A Beam bridge or "Girder" bridge is the simplest kind of bridge. As its name implies it is made from beams or girders. These beams are designed to resist the forces or load applied to the bridge by traffic. The 'Truss' bridge has beams that are arranged in triangles, which make the bridge even stronger.
In the past a simple beam bridge may have taken the form of a log or slab of stone placed across a stream.
There are lots of different types of beam bridges. Have a look at the images shown opposite.
Can you find more images of beam bridges? Can you produce a labelled sketch of a Beam, Girder or Truss Bridge? Where is the beam bridge? What materials have been used to make it? Are there any beam bridges near where you live?
A beam bridge needs to be stiff. It needs to resist 'twisting' and 'bending' under load.
In its most basic form, a beam bridge consists of a horizontal beam that is supported at each end by piers. The weight of the beam pushes straight down on the piers.
Under load, the beam's top surface is pushed down or compressed while the bottom edge is stretched or placed under tension. If we imagine that there is an imaginary line running down the centre of the beam this line remains at its original length while the material above is compressed and the material below is stretched. This line is referred to as the neutral axis.
As the supports or piers move further apart, the weaker a beam bridge gets. As a result, beam bridges rarely span more than 250 feet or 80 meters. This doesn't mean beam bridges aren't used to cross great distances, it only means that there may be a series of beam bridges joined together, creating what's known as a "continuous span''.
Opposite there is an image of a a Large Box Girder Bridge or Truss Bridge. The beams have beam arranged into triangles to make the bridge even stronger
Bridges are made using BEAMS
The design of the beam and the materials used to make the beam is very important.
The Engineer uses knowledge of Science, Mathematics and Technology to make bridges from beams that are strong enough to transmit all of the forces through the beam to the surrounding earth.
Beams used in bridges and buildings may vary in cross sectional shape. Some may be solid or hollow. Opposite there are images of three different shaped beams. The first beam is a box section beam, the second is an I section beam and the third is an L section beam. Solid beams are heavier than hollow beams. Beams like the one's shown are given a special cross section for strength and rigidity. They may be as strong as the solid beams but are a lot lighter.
We may describe them as having a good strength to weight ratio.
BEAMS: CHALLENGE YOURSELF....MAKE AND TEST CARD BEAMS
- Using card and glue make the three different beams. Test the beams individually for structural strength and resistance to loads (stress) in compression and tension. You can achieve this by placing the beams between either desks or chairs. You may use one of the pre made 300mm gaps in the classroom. If weights are hung or placed on top of the beams you should eventually be able to witness the shape of the beams altering. It will indicate the points of weakness. They may fold or bend depending upon how much weight is hung or placed on the beam.
- Test each beam with increasingly heavier weights. Whilst hanging the weights, or placing on top, record both the size of the weight and the structural damage if any it has upon the beam. Place these results into a spreadsheet or record your results in a chart with notes describing the changes.
CHALLENGE YOURSELF....Measure deflection
As a developing young experimental engineer you will need to give thought to how you will measure the movement or bend of the beam.
BEAMS : STRETCH AND CHALLENGE YOURSELF....
Once you have completed your experiment explain which beam held the heaviest weights for the longest length of time without folding or crumpling in any way.
Explain why this type of beam appeared to be the structurally stronger than the other two?
Suggest where this type of beam is used in architectural structures?
Using additional card attempt to increase the strength of the beam? Remember too much additional card will make the beam too heavy. It will end up with a low strength to weight ratio.
Reinforced Concrete Beams
Concrete beams that are used in buildings as horizontal supporting pieces above doors and windows. These are called 'lintels'. These lintels are reinforced with steel rods cast in the concrete. The steel rods are normally placed below the neutral axis. The combination of more than one material makes the reinforced concrete a composite material. The steel enhances the strength of the concrete when stretched under tension.
Concrete beams that have been reinforced with steel rods are used to construct Beam Bridges.
Concrete is very strong in compression but weak in tension. We describe this as one of the properties of concrete.
REINFORCED CONCRETE BEAM : CHALLENGE YOURSELF TO CAST A COMPOSITE CONCRETE BEAM.
You are being challenged to work with a partner to cast a concrete beam. The beam should have wire going through it.
The beam will be 300mm long, 30mm wide and 30 mm deep.
Make a Mold using the wood that you are supplied with.
Drill correct holes in both ends of the Mold to allow for the wire to pass through. Use blue tack to ensure holes do not allow liquid cement to pass through.
STRETCH & CHALLENGE yourself and your partner. Think of a way to add tension to the wire.
Mix and pour the ready mix cement you have been supplied with. Ask your teacher for advice on how to do this safely.
THINK SAFETY WHEN WORKING WITH CEMENT
Wear a mask.
Avoid breathing in fine cement dust.
THE TRUSS BEAM
This beam uses the proven structural strength of the triangle. The triangle is a geometrical shape that has been recognised as a very strong structural component.
If you take a solid beam and remove lots of material by drilling holes, you can create a beam that has a very high Strength to weight ratio.
If all of the holes are triangles then you have made a Truss Beam. Truss Beams are much lighter than solid beams. Truss beams are very strong.
TRUSS BEAMS: CHALLENGE YOURSELF....MAKE AND TEST A CARD TRIANGULAR SECTION BEAM
Using card and glue make a triangular section beam. Test the beam for structural strength and resistance to loads (stress) in compression and tension. You can achieve this by placing the beams between either desks or chairs. You may use one of the pre made 300mm gaps in the classroom. If weights are hung or placed on top of the beam you should eventually be able to witness the shape of the beams altering. It will indicate the points of weakness.
Test the beam with increasingly heavier weights. Whilst hanging the weights, or placing inside the beam, record both the size of the weight and the structural damage if any it has upon the beam. Place these results into a spreadsheet or record your results in a chart with notes describing the changes.