Rua Augusta: The Cage Inside the Walls

Why is it significant?

Stop in the middle of Rua Augusta, among the street performers and café tables, and look at the buildings on either side: four or five storeys, identical windows, wrought-iron balconies, shops at street level. They look like ordinary city blocks. Inside the walls, they are machines.

Key Facts

Open the walk

Stop in the middle of Rua Augusta, among the street performers and café tables, and look at the buildings on either side: four or five storeys, identical windows, wrought-iron balconies, shops at street level. They look like ordinary city blocks. Inside the walls, they are machines.

After 1755, the engineers of the rebuilding asked a simple question: how do you make a house that will not kill the people inside it the next time? Their answer was the gaiola pombalina, the Pombaline cage. Inside the masonry walls, builders set a three-dimensional frame of timber, braced with diagonal crosses. When the ground shakes, the wooden lattice flexes and holds the building together, instead of letting the stone simply collapse on the people inside.

The story goes that the system was tested by building a wooden model and having soldiers march around it to imitate an earthquake. Whether or not that happened exactly so, the gaiola is counted among the earliest building systems in Europe designed against earthquakes. And the parts were standardised: doors, window frames, stonework and ironwork were made to measure in series, almost like an eighteenth-century flat-pack city.

Now think about what lies under your feet. The Baixa sits in a low valley where a stream once ran down to the river, on soft, waterlogged ground. So the new blocks were set on thousands of pine piles driven deep into the mud. As long as they stay wet and away from the air, the wood does not rot, and the piles have been holding up these streets for more than two centuries. A few blocks south, in the basement of a bank, an archaeological site shows some of them beside Roman fish-salting tanks.

The cage has a weakness, though, and it is us. Over the years, owners have cut away timber to open up shop fronts and added heavy extra floors on top, and engineers worry about how many of these buildings would still behave as designed if the ground shook again. The street's name, Augusta, honours the majesty of the king who reigned when it was rebuilt. Now carry on south down Rua Augusta towards the great arch at the end, and stop just beneath it.