| [ Form of Bridge | Connections | Properties of Balsa Wood | Additional Resources | Further Reading ] |
So, you've been given a project to design and construct a bridge that will hold the most weight for a given span. Now you are probably wondering where to start. What type of bridge is the strongest? (We get that question frequently from kids.) That question doesn't really have an answer. There are too many variables involved for the question to be meaningful. For example, what are your material constraints? Certainly, it would not be of much use to you were I to tell you that stone arches were the strongest, while you were instructed to build your bridge out of toothpicks.
The idea of "constraints" is an important one. The teacher did not tell you that you must build your bridge out of toothpicks (or popsicle sticks, balsa wood, or maybe even spaghetti) for no reason. Certainly one reason was so that it would be easier to compare your bridge with that of your fellow students. But the main reason that you were given the rules that you were was to "constrain" you just like bridge engineers everywhere. Before a bridge project begins we bridge engineers are told what the constraints are - what we can and can not do. It varies from job to job, and that's what makes it fun.
I will assume that you have been told that the bridge must span a certain distance and hold a certain weight (or the most weight compared to the other student's bridges). Am I right so far? The first thing that we must do is to pick a material. That was probably done for you by your teacher. The next thing that we must do is to pick a bridge form or type.
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| Picture courtesy of Overland Models |
For the type of project that you most likely have been given, probably the best design would use the Warren truss bridge as shown above. The Warren truss is a favorite of the railroads. Each truss (there are two, one on each side of the bridge) is composed of equilateral triangles. The bridge shown in the picture is a "Warren truss with verticals" so each equilateral triangle is divided into two smaller triangles. You can accommodate various span lengths by varying the length of your members (the sides of the triangles) and/or the number of panels (the number of triangles). If you decide to add panels you should add them to the center section where the chords (the upper and lower members) are parallel and the structure is the deepest.
 A nicely constructed 34 inch-span Warren truss with verticals built by Ocean County College student Andy McConnell. It supported 188 times its weight! |
| Picture courtesy of Ocean County College |
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An interesting adaptation of the Warren truss is the bridge built by high-school student Tim Kaucher. It was made out of 400 toothpicks and Elmer's Wood Glue, and it supported his teacher |
| Picture courtesy of Tim Kaucher |
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Another use of the Warren truss can be seen here in one of the spaghetti bridges built by students at Johns Hopkins University. |
| Picture courtesy of Professor Michael Karweit, Johns Hopkins University |
Look at the picture at the top of the page. Notice the "gusset plates" at the connections? This kind of detail is extremely important. Stresses flow like water. Where members come together there are stress concentrations that can destroy your structure. It would be very useful if you could visit a truss bridge and take a look at the connections there. If you are building the bridge out of balsa wood cut out and glue gusset plates similar to what Andy McConnell did in his well built bridge shown above. If you can use only toothpicks you may wish to glue little triangles of parts of toothpicks around the joints to make a kind of gusset "plate".
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Here is a connection detail of one of the spaghetti bridges. |
| Picture courtesy of Professor Michael Karweit, Johns Hopkins University |
Balsa wood is so light because the cells of the wood are big and very thin walled, so that the ratio of solid matter to open space is very small. Woods typically have a gooey cement, called lignin, holding the cells together. In balsa, the lignin content is at a minimum. Only about 40% of the volume of balsa is solid substance. In a living balsa tree, the remainder of the volume is filled with water. That gives the tree, which can grow to 60 to 90 feet tall, the rigidity to stand. Each balsa cell is naturally pumped full of water until it becomes rigid much like a tire full of air. Green balsa wood must be kiln dried to remove most of the water before it can be sold.
If you are building a structure out of balsa wood (or any other wood for that matter) you should bear in mind that as a natural material its properties will vary considerably from piece to piece. Some of the variables involved include where and when it was grown, the orientation of the grain, the presence of irregularities, and the density of the individual sample.
You should select your pieces so as to eliminate any obvious imperfections.
You can use the following property values for design purposes:
| Density | 163 ± 10 kg/m³ | 0.00589 ± 0.00036 lb/in³ |
| Compressive Strength¤ low density medium density high density |
4.7 MPa 12.1 MPa 19.5 MPa |
680 lb/in² 1750 lb/in² 2830 lb/in² |
| Tensile Strength¤ low density medium density high density |
7.6 MPa 19.9 MPa 32.2 MPa |
1100 lb/in² 2890 lb/in² 4670 lb/in² |
| Elastic Modulus - Compression Elastic Modulus - Tension |
460 ± 71 MPa 1280 ± 450 MPa |
66,700 ± 10,300 lb/in² 185,300 ± 65,400 lb/in² |
All else being equal, you should select pieces of greater density because the strength increases more than the mass does. If you really want to optimize your project, you can calculate the forces in your trusses and then select your highest density wood for the members with the highest stresses.
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Building Big: Bridges From PBS |
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Balsa Bridge Building Contest A super website by the Notre Dame Regional Secondary High School, Vancouver, B.C.. |
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Here is a tremendous design tool for you: Johns Hopkins Virtual Laboratory: Bridge Designer You can use it to check the forces in your trusses. |
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From the United States Military Academy: West Point Bridge Designer |
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Check out the excellent tutorials on structures using the PolymorfTM system. |
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Truss Fun
by David W. Harris, PhD., P.E. Paperback - 144 pages (July 2000) BaHa Enterprises; ISBN: 0967549507 ; Dimensions (in inches): 0.40 x 11.00 x 8.50 High school level. A fun and interesting book based on wood and metal trusses from the 1850's to 1900's, the book includes both easy explanations and a workbook for students. "Written for high school students, (and early college students), this book is for kids of all ages who still wonder about the things around them and how they work. Modelers, hobbyist, historians alike will find useful." Price: $19.95 |
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Newspaper Truss by David W. Harris, PhD., P.E.
CD-ROM - 250 pages CD-ROM edition (August 21, 2001) BaHa Enterprises; ISBN: 0967549515 ; High school level. CD contains PowerPoint modules covering geometry, bridge analysis using vectors, video clips, and other resources to enable you to build your own truss bridge out of newspaper that you can walk across. Price: $12.95 |
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Bridges : Amazing Structures to Design, Build & Test
by Michael P. Kline (Illustrator), Carol A. Johmann, Elizabeth J. Rieth Paperback - 96 pages (October 1999) Williamson Publishing; ISBN: 1885593309 ; Dimensions (in inches): 0.33 x 10.08 x 10.02 Reading level: Ages 7-14 This is a wonderful book. It covers the theory and history of bridges. The behavior of arches, beams, trusses, cantilevers, and suspension bridges is explained in a way that is fun and intuitive. The numerous projects, including fabricating arches, trusses, and coffer dams, are well explained. Notable bridge failures (including the Silver Bridge, the Ashtabula Bridge, and Tacoma Narrows are covered as are the contributions of men like Ellet, the Roeblings, Whipple and Ammann. Price: $8.76 |
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