Seismic Retrofit of Bnai Zion Hospital Using An Innovative Damped Rocking-Mass System

Abstract eng:
This paper presents the seismic retrofit design of the first major hospital to be retrofitted in Israel. This is the first out of four projects initiated and supported by the Israeli Ministry of Health, in accordance with a decision made by the Government of the State of Israel in 2010 to retrofit thousands of public buildings, assigning a $1,000,000,000 budget over a time period of 25 years. B’nai Zion hospital was built in the 1950s-1960s in Haifa, which is situated near an active fault line, and is considered an area of high seismicity. The existing seven floor, 9,000 square meter Central Wing was originally constructed using “Debesh" walls, a unique Israeli building method, which was widely used in past decades. The concrete used in these walls contains very small amounts of cement paste, almost no steel reinforcement and is composed mainly of sand, stone aggregates, and impurities such as seashells. The seismic retrofit of the structure, which includes a 3,500 m2 new extension, required detailed design and unique engineering solutions. The existing structure was found to behave poorly during a seismic event. The predicted high seismic demands could not be accommodated in view of the low shear and bending capacities of "Debesh" walls. Hence severe damage was expected, mainly at low floor levels. The seismic demand imposed on various concrete elements reached an alarming ten times their capacity. An efficient retrofit scheme, designed to prevent severe damage while avoiding extensive retrofit measures and thereby avoiding disruptions to ongoing hospital operations, was initially suggested. This included an innovative use of the new wing extension as a "Damped Rocking-Mass" system, by assembling 20 fluid viscous dampers at the gap between the existing and new structures. The limited horizontal displacements of the old concrete structure required developing the necessary velocities for efficient viscous damper action by taking advantage of the differential vertical displacements developed within the gap segments. In addition to the rocking base, a reversed toggle damper configuration was utilized to improve damper action at small relative velocities. The project is in progress and requires a special coordination so as to ensure full operation of all hospital activities at all times. This can only be achieved with entire cooperation of hospital management and staff, to schedule the works and occupy areas as they are upgraded. This paper presents the parts of the project that have been already completed, including the unique toggle setup, which also enabled the creation of a critical passageway under the newly installed damper steel frames, and the installation of the “rocking foundations”. Several unique construction problems, related to the need of ensuring full ongoing operations at all times are discussed, as well as the solutions that were provided on-site.

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