cantilever footbridge

Niagara Cantilever Bridge (1892)

Cantilever bridges are built using cantilevers—horizontal beams supported on only one end. Most cantilever bridges use a pair of continuous spans that extend from opposite sides of the supporting piers to meet at the centre of the obstacle the bridge crosses. Cantilever bridges are constructed using much the same materials & techniques as beam bridges. The difference comes in the action of the forces through the bridge.

What’s the problem?

This impressive 5-part footbridge with a novel cantilevered fan design is proposed for central London. The 3 meter wide footbridge is split into five separate parts. The five steel beams forming the deck open in sequence and shaped counterweights assist the hydraulic mechanism and reduce the energy required to move the structure.

At its fully opened position, the footbridge looks strikingly like a Japanese fan as each of its five sections are arrested at separate angles that fall between 20 and 80 degrees.

  • Ask – What is the basic function of the bridge structure? How large is the gap being bridged? The key point of this bridge design is its aesthetic shape.
  • Imagine – What modifications can be made to an ordinary cantilever bridge? What would be an attractive design? The new pedestrian bridge is to be located in an area where there are many new ultra-modern high-rise buildings. The designers want their new bridge to be functional and attractive.
  • Design, Build – What are the requirements for anchoring the ends of the bridge? How do cantilever bridges operate? What are the basic components needed to operate the bridge? Most of the draw bridges use a counterweight to reduce the torque needed to lift the bridge. This bridge is divided into 5 parts and each has its own counterweight. For the aesthetic purpose each part is lifted to a different angle. It is not clear but assumed that one motor is used to lift the entire bridge. Each part is pulled up in sequence. There is no engineering significance of having five parts. Each part is similar to a lift gate at the entrance of a parking garage.
  • Improve – Besides being attractive, what other improvements can be included in the design? It is also claimed that it reduces the energy required to raise the bridge. It is hard to justify the energy reduction claim since it depends on what else it is compared to.

That’s engineering

  • Strength of Materials: For the designed pedestrian load, how big a beam does each finger have to have?
  • Engineering Materials: What kind of material should the bridge made of?
  • Engineering Statics: What is the force acting on the support (pivot) of the bridge? How big a counterweight should be chosen? What will happen if the counterweight is too heavy? How much torque is needed to lift the bridge? Does torque relate to the bridge weight, counterweight, and the load?
  • Engineering Dynamics: What is the effect of wind on the bridge? What will happen if a flash mob begins running and jumping on the bridge? Can we use one motor to lift the entire bridge (all five fingers at different angle)?

Engineering ideas

  • cantilever, hydraulic, counterweight, torque, rotational equilibrium, beam, anchor, load, force, moment, shear stress

Do it
Here are some challenges for you to work on…

  • suggest some other uses for cantilever structures. When would it be important to have part of the structure just sticking out in midair?
  • design and build a model to show how cantilevers work and what the limitations are.
  • What are the different kind of draw bridge designs?
  • Can you build a wood model of a drawing bridge like this?

Learn more…