Background - Factors in Bridge Design

A number of factors must be taken into account when an engineer is designing a bridge. Failure to consider any important factor could result in failure (breaking) of the bridge.

• What is the length/span of the bridge? Span is the length of the bridge from one side (bank) of the river to the other.

• What will be the load on the bridge? What type of load(s) will it experience?
The engineer considers two types of loads: dynamic (live) the weight of the cars, trucks, or people expected on the bridge; or static (dead) the weight of the bridge itself.

• What environmental factors will effect the bridge? Will the bridge need to withstand strong winds, ice, snow, extreme cold or heat?

• What is the budget for the bridge?

• What are the soil characteristics of the riverbanks? Sandy? Clay-like? Rocky?

• What are the soil characteristics of the bottom of the river if supporting columns are required in the bridge design?

• What is the time frame in which must be built? “Galloping Gertie”
Destroyed by a 42-mph wind

After considering these and other factors, including the aesthetics (attractiveness) of the bridge, the engineer is ready to begin the design.

The Physics of Bridge Design

The terms Load, Compression, and Tension are terms used in design. When a load is placed in the middle of the beam, it tries to compress on the top of the bridge (compression). The bottom of the bridge tries to pull apart, creating tension.

Racking is a kind of stress that distorts a square or rectangle, causing it to be a parallelogram as shown.

To strengthen a square or rectangle, a diagonal brace converts (changes) the rectangle into two triangles, making the figure much stronger. This action minimizes the racking effect.

Activity – Construction Techniques

When making wooden models of bridges, joints are the most critical aspects of making a strong model. Once a joint fails, the structural integrity of the entire bridge is weakened. This weakening starts a chain reaction, which cause the total failure of the structure.

Using different types of joints can increase wood joint strength. While certain types of joints are more effective than others, you also need to consider your skill level. Notching a joint will make it stronger, but only if the joint is made accurately. Otherwise the joint will fail.

Activity -- Requirements:

You will be using regular white glue for your bridge, no other glue will be allowed. Keep in mind that glue does not work as well on the end grain of wood. You will make a stronger bridge if your joints are made with the grain side of the wood.

Your completed bridge should not weigh more than 5 ounces.

No continuous lamination except where the sides are joined to the bottom piece. (Continuous lamination is the joining of two layers on top of each other). There must be at least the width of 1/8th inch (the width of a piece of the wood) of space between two layers of wood, otherwise it will be considered continuous lamination.

Once you have designed and built your bridge it is time for testing. Once you have the load of the bridge (from the tester) then you will calculate the efficiency of your bridge. The load, in pounds, that your bridge holds is multiplied by 16. This will give you a failure rate. This failure weight will then be divided by the weight of your bridge. The formula is:

Efficiency = Load (lbs.) x 16 (oz/lb)
Mass of bridge (oz)