On the effect of seismic base isolation on seismic design requirements of RC structures

According to the basic concept of seismic isolation, the base-isolated structures must remain elastic during an earthquake. On the one hand, the existing technical documentation and codes strongly noted that the superstructure of isolated buildings should be designed the same as a fixed base structure. The only difference is the isolator. So, its period is higher, and consequently the applied acceleration is lower. One of the criteria for designing fixed-base structures is a seismic design and especially compact transverse rebars for beams and columns near the support. In the present study, 12 structures of 8 to 24 stories were designed in both fixed and isolated forms. Then, a program was written in MATLAB that calculated moment-curvature and P-Mx-My interaction curves for concrete beam and column sections with any amount of transverse rebars (Av/s). Having these curves, the structures were push-over analyzed in three sway conditions ordinary, intermediate and special. The results also showed that, unlike fixed-base structures, the response modification factor (Ru) of the isolated structures was not affected by the seismic design level of beams and columns. Also, in the two target and final displacements, the number of plastic hinges formed in IO or thereafter, in the isolated structures was much less than the fixed ones.