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Saving grace of the Basilica of St Francis at Assisi


THE SAVING of a priceless early rennaissance church from oblivion after an earthquake provided a gripping tale at the ICE last week.

The strengthening of the Basilica of St Francis at Assisi by Professor Giorgio Croci of Universita di Roma explained the bold decisions, careful engineering and hi-tech solutions that protected some of the greatest cultural and spiritual treasures of Italy from further destruction by earthquakes.

Exactly two years ago, a moderate earthquake caused partial collapse of the main vaulting in the Basilica at Assisi, and the loss of rare frescos by Giotto, one of the greatest painters of the early Renaissance. Overall, the damage to the Basilica was extensive and parts of the structures were clearly in a perilous condition, a fact confirmed when a large aftershock caused further damage. As earthquakes continued to strike the region, urgent action was clearly needed; Croci, one of Italy's leading experts on the restoration of historical monuments, was called in to take charge.

Of most immediate concern was the triangular masonry parapet (tympanum) high above the south east end of the building; it was severely damaged, and threatened to collapse onto the roof of a side chapel with important frescos 40m below. A very large mobile crane was needed in the courtyard beneath to erect the shoring required, but a tall medieval wall with small openings prevented entry.

The bold - and risky - solution was to get another very large mobile to lift the first one up and over the wall into the courtyard beyond. The plan worked 'but it was not a moment that I would want to experience again,' said Croci. The permanent fix was to support the tympanum off reinforced concrete beams via a recently developed energy absorbing device incorporating tensioned wires made of shape memory alloys.

The other urgent problem was the main masonry vaulting bearing the priceless Giotto frescos. Two bays had collapsed, but five remained, albeit badly cracked. It turned out that the spaces between the main longitudinal walls of the Basilica and the vaulting had become filled with debris over seven centuries. Croci surmised that under the cyclic movements of successive earthquakes over the ages, the debris had become more and more firmly wedged against the vaulting, causing loss of curvature and severe bending stresses.

He believes that the reason such a moderate earthquake in 1997 caused such extensive damage was that it merely gave the last push to a severely overloaded system. The action was therefore to remove 1000 tonnes of the accumulated debris and then hang the vaults off temporary bridges spanning between the main walls via low spring rate devices, giving an effectively uniform support pressure, while allowing some movement of the vaults. Too rigid a restraint could have led to very high restraint forces and local failures. Grouting of the cracks in the vaulting then followed, using a carefully devised cementitious mortar designed to avoid damage to thefrescos.

A permanent strengthening solution for the vaulting was also needed. Croci chose to add strengthening ribs to the extrados (top side) of the vaulting, similar to those seen on the intrados of traditional gothic roof vaults. However, Croci's ribs were not masonry but were built up in-situ from thin strips of wood, following the curved (and distorted) profile of the vaulting and glued together to form a laminate.

The ribs were surrounded by an aramid reinforced fabric to provide additional strength, and then connected down to the vault by glue and by steel stirrups. It was a solution sympathetic to the original structural form, which was transparent and potentially reversible, while not interfering with the works of art below.

Such novel solutions would probably not have been accepted without interminable debate and discussion had it not been for the urgency of the situation. However, it has still not been decided how to decorate the intrados of the bays which had been rebuilt completely; Croci wondered ruefully how much of the next millennium might be needed to settle that debate.

Finally, Croci had some comments on the reinforced concrete beams added to the main roof structure in the 1960s. Commentators at the time of the earthquake had surmised that these represented rigid restraint points to the vaulting and had been instrumental in the partial collapse.

Croci is no great advocator of rc for strengthening historic masonry, but he is adamant that the accumulated debris was the main culprit, not the recent rc beams, which changed the roof stiffness very little.

The meeting was organised by SECED (Society for Earthquake and Civil Engineering Dynamics).

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