal left! Please review this 🙂 It’s bound do drive any OCD engineers who see the kit crazy.

    • Jen, thank you for pointing this error out and I will get it fixed with new photos. I also appreciate your design feedback about having even bays. I’m actually in process of updating all of the models with slight tweaks, and will take this into consideration for the 100 Stick Howe Truss model.

  2. The diagonal members are wrongly shown. They should been red instead of blue; they are in compression and not in tension

    • Sorry for the confusion. These diagrams were created with the now defunct JH Bridge Designer, which represented tension in red and compression in blue. However, it looks like I forgot to notate this.

  3. Thank you for all this lovely information on Warren, Howe, and Pratt trusses. In high school we are learning each and your cite has helped me complete my tasks and goals.


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