COMPDYN 2011 III ECCOMAS Thematic Conference on Computational Methods in Structural Dynamics and Earthquake Engineering M. Papadrakakis, M. Fragiadakis, V. Plevris (eds.) Corfu, Greece, 25–28 May 2011 RETROFITTING BY BASE ISOLATION OF EXISTING BUILDINGS IN ARMENIA AND IN ROMANIA AND COMPARATIVE ANALYSIS OF INNOVATIVE VS. CONVENTIONAL RETROFITTING Mikayel Melkumyan1*, Valentin Mihul2, Emma Gevorgyan3 1 Armenian Association for Earthquake Engineering (AAEE) 1st lane of Nansen str. 6, 0056, Yerevan, Armenia e-mail: [email protected] 2 MIHUL S.R.L. Stroici str. 8B, 700373 Iasi, Iasi county, Romania e-mail: [email protected] 3 Armenian Association for Earthquake Engineering (AAEE) 1st lane of Nansen str. 6, 0056, Yerevan, Armenia e-mail: [email protected] Keywords: Base Isolation, Innovative Retrofitting, Existing Building, Conventional Retrofit- ting, Comparative Analysis, Cost Effectiveness Abstract. In recent years seismic isolation technologies in Armenia were extensively applied in construction of new buildings, as well as in retrofitting of existing buildings. Three re- markable projects on retrofitting by base isolation are described in the paper. One of them is retrofitting of a 5-story stone apartment building. The operation was made without resettle- ment of the occupants. World practice provides no similar precedent in retrofitting of apart- ment buildings. The other project is retrofitting of the 60 years old non-engineered 3-story stone school building. This building has historical and architectural value. Unique operations which were carried out in order to install the isolation system within the basement of this building and to preserve its architectural appearance are described. Accumulated experience created a good basis for participation in the international competition announced by the Government of Romania for development of the design on retrofitting by base isolation of the 178 years old historical building of the Iasi City Hall. The structural concept, including the new approach on installation of seismic isolation rubber bearings in this building, is de- scribed and detailed results of the earthquake response analysis for two cases, i.e. when the building is base isolated and when it has a fixed base, are given. For all three buildings com- parative analyses of innovative vs. conventional retrofitting are carried out. Different meth- ods of conventional retrofitting are considered and their costs are compared with the cost of innovative retrofitting by base isolation. Mikayel Melkumyan, Valentin Mihul, Emma Gevorgyan 1 APPLICATION OF BASE ISOLATION FOR RETROFITTING OF BUILDINGS WITH STONE BEARING WALLS Isolation of structures from horizontal ground motions is gradually becoming a more com- mon method of providing protection from earthquake damage. By reducing the seismic forces transmitted, isolation protects the contents and secondary structural features as well as the main structure; the safety of occupants and passers-by is thus also enhanced. Moreover, it is practicable to design the isolation system so that the structure responds elastically to the de- sign level earthquake. Thus repair coast should be greatly reduced and continued serviceabil- ity of the structure assured [1]. The rehabilitation of existing structures by the insertion of isolators at foundation level has been carried out on historic buildings in California such as the Oakland City Hall, San Francisco City Hall [2, 3], Salt Lake City and County Building [4, 5]. For these, isolation may provide the only viable means that is not unduly intrusive and damaging for the appearance of the building. Together with that the retrofitting technique us- ing base isolation has great potential for rehabilitation of ordinary civil structures such as apartment blocks. The first retrofit of stone apartment building of series 1A-450 has been car- ried out in Armenia [6, 7]. 1.1 Retrofitting of the Existing Fife-Story Stone Apartment Building Buildings on typical design of series 1A-450 have been erected in all regions of Armenia. They have the plan dimensions 52x15 m, symmetrical about the center of the long side and the bearing walls with 45-50 cm of thickness located mainly in transverse direction. The hori- zontal stiffness in the longitudinal direction is provided partly by the R/C frames with strong beams and columns, made inside the body of walls, and by longitudinal walls at the edge parts of the buildings. The analysis of consequences of the 1988 Spitak earthquake has shown that the most vulnerable zones in these buildings are the edge parts where the direction of bearing walls had been changed. It is in these very zones that intensive plastic deformations resulting in failure of the buildings have been developed due to the weak connections between longitu- dinal and transverse walls. The developed structural concept aims at retrofitting an existing building by means of seismic isolators using simple working technology [8]. This is a unique pioneering seismic isolation project