Creation of a simple beam bridge for road traffic
A beam bridge is the simplest type of bridge, consisting of a deck resting on vertical columns. Despite their simplistic physics and appearance, the construction of such a structure in the modern world takes more than just resting a plank on a column. I’ve made a short video showing an example of the construction process.
The main materials of interest are:
Reinforced soil. This is a specialist technique whereby the nearby soil is made stronger by the addition of a grid of a different material – usually metal or composite layers. This is a vital process in weak soils and is usually carried out by a specialist subcontractor on a big project.
U beams. The deck consists of these concrete beams, whose cross section is a U shape. This has a large second moment of area (see below for calculation), making the deck very resistant to torsional and bending stresses. The beams tend to be large enough that construction workers are able to stand inside them to attach the next layers of material.
Temporary works scaffolding. Usually consisting of metal bars which can be assembled and disassembled quickly, temporary works structures are needed to prop the structure up before all structural elements are completely secure.
Rebar. Rebar is a type of steel shaped into rods of about 2cm diameter. When worked into a cage shape and inserted during the concrete pour, the resulting ‘reinforced’ concrete, is much stronger in tension that its raw counterpart. This is because concrete tends to be strong in compression but not tension; using pure steel would be way too heavy, not to mention way too expensive. Combining these properties creates an excellent new material which is used in all large modern infrastructure projects.
Permadec plastic panelling. This is a very strong overlay material that is manufactured by a specialist company. It contains a shell of fibreglass with steel strips inside and is placed on top of the U-beams.
Composite top layer, consisting of some of the materials described above. This diagram is an expansion of the diagram of the U-beams above. It is a zoomed in version of the green box.
Calculation of the U-beam second moment of area:
We have to split the cross section up, calculate the value of I about the centroid of each section, then use the parallel axis theorem to find the total I about the centre of the section.
