Bucharest City Hall seismic protection

Heritage structure

The City Hall was built in two phases; construction of the basement, first and second floors began in 1906 and was completed six years later, while the two upper storeys were not added until the end of the 1940s. The structure is masonry construction with concrete slabs, and walls of up to 1.1 m thick; U-shaped in plan, the main dimensions of the building are 89 m and 65 m, and it rises to a height of almost 26 m. Critically, it is located in an area of very high seismic activity.

An active past of seismic activity

During its lifetime, the building had already been subjected to four large earthquakes of between 6.7 and 7.4 on the Richter scale, and was considered to be unsafe and at high risk of collapse. A traditional seismic retrofitting design that was proposed at the end of the 1990s was subsequently replaced by a design for base isolation, once the renovation contract was ready to proceed. By this time seismic codes had been updated and the original proposal did not meet the new standards.

Special seismic protection for this heritage structure

Freyssinet’s scope of work was to design and install the special seismic protection elements that were required for the project – a total of 262 isolators and 36 dampers. A particular feature demanded in this highly-seismic region was the significant displacement – 1.5 m in total – that these elements had to be able to accommodate.

Large-scale tests for seismic protection devices

The two types of lead rubber bearings that were designed for the base isolation each had diameters of around 1 m and heights of 0.5 m to accommodate the anticipated loads of up to 760 kNm. At the time there was no facility in Europe capable of handling such large units, and they had to be shipped to the USA in order to undergo the necessary testing.

Works by the main contractor saw micropiles installed to strengthen the raft foundation under the building, and reinforced concrete beams constructed below the main walls of the structure. Once the isolators and dampers had been introduced into their specified locations below the new beams, the loads were transferred onto them using flat jacks that were subsequently left in position.