Concrete Bridges
Within this page we are going to look at those bridges which outwardly, at least, are constructed of Concrete. In fact, Concrete is a pretty versatile material which enables most bridge types to be constructed with it.
Structural Properties
Concrete is made from a mix of stone, sand and cement, bound together as the result of a chemical reaction between cement and added water. The strength of a concrete mix is dependent on the relative proportions of stone, sand, water and cement. Concrete is very strong in compression, but much weaker in tension, so to compensate this, steel bars are added to create reinforced concrete. An alternative method is to pre-stress the concrete by running cables from one end of the beam to the other, and tensioning them so that the concrete is permanently compressed. This is the same principle as picking up a line of wooden blocks by pushing the two end ones towards each other.
Arch Bridges
Most people who see an Arch Bridge will automatically assume that it is constructed of Stone, or at least Masonry at first glance. However, there are a surprising number of arch bridges that were constructed in Concrete. In the 1930s in particular a lot of roads in Northern Scotland were upgraded and improved, and with them new bridges were built. A prime example of this is the new bridge at Invergarry on the A82. A quick glance would convince the casual passer-by that the bridge is Stone, and this is the impression that the designers wanted to give. However, on closer inspection it is proven to be Concrete - a quick glance underneath proves it.
It can therefore be seen that simply by moulding the concrete to form an Arch ring, and some embellishment to the parapet and abutments, can turn a plain concrete structure into something that suits its environment.
Girder Bridges
Just as Concrete can be used to mimic an Arch Bridge, so it can also be used to form Girder Bridges. Whilst true girder bridges have a visible metal girder under the deck, the concrete version will have a Concrete beam between the piers and deck. This, inevitably is formed from Reinforced Concrete, which means that there is a large amount of steel inside the concrete to maintain its strength and rigidity.
Box Girder Bridge
This is a special kind of girder bridge, where the concrete is formed into a box with a void in the middle to reduce weight. This works because the highest stresses in a bridge beam occur at the top (compression) and at the bottom (tension). In the middle there are no forces along the line of the beam, so the sides of the box are there to stiffen the deck rather than to resist tension or compression.
Often box girders are prestressed in order to allow longer spans, and many multi span concrete structures take this form,for example the Hammersmith Flyover.
Concrete Deck Bridges
The one bridge type that seems to be unique to Concrete is the Concrete Deck Bridge. These bridges are very simple in visual design, with a monolithic lump of concrete spanning between the abutments and piers. OK, so that is rather simplistic, but a quick visual inspection will reveal nothing more. Inside that concrete, the reinforcement may take the form of Girders, Trusses or even some hybrid form. They are most commonly found on single-span river crossings, although can be used in almost any situation.
Concrete Lattice Bridges
This is the Concrete version of a Truss Bridge. Most of this type of bridge were built in the early days of Concrete, and few if any appear to have been built since the war. There are two basic sub-types, those where the bridge parapets were formed from Concrete Trusses, as at Advie Bridge (right), and those where the supports underneath were a mass of piers and cross bracing. Some of the Concrete Parapet-Trusses, however, are more decorative than structural.
See Also
- Arch Bridges - as copied in Concrete.
- Girder Bridges - as copied in Concrete!
- Truss Bridges - you get the idea!
