Investigating the Amount of Structural Gap For Base-Isolated Structures, Subjected To Near-Fault Earthquake Records

One of the important effects of earthquake is impact of adjacent buildings. To eliminate or reduce the damage and destruction caused by the impact of adjacent buildings to each other, they should be separated by a gap or should be constructed with a minimum distance from the border. In this issue, the properties of near field records of earthquakes, with close distance to the source of the wave propagation makes them different from other records. Seismic Code 2800 Iran has made no mention of near-field earthquake response spectrum and seismic design spectrum in 2800 code is for far field earthquakes. Furthermore, a new method to reduce the effects of earthquakes on structures is using base isolators. In this study, the effects of base isolators in prediction of the behavior of concrete moment frames (medium ductility) located near the source of earthquake have been investigated and two-dimensional frames with same bays and different number of floors (2 4, 6, 8, 10 and 15 story frames) which represent the short, medium and high rises buildings have been considered. Results indicate that the required gap for isolated buildings located near faults is more than the predicted amount in Iranian Seismic Code (2800 standard). All the models are intermediate reinforced concrete frames which are separated from regular structures. One of the modern techniques for designing seismic resisting structures is using base isolations. There are several types of isolators which can be used practically in the structures. The damping of the isolators depends on the type of them and there are wide ranges of damping amounts based on isolators’ types. The main idea in this regard is to reduce the strength demand in the structures and also reduce the nonstructural damages even during severe earthquakes. Also it is possible to reduce non-structural damages by using base isolators. Base isolators with high damping, absorb earthquake energy using displacement. Although it is very good idea to use base isolators in seismic areas, but the gap should be produced between adjacent buildings to prevent buildings contact. This is very dangerous especially when the story levels are not same in the two buildings. In this case a concentrated load will induce to middle of column of another building. In this research it is tried to find the suitable amount of gap for base isolated structures. Also for considering near-fault effects, some of records with near-fault behavior have been selected and used. The structures considered to be retrofitted by base isolators. For calculating displacement demand of the structures under earthquakes the nonlinear time history analyses have been used. All the analyses performed using opensees software and scaled records. The results shows that the amount of Gap in Iranian Seismic Code (2800 standard) is suitable for structures which are not in near-fault regions. Of course there are some differences between the code and the results achieved in this research. But for structures in near-fault regions the proposed amount in the code is less than required amount of gap from the results of this research.