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

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/

There was one major complication. Instead of being provided with sand to load our towers as the rules specify, we were given pebbles. You can probably imagine the thoughts running through my mind.

But I was surprised at how much my tower held, 14.6kg, and took home a gold. Now on to states. I am hoping for a sub 7 gram tower.

Here is an outline I made for the presentation:

• History of Bridges
• Nature
• Greek
• Roman

• Types of Bridges
• Arch
• Da Vinci Bridge
• Truss Bridge
• Suspension Bridge
• Cable Stay Bridge
• Moveable Bridges

• Tacoma Narrows Bridge

• For A Contest
• Read the Rules!
• Design Within an Envelope
• Why use Triangles
• Compression and Tension
• Building: Wood and Glue
• Joints
• Bending Wood
• Construction Techniques
• Check Bridge Against Rules
• Weigh Bridge

• Testing a Bridge
• Different Techniques
• Safety
• Efficiency

What do you think about this presentation? Is there anything you disagree with?

Important Link in Video:

1997-2006 Academic Years – Mechanics: Bridges

Arch: A structure that is curved and carries weight in a vertical manner primarily by using x-axis compression.

Beam: Horizontal structures that hold a vertical weight while not bending. Girders are multiple beams placed together and are usually the foundation of a truss.

Bridge: A structure used in aiding humans and animals in transportation over gaps, rivers, etc.

Column: The part of a bridge that connects the footing to the bottom of the bridge’s deck.

Deck: The surface of a truss or bridge that people or things drive and walk across.

Fixed Arch: A structure that is permanently in a single area/position.

Footing: The part of a bridge that is under ground level.

Member: A part in a structure, most especially a truss.

Portal: The open ends on a Through Truss, a.k.a the entrance.

Span: The length in between the inner edges of two of the “legs” of a structure.

Strut: A member that is compressive.

Substructure: Bridge parts below deck.

Superstructure: Bridge parts deck and above.

Suspenders: Tension members on the cable from the main cable to the deck of a suspension bridge.

Tie: Tension member of a truss.

Tower: A large frame holding the cables of a suspension bridge.

Truss: A stronger form of a beam or girder made with a web of members.

I hope this helps you build and describe your bridges for your classmates, students, and/or professors. Happy building everybody! If there is any other term that you don’t understand, post a comment and I will attempt to define it for you.

These two questions are dealing with making an elevated bridge, very similar to the Science Olympiad challenge.

1: I’m currently building an elevated bridge (for my physics class, not for scioly) and I have finished the two trusses and connected them and now only need to add the towers. Would it be a good or bad idea to make the towers slant outwards or should I just have them go straight up and down?

The only benefit to slanting the towers is that the main trusses can be a little shorter. This will reduce the weight of the bridge. The forces will increase on the towers if they are slanting, and then the towers will have to increase in weight. There is a balance to find. Since you have already built the main trusses, slanting the towers will not really help you. Also, it is significantly easier to build the towers so they are straight up and down.

2: For the bracing on the towers, I want to make X’s but I’m not sure how to do it exactly. Would I put one diagonal piece on one side of the corner and the other piece on the other side or put them on the same side and have one overlap the other?

Good question. I would recommend that you overlap the X’s. The X’s themselves do not need to be large pieces and thus won’t add a lot of weight. Rectangle shaped pieces work best. 1/4″ wide and 1/16″ thick should be more than sufficient. In one tower I built, the X’s were 1/32″ deep and 3/32″ wide. I never had a problem with the X’s breaking, it was always something else that failed.

Here is my Science Olympiad Tower.

Elevated Bridge Resources:

Diary of a Bridge Builder

SciOly Elevated Bridge Thread

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