Introduction: (Initial Observation)

Bridges are amongst the most fascinating commodities of engineering and physical science. If designing and building a bridge you have to overcome many difficulties and constraints.

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In a bridge task you will be asked come design and construct a leg that will hold the many weight for a offered span. Now you are more than likely wondering where to start. What form of leg is the strongest? (We get that question typically from kids.) That question doesn’t really have an answer. There space too plenty of variables associated for the question to be meaningful. For example, what are your product constraints? Certainly, it would certainly not it is in of much use come you were i to tell friend that rock arches to be the strongest, when you were already instructed to construct your leg out the toothpicks.

The idea the “constraints” is an essential one. Her teacher may have actually told you the you must build your bridge out that toothpicks, or Popsicle sticks, balsa wood, or maybe also spaghetti. The key reason the you were given the rule is to “constrain” you similar to bridge engineers everywhere. Prior to a bridge project begins bridge designers are called what the constraints space – what us can and also can no do. It varies from job to job, and also that’s what provides it fun.


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This project guide consists of information that you require in bespeak to start your project. If friend have any type of questions or need an ext support around this project, click the “Ask Question” switch on the top of this page to send me a message.

If you are new in doing scientific research project, click on “How to Start” in the main page. Over there you will find helpful links that describe different species of scientific research projects, clinical method, variables, hypothesis, graph, abstract and also all other general basics the you must know.

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Information Gathering:

The complying with is some basic information the can aid you design and construct her bridge. I suggest a balsa hardwood bridge and also initial drawing on the paper. The key idea in making the leg is to redirect all pressures to tensile and compressive forces. The factor that we carry out that is that most building material have actually a high tensile strength or a high compressive strength, yet they have tiny strength if you try to bend them. Converting weight pressure to tension and compression is done by creating triangles. A great on-line program to help you learn just how weights distribution in a triangle is build a Truss Program. This program enables you to develop a truss bridge model and also test it!

You can additionally click Bridges come see pictures of bridges make by other students.

Structure Bridges selling kits and also videos and additionally has a tips page.

Form of Bridge

There room three major types that bridges: (See much more details and diagrams)

the Beam bridgethe Arch bridge

The biggest difference between the three is the distances they have the right to each cross in a solitary span. A expectancy is the distance in between two bridge supports, even if it is they are columns, towers or the wall of a canyon. A modern-day beam bridge, because that instance, is likely to expectancy a distance of as much as 200 feet, if a modern-day arch have the right to safely expectancy up come 800 or 1,000 feet. A suspension bridge, the pinnacle of bridge technology, is capable of covering up come 7,000 feet.

What allows an arch bridge to span greater ranges than a beam bridge, or a suspension bridge to expectancy a distance practically 7 times the of an arch bridge? The prize lies in just how each bridge kind deals through two vital forces called Compression and Tension:

Compression is a force which acts to compress or shorten the point it is acting on.Tension is a force which acts to broaden or lengthen the thing it is acting on.

Beam bridges come in dozens of various styles. The design, location, and also composition the the truss is what determines the type.


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I will certainly assume that you have been told the the bridge must span a particular distance and hold a particular weight (or the most weight compared to the various other student’s bridges). To be I right so far? The first thing the we have to do is to pick a material. That was probably done because that you by her teacher. The following thing that we must do is to pick a bridge kind or type.

For the form of project that you many likely have actually been given, the best style would more than likely be the Warren truss bridge as shown above. The Warren truss is a favourite of the railroads. Each truss (there room two, one on every side the the bridge) is composed of it is provided triangles. The bridge displayed in the photo is a “Warren truss v verticals” so every equilateral triangle is split into two smaller triangles. You deserve to accommodate various expectations lengths by differing the length of your members (the sides of the triangles) and/or the number of panels (the number of triangles). If you decide to include panels friend should include them come the center section wherein the chords (the upper and also lower members) space parallel and also the structure is the deepest.

To see samples that nicely created 34 inch-span Warren truss v verticals developed by s County university students click the link. Few of these models assistance close to 200 time their very own weight!

Another usage of the Warren truss can be seen here in one of the spaghetti bridges.


The importance of Connections

It is difficult to overstate the prominence of relations to the stamin of your bridge (or any type of structure for that matter). Really!Look at the photo at the peak of the page. Notification the “gusset plates” in ~ the connections? This kind of information is very important. Stresses flow like water. Whereby members come with each other there are stress concentrations the can damage your structure. It would certainly be very useful if you can visit a truss bridge and take a look at the relations there. If you are structure the leg out that balsa wood cut out and also glue gusset plates similar to what Andy McConnell go in his well developed bridge shown above. If you deserve to use just toothpicks you might wish to glue little triangles of parts of toothpicks approximately the joints to make a sort of gusset “plate”.

Here is a connection detail of one of the spaghetti bridges.


The properties of Balsa Wood

It is probable that your task will use balsa timber (Sold in understanding stores and some craft stores). The lightest of the commercially easily accessible hardwoods, it grows naturally in the humid rain forests of central and south America. The best stands that balsa usually show up on the high ground in between tropical rivers whereby there is many of rainfall and an excellent drainage.

Balsa hardwood is so light due to the fact that the cell of the hardwood are big and an extremely thin walled, so the the ratio of solid matter to open space is really small. Woods generally have a gooey cement, dubbed lignin, stop the cell together. In balsa, the lignin content is at a minimum. Only about 40% the the volume that balsa is solid substance. In a living balsa tree, the remainder of the volume is filled v water. That gives the tree, which can thrive to 60 come 90 feet tall, the rigidity come stand. Every balsa cabinet is normally pumped complete of water until it i do not care rigid lot like a tire full of air. Environment-friendly balsa wood must be kiln dried come remove most of the water prior to it deserve to be sold.

If friend are building a framework out of balsa wood (or any other lumber for that matter) you must bear in mind that as a herbal material that is properties will vary considerably from item to piece. Some of the variables associated include where and when it was grown, the orientation the the grain, the existence of irregularities, and the thickness of the separation, personal, instance sample.

You should select your piece so as to eliminate any kind of obvious imperfections.

You can use the following property worths for design purposes:


Density0.00589 ± 0.00036 lb/in³or 163 ± 10 kg/m³
Compressive Strength ¤low densitymedium densityhigh density 680 lb/in²1750 lb/in²2830 lb/in²
Tensile toughness ¤low densitymedium densityhigh density1100 lb/in²2890 lb/in²4670 lb/in²
Elastic Modulus – CompressionElastic Modulus – Tension 66,700 ± 10,300 lb/in²185,300 ± 65,400 lb/in²

¤ Low density = 75 kg/m³ (0.0027 lb/in³); Medium thickness = 150 kg/m³ (0.0054 lb/in³); High density = 225 kg/m³ (0.0081 lb/in³)

All else being equal, girlfriend should choose pieces of better density since the strength increases much more than the massive does. If girlfriend really desire to optimize your project, friend can click the attach to calculate the forces in her trusses and then select your highest density wood for the members through the highest stresses.


Computers can be offered in making bridges and also other design structures. The West point Bridge Designer is just one of the software application programs obtainable to student for designing bridges. This program will present you to engineering through an authentic, hands-on architecture experience. This software gives you through the devices to model, test, and also optimize a steel highway bridge, based upon realistic specifications, constraints, and performance criteria.


You can likewise use WPBD to style bridges and also participate in the West point Bridge style Contest.

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Note that usage of software is not required for this project; however, trying this software is a great step to familiarize yourself v engineering architecture programs.