BRIDGES

 

versión española

 

When the Romans made their road network they erected along their empire about 2.000 bridges, engineering works with an incredible technical skill; these bridges are probably the most evident symbol of that road network and some times the only symbol preserved; however, they needed works of remodelling, maintenance and restoration along 2.000 years.  In the republican age there were in Rome only 4 bridges: the Sublicius bridge, the more ancient, built in iron and wood in order to be easily destroyed in case of war –there are no remains of this bridge-; the Aemilius bridge, the firs Roman bridge built in stone in the year 179 b. C.; nowadays it is known as the Ponte Rotto –“the broken bridge”-, because it was partially destroyed and its remains are still visible in the Tiber bed; the Fabricius bridge built in the year 62 b. C and the Cestius bridge, built few years later to connect the Tiberina island with the two banks of the river.  In the imperial age the Roman built 5 new bridges, like the Agrippa’s bridge or the Milvius bridge; the Nero’s bridge kept outside the limits of the walls and was destroyed in the VI century a. C.

 

Pont du Gard (Provence, Francea), bridge and aqueduct at the same time. 

(Photo: Roberto Lérida Lafarga 06/08/2007)

 

 

 

The basis of a Roman point was the arch, a system of construction that the Romans inherited from the Etruscans and that they developed and perfected.  The arch used by the Romans is the semicircular arch.  The success in building an arch lays in keeping an equilibrium between the strengths and weights of the stones that formed it; concretely an arch deflects the weight and the strength of the stone to the sides; so, it is necessary to have good foundations, good buttresses or supports that contained that weight and that strength, an exact centre and a perfect adjust of the voussoirs –the wedge-shaped stones that compose an arch-.  An added difficulty for the bridge builders was the weight of a Roman road on the bridge.  To build an arch it was necessary to make a wooden shell that provisionally held the structure until all the voussoirs were put; then the shell was dismantled and the arch held on the buttresses.

 

Section of the building of a bridge, according to  HAMEY, L. A. y HAMEY, J. A.: Los ingenieros romanos, Madrid, 1990

 

 

 

The first stage to build a bridge was to dig some holes to check the subsoil and its resistance to support the weight of the bridge.  After selecting the place where the bridge should be built, the engineers marked where they erected the buttresses and then the workers started to make the foundations.  Very often some of the buttresses had to be made inside the river bed, so that the water became a problem; so, the engineers created a system called cofferdam, i. e., a hermetic dam made with trunks stuck in the river bed like a coffer; the space inside the cofferdam was bigger than the foundations of the buttress; they need to evacuate some water that entered in the cofferdam with pump or with buckets.  Vitruvius indicated that it was necessary to built a double cofferdam, joining trunks with planks and chains and to refill the space between the two cofferdams with well rolled clay; afterwards, they had to evacuate the water that entered or filtered with hydraulic mechanisms similar to our water pumps; if the foundations were not too big or the river bed was not too deep, it was necessary only buckets.

 

Construction of a bridge, picture from HAMEY, L. A. y HAMEY, J. A.: Los ingenieros romanos, Madrid, 1990

 

 

 

When the space for the buttress was delimited, it was usually necessary to level and to consolidate the ground with mortar.  Afterwards, the building in the strict sense started.  Sometimes the Roman built their bridges with big blocks of stone not joined with mortar; then they needed to cut and to sculpt these blocks to assemble them perfectly and to keep firmly in their place; the strength of the water could drag these blocks and collapse the bridge; so the mortar became more useful and it allowed the use of blocks of stone with smaller sizes.

 

Roman bridge in Vasio (nowadays Vaison la Romaine, Provence, France). 

(Photo: Roberto Lérida Lafarga 06/08/2007)

 

 

 

Some of the blocks in the bridge and the cornices were projected outside the structure in order to be used as base of the shell and to centre the arch.  The quarrymen raised up the buttress or the supports up to the impost, where the carpenters raised up the wooden shell to make the arch, propping it up on the cornices or the imposts; the exterior side of this semicircular shell had the same measure and shape as the interior side of the arch –intrados-; logically, this wooden shell was strong enough and was fastened enough to support the tons of the arch voussoirs.  The workers raised up these voussoirs with cranes and they put one after the other, until they put the central voussoir, the keystone; then they moved away the wooden shell.  First they only moved it down, taking away the wedges that supported the shell on the cornices or imposts; when they made the spandrels and they levelled the curve of the extrados –the exterior and superior side of the arch- with mortar and slag, the moved away completely the wooden shell and finally they made the road on the bridge.

 

 

 

 

As a curiosity we can say that the Romans always were very superstitious and the primitive Romans had such a fear to the spirits that they had to make some rites, even when they had to cross a river; so, they appeal to priests, some of them were called pontifices, i. e., “those who make bridges”.

     

In Aragón there are preserved some Roman bridges, some of them complete; in other cases we can only see their remains.  Especially well preserved it is the bridge in Luco de Jiloca (Teruel).

 

Roman bridge over Jiloca river near Luco de Jiloca.

(Photo: Roberto Lérida Lafarga 23/04/2008)

 

Roman bridge in the Cañón de Añisclo (Huesca), over a gorge 100 metres deep. 

(Photo:  Roberto Lérida Lafarga 10/10/2005)

 

 

 

 

 

SOURCES:

- GABUCCI, Ada: Roma, Barcelona, 2006

- HAMEY, L. A. y HAMEY, J. A.: Los ingenieros romanos, Madrid, 1990