introduced for an existing 5-story stone building (Fig.1). a. b. Figure 1: General view of the retrofitted by base isolation existing 5-story stone apartment building (a) and a fragments of its isolation system (b) 2 Mikayel Melkumyan, Valentin Mihul, Emma Gevorgyan The idea is to supply this building with seismic isolation in the foundation by gradually cutting the isolators into the walls at the level of foundation upper edge by means of a two- stage system of R/C beams. Base isolation method for existing buildings with bearing walls that envisions placing of seismic isolators at the level of the foundation or the basement solves the problem as shown on Figure 2. According to the innovative technology developed by the first author of this paper (Patent of the Republic of Armenia #579), in the basement bearing walls openings with certain spacing are made to accommodate reinforcement frames of lower and upper pedestals with seismic isolators. Then reinforcement frames are placed between pedestals thus forming lower and upper continuous beams along all bearing walls of the build- ing. The parts of existing walls between seismic isolators are removed creating gaps and the building appears separated from its foundation and linked to it only by seismic isolators. It is very important that openings in walls are made so that two adjacent openings are not made simultaneously; parts of walls existing between seismic isolators should be cut off beginning from the middle of building plan. The above described operation was made without re-settlement of the dwellers. The world practice has had no similar precedent in retrofitting of apartment buildings. The project was implemented in 1995-1996, was financed by the World Bank and co financed by UNIDO. The high damping rubber bearings (HDRB) for this retrofit project were designed with sig- nificant help and support of the UK based Malaysian Rubber Producers’ Research Associa- tion (MRPRA), particularly of Dr. K.N.G. Fuller. To implement the project 60 HDRBs were used, 28 bearings have been manufactured in MRPRA and 32 in Malaysia by Min Rubber Products Sdn. Bhd. and Sime Engineering Rubber Products Sdn. Bhd. 1.1.1 Design of Isolation System High damping rubber bearings are a simple economical means of providing isolation. They have the low horizontal stiffness required to give a long vibration period (typically 2 s) to the structure when mounted on the bearings. Their vertical stiffness is high, thus minimizing any rocking of the structure during an earthquake. The damping needed to limit the displacement of the structure and reduce the response at the isolation frequency is incorporated into the rubber compound so that there are generally no needed auxiliary dissipation devices. The bearings can be designed to withstand safely the large horizontal displacements imposed dur- ing an earthquake. The service life of the bearings is expected to be several decades [9], and they should require no maintenance. There has been much emphasis on the suitability of base isolation for critically important structures such as hospitals and emergency centers. The pro- tection of both structure and contents, however, would be advantageous for civil structures such as apartment blocks. The isolators are located by upper and lower recesses provided by annular steel rings bolted to outer steel plates which are connected to the reinforcement in the upper continuous and lower foundation beams; the isolators themselves are not bolted to the structure. This method of connection helps to minimize the cost of the isolators themselves and simplifies their installation on site. Because the bearing is simply located in a recess, no tapped holes for bolted connections are needed in the end-plate. The side, top and bottom rubber cover layers ensure the steel plates are protected from corrosion. In the considered existing building the bearings were not be located in an enclosed, heated basement, but would be exposed to the outside environment. The severe winter weather at the site meant that particular attention had to be paid to the low temperature crystallization resistance of the rubber compound. The plan of foundation is given in Figure 3, along with the location of the HDRBs and the total static load on each. The range of the vertical loads on bearings is quite high so it was de- cided to have two types of bearing, but differing only in the shear modulus of the rubber. 3 Mikayel Melkumyan, Valentin Mihul, Emma Gevorgyan Figure 2: Stages of installation of seismic isolation system in the existing building with stone bearing walls One type (designated Hard) would support loads in the range 740-900 kN and the other (Soft) loads in the range 500-680 kN. This strategy would lead to a somewhat more uniform 4 Mikayel Melkumyan, Valentin Mihul, Emma Gevorgyan distribution of the horizontal seismic forces, and help ensure that the centre of stiffness of the isolation system is close to the projection of the centre of mass of the building on to the ground plan.