In India, Unreinforced Masonry (URM) infills are invariably used in frame buildings for their low cost, ease in construction, and good sound and heat insulation properties. However, safety of these infills during earthquake is a major issue, particularly in important buildings like hospitals. Infills are subjected to in-plane forces due to inter-storey drift and out-of-plane forces due to floor acceleration. Base isolation is a very promising technique, which controls the inter-storey drift and floor accelerations, simultaneously. The present study explores the efficacy of base isolation for achieving the desired seismic performance of URM infills. A procedure is presented for the design of base isolation systems for URM infilled frame buildings. The efficacy of base isolation is studied for two URM infilled RC (Reinforced Concrete) frame hospital buildings, four and eight-storeys tall. The effect of isolators on dynamic characteristics, seismic performance, inter-storey drift ratio and peak floor acceleration is presented. It is found that it is possible to design a viable isolation system to achieve immediate occupancy performance level of URM infills in the four-storey building; however, only life safety performance level could be achieved for the eight-storey building.

Unreinforced Masonry (URM) infilled RC (Reinforced Concrete) frames are the most common structures for multistorey buildings in India. Masonry infill is used in buildings for its good sound and heat insulation properties, low cost and ease in construction. During earthquakes, the infills are subjected to both in-plane as well as out-of-plane forces, and their behavior is quite complex. As reported by Pauley and Priestley (1992), generally, the infills first crack under in-plane forces and then collapse due to out-of-plane inertia forces. It has been observed that even in the buildings for which the frame is intact after earthquake, damage of infill is almost unavoidable. This is not at all acceptable for hospital buildings, which are of utmost post-earthquake importance and must remain functional after the earthquake. Other nonstructural components, such as medical equipment and complex network of electrical and mechanical facilities also must be intact and operational after the earthquake. Floor response and inter-storey drift, together are responsible for damage to the nonstructural components, building contents and services. For hospital buildings, inter-storey drift ratio for infill should be below 0.1% for Immediate Occupancy (IO) level, and floor accelerations should be such that infills do not fail in out-of-plane action. The only practical way of reducing floor accelerations and inter-storey drift simultaneously is to use base isolation.

Kelly and Tsai (1989) conducted a shake-table test to evaluate the seismic response of a base isolated building and its effect on the primary and secondary components. This experimental study shows that the first base isolated mode not only controls the response of the superstructure, but also dominates the response of high frequency attachments. The contribution of the higher modes to the response of base isolated structures is very small.

Dolce et al. (2007) performed shake-table tests on URM infilled RC frame scaled models. The results of the experimental tests show that base isolation can provide outstanding structural performances, with no damage to the infill walls, even under strong earthquakes. Moreover, it was concluded from their study that seismic isolation could also lead to significant improvement in the protection of building contents, if a proper choice of the isolation system is made, depending on the dynamic characteristics of the content.

Structural Engineering Journal, Seismic Safety, RC Frame Buildings, Base Isolation, Isolation Systems, Performance-Based Design, Pushover Analysis, Unreinforced Masonry, Heat Insulation, Out-Of-Plane Inertia Forces, Inter-Storey Drift Ratio, Lead-Rubber Bearings, LRB, Friction Pendulum System, FPS, RC Frame Scaled Models.