The Pombaline cage (“gaiola pombalina”): an European anti-seismic system based on enlightenment era of experimentation

On November 1st, 1755, the lower part of Lisbon, a city of medieval character, was badly destroyed by the action of a violent earthquake, followed by a tsunami and violent fires. At the time, the Marquis of Pombal, ruled Portugal, and took several measures to safeguard the inhabitants and rebuild the city. It was decided to rebuild the city on the same site, according to a new urban plan with orthogonal streets. The conception of the new urban mesh was an Enlightenment Era design. The buildings were endowed with a series of unique characteristics, including an anti-seismic wooden structure - the Pombaline cage. This solution sought to resist earthquakes, making the building structure less rigid, aiming at dissipating the seismic energy through the lower weight and elasticity of the wood, the flexibility of the connections between the elements and even the lightening of the massive elements. This way, the total ruin of the building was avoided, in the event of an earthquake, and an interior area was created that could remain intact, serving as a refuge for the inhabitants. This anti-seismic system had great importance and influence in the panorama of engineering history, considering it could be at the genesis of seismic engineering in Europe. The perception that 18th century engineers had of the system to resist earthquakes through flexibility, should be the object of in-depth study, as it could be adopted in the reinforcement of constructions against possible earthquakes, especially the oldest ones, avoiding the introduction of rigid reinforcements in buildings otherwise characterized by their ductility. The present study seeks to explore the performance of this system, and to better understand how the increase in seismic performance is processed through flexibility, instead of the increase of the buildings stiffness.