DESIGNERS OF earthquake-resistant structures should consider the risk of multiple low intensity events as well as single big tremors, leading experts warned last week.
Two teams of UK earthquake specialists flew out to the central Italian region of Umbria-Marche immediately after last year's quakes. Last week they presented their initial findings at the Institution of Structural Engineers. The meeting was held by ICE's Society for Earthquake & Civil Engineering Dynamics with the Earthquake Field Training Unitand the Earthquake Engineering Field Investigation Team.
The region suffered a month of tremors spanning September and October, with three main quakes registering up to six on the Richter Scale. It is the cumulative effect of these tremors that caused the collapse of hundreds of masonry structures and the destruction of priceless works of art, claimed EFTU representative Professor Nicholas Ambraseys. 'Despite the low magnitude of the quakes, the activity on structures was high,' he said.
Each of the quakes was 'multi- event' - a number of consecutive tremors of decreasing frequency. In brittle structures such as masonry buildings, damage caused during the initial phase lowers the structure's natural frequency which means it is shaken more violently during the next, lower frequency phase.
The quakes left 13 dead and more than 150 injured. As many as 10,000 residents are still without satisfactory accommodation, said EQE International senior engineer Tim Allmark. Also in the affected zone are more than 1,000 historic monuments. 'Almost all suffered damage,' he said. In some parts, 90% of listed buildings were rendered unsafe or unfit for habitation.
Many domestic masonry buildings were badly damaged. They are brittle structures built to low standards. 'Initial damage was heightened by the aftershocks,' said Allmark.
There is no proper shear reinforcement in these buildings. The walls act independently, pounding against each other. Said Cambridge University's Dr Robin Spence: 'These buildings are lethal. Unreinforced masonry disintegrates into rubble'.
But there is little that can be done to minimise these effects. This part of Italy has thousands of these buildings, explained Allmark, and 'repair and retrofitting is prohibitively expensive'.