Howe Truss

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 trussed 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

Pratt Truss

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.

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.

Additional Resources

Pictures of real Pratt Bridges
History of Truss Design

Warren Truss

How the forces are spread out

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

Interestingly, there is a significant difference. When the load is concentrated on the middle of the bridge, pretty much all the forces are larger. The top and bottom chord are under larger forces, even though the total load is the same. Thus, if you want your school project bridge to be able to hold more weight then try to spread out the force across the top of the bridge.

For a real life Warren Truss bridge, the forces often will be very localized and not spread out along the bridge. Thus, engineers must calculate how strong to make each member of the bridge and build accordingly. Unfortunately, not many Warren bridges are made anymore.

Warren Truss for model bridges

I have definitely used the Warren truss design for many balsa and basswood bridges. I have also used for some popsicle stick bridges. In fact, you can purchase blueprints for a couple in my store. I think the Warren is a very solid choice when designing a model bridge. If you do not know how to start designing your own bridge, I would recommend the Warren, or the Pratt or Howe trusses.

The Warren truss is easy to use with Lap Joints, which are very strong joints. Find out more on my Bridge Joints page. All you have to do is lay down your top and bottom chords, and glue on the truss members directly on top of the top and bottom chords. The example bridge that I build in my 5 Steps to Building a Model Bridge ebook is a Warren Truss design.

Additional Resources

Pictures of real Warren Truss Bridges
History of the Warren Truss

A well done introduction to the practical steps of truss design.

Written in 2004. It has 448 pages and has an average of 4.5 star rating on Amazon.

Contains Illustrations.

Here is the link to purchase this book at Amazon:
Design of Building Trusses (Parker/Ambrose Series of Simplified Design Guides)

The Dr. Software is very comprehensive and in-depth. You can do a lot of different kinds of analysis on your truss design. One of the coolest features is that all your modifications are shown in real time. Even while you are dragging a joint to a new location the program shows you the changes in the forces and moments of inertia. I think that the Dr. Software can be a serious help to the serious model builder who wants to better understand the physics of truss design. Check out their website:

http://www.drsoftware-home.com/

K Truss

How the forces are spread out

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

The K truss shows the smallest amount of change from the two types of loads on the top and bottom chords. In fact, there is very little difference between the two for the top and bottom. For the internal members, however, there is a large change. As usual, the concentrated load increases the forces on most members. Interestingly, on the K Truss, some members change from tension to compression. Notice this on the top half of the vertical members.

K Truss and model bridges

I think the K truss, while being more complex and more difficult to build, could be a good option for model bridges. I have not build a K Truss bridge yet, but if I get the chance I would like to try it out. Many of my readers have reported success with this design.

Additional Resources

1. Pictures of Real K Truss Bridges
2. Some K Truss History

The answer to that question is not simple and probably is not going to be what you expect. The truth is, I cannot answer the question. There are too many variables that are not being defined.

Example:
Is the the Pratt truss or the Warren truss stronger? Actually, the answer is up to you. You can make the Warren truss stronger. Or you can make the Pratt stronger. It depends simply on the strength of the wood you use. You can use 2×4′s to build the Warren, and toothpicks to build the Pratt. Obviously the Warren is going to be stronger in this case.

A Better Question

I think though, many people are trying to ask whether or not one of these trusses has an inherent advantage over the others. In my mind, none of them do. If you look at my truss design page, you will see that each truss spreads a load out differently, but none with an apparent advantage. Conclusion: no bridge design has an inherent strength over another.

However, this conclusion only applies to this general setting. It may be that in a specific situation one bridge design would be better suited than another. It is up to you to examine how each truss works and decide which one to use.

h3>How Changing Height Affects a Bridge

Control Bridge

Now I will show you another bridge, with the same design. The only difference is that this bridge is 4 inches tall, one inch taller than before.

1 inch taller

What does this mean to you?

As you saw in the example bridges, by increasing the height of the bridge you decrease the load on the top (and bottom) chord. A decrease in load means you can make it smaller. Smaller means lighter in this case. However, there is a catch.

Look back at the two examples. By increasing the height, the load decreased on the top and bottom chords but remained the same on the middle “truss members”. Why is this a drawback? By increasing the height, the middle members have to become longer. That inherently adds weight. However, bridge builders have another problem when pieces become longer. The amount a piece of wood can support in compression before buckling decreases with length. So that means the middle members will have to be made stronger the taller the bridge is. Adding strength usually means adding weight.

