Tag: road

Creation of a simple beam bridge for road traffic

Creation of a simple beam bridge for road traffic

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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.

French alpine viaducts – the old and the new

French alpine viaducts – the old and the new

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The A40 motorway in southern France is a busy highway through the mountains that provides the most direct route from Geneva to the mountain resort of Chamonix and surrounding towns. Together with a railways along a similar route, they are used all year round by both French and international tourists and local people; the area is famous for skiing, mountain hiking, climbing and local French culture – so the roads and railways along this route are essential.

Two prominent and impressive viaducts caught my eye as I drove down the motorway. For each, I sketched a front-on, fine-lined shape and a more visual sketch of what the bridges actually look like when you’re driving on the road.

First, a modern, slender concrete structure – the Viaduc des Egratz de Passy. This one is part of the westbound A40 motorway.

^ see the column cross sections at the bottom of the drawing

Viaduc des Egratz de Passy, 1981 (road)

  • The (presumably reinforced) concrete posts are generally rectangular, with their short side aligned with the length of the motorway, with one exception. The column on the right of the drawing above is hexagonal instead – the reason is not clear, but it may be required because of the harsh bend at that point on the structure.
  • The deck does not appear to be simply fixed straight on to the columns – at a glance, it looks like it is levitating slightly. This is probably because of the damping system between the deck and the columns. Allowing some small damped rocking, rather than rigidly fixing the two together, helps the structure deal with the vibrations of the road without sudden plastic collapse or fast fracture of the joints.

Second, a more traditional, heavier-weight arch design – clearly from a much older era – the Viaduc de Saint-Marie.

Viaduc de Saint-Marie, opening date unclear (rail)

  • Straight, sturdy columns form the bottom section. Semi-circular arches have been utilised for structural stability in the top section.
  • The main material is masonry – probably stone masonry by observation.
  • Arch bridges are excellent at dealing with the continuous vibrations of railway traffic without the need for external damping systems (which were likley not developed at the time of construction).
  • The project would have been advanced for its time, fitting in with such an uncertain landscape – not to mention massively expensive as it would have been build by human power alone. Impressive!