So we have a draw

By increasing the height of a bridge, you can make the top and bottom chords lighter. But at the same time, the middle truss members have to be made heavier. The goal is to find a balance. I believe that there is an optimal height for a model bridge in every situation. The trick is to find that height, by balancing out these two factors. I was once told that the optimal height for an arch bridge is 1/6th the length of the span. That means if your bridge is 6 inches long, it should be 1 inch tall. My experience has confirmed that this is a very good starting place for an arch bridge. And I believe the same ratio of height to length can be applied to regular truss bridges.

The Optimal Height

Interestingly, the Golden Gate Bridge uses the 1/6th ratio almost exactly. Perhaps you can do some studying on other real bridges and see if they follow this idea also.

Data taken from the Bridge Designer.

Common trusses used in engineering:

Warren Truss

Warren Truss

Go to a more in depth analysis of the Warren Truss.

Pratt And Howe Truss

The Pratt and Howe trusses are very similar. In fact, the only difference is the direction the slanted members are angled. This changes which members are in compression and tension. On the Pratt truss, the shorter, vertical members are in compression. However, on the Howe truss, the longer, angled members are in compression. Because most materials (especially wood) that model bridge builders use decrease in the ability to resist compression the longer they are, I think the Pratt truss has an advantage.

There are more factors to consider, however. The Pratt and Howe trusses also differ in how they spread the load to the top and bottom chords. The Pratt truss has larger forces on the top and bottom chords than the Howe. Thus. you’d have to use bigger top and bottom chords.

Go to a more in depth analysis of the Pratt Truss.

Go to a more in depth analysis of the Howe Truss.

K Truss

K Truss

Go to a more in depth analysis of the K Truss.

The one thing I don’t like about this truss is the long vertical compression member in the middle of the bridge. If that one member could be shortened or even eliminated, I think the bridge would become more efficient.The K truss would be the hardest of these trusses to build. This is something worth considering. Making a strong joint that would make the most of the switch between compression and tension of the vertical members would be difficult.

If you are interested in learning more about trusses and truss design, check out Truss Fun, Second Edition from amazon. It can be purchased online though some simple credit card processing from Flagship Merchant Services. This is a comprehensive study on the engineering principles behind the design of bridges. It is easy to understand and to follow, and is a great fit for students who are just learning, but advanced enough to be a great resource to those with more experience. For more great resources, see this list of other great bridge books.

I was first introduced to the Bridge Designer by a civil engineer, who explained to me the usefulness of the free program. However, it took me, a 6th grader at the time, over a year of playing around with it to finally figure it out. With all of the time that I have spent using this program, it probably would have been to my benefit to hire a live answering service to intercept all of the calls I’ve missed while using it. This program has some limitations that you must understand if you are going to use it properly. Watch the video tutorial or read through the steps.

Link to the Bridge Designer

The Bridge Designer allows you to create a virtual truss, put a load on it, and see how the load is spread out. It is very useful to use after you test a bridge to failure, then plug in the bridge design, and see how much force it took to break it.

Here is a text version of the video. When you first load the program, you are presented with this:

To begin, you will need to click the “Add Nodes” button. A “node” is simply a joint. You must add the joints before adding the actual members.

Usually, I start with the bottom left corner of the bridge, and then count over to the right however many squares as my bridge is long. I count one square per inch. If your bridge is really short, you might do two squares per inch. Or if it is really long, 1/2 square per inch.

After I have the bottom length, I add the top nodes. It might look something like this:

Now it is time to add the members. Click the “Add Members” button. Left click on one of the nodes you have added, and drag the mouse over to the next one. You must hold down the left button to do this. It doesn’t matter what order you add the members, but you must connect every node.

After adding all the members, click the “Calculate” button on the bottom. Now look just below the top buttons. In red, you should see something like “There must be one fixed and one roller node”. If you see “Members +3 must equal twice the Nodes”, you need to adjust either the number of nodes or members.

Once everything is okay, click the “Fixed node” button which is one the bottom left. Then click the bottom left node. That node should turn yellow. Make the bottom right node the “Horizontal rolling node”, it should turn red.

Alright, you’re almost done. Click the “Add a Load” button, and choose which node to put it on. You can add more than one load if you want. Once you click that node, Pull your mouse straight down. You should see a number right next to the load increase as you go farther down. Once the load is correct, click once. If you mess up, don’t worry. You easily remove any load, member or node using the buttons.

Now click the “Calculate” button again, and also click anyway on the grid. If everything is set up right, the members should change colors to red, blue, and sometimes green.

You might get the message, “Cannot compute, matrix is singular”. If you do, then somewhere a member might be missing. You need to create triangles for this program to work correctly. If it doesn’t think that your design is “safe”, it won’t work.

Occasionally the program thinks I have an extra node somewhere, and the only fix is to clear everything and start over. Don’t worry, with time you will get the hang of it and create designs in just a few minutes.