Effects of site condition in near-fault area on the nonlinear response of fire-damaged base-isolated structures

Structural damage due to resonance, when the predominant vibration periods of both site and structure are close, is expected in the case of base-isolated structures subjected to near-fault earthquakes. Little is known, however, about the effects of different site conditions on the nonlinear seismic response of fire-weakened base-isolated structures. To analyze this aspect, eight five-storey reinforced concrete (r.c.) office buildings, base-isolated with High-Damping-Laminated-Rubber Bearings (HDLRBs), are designed in line with the Italian seismic code (NTC08) assuming four subsoil types, ranging from rock-site to soil-site, combined with relatively low and high values of the ratio between the vertical and horizontal stiffnesses of the HDLRBs. Three fire scenarios are considered at 45 (i.e. R45) and 60 (i.e. R60) minutes of fire resistance, with the fire compartment confined to the area of the first level (F1), the lower two levels (F1/2) and the fifth level (F5). A finite element thermal model to measure temperature distribution in the cross-section for columns, exposed to fire on one and four sides, and beams, exposed to fire on one and three sides, is considered in combination with the parametric time–temperature fire curve evaluated in line with Eurocode 1. Then, a nonlinear incremental dynamic analysis is carried out which takes into consideration the horizontal and vertical components of near-fault ground motions recorded at different site conditions from rock- to soil-site and normalized with respect to the NTC08 ones in accordance with the Modified Velocity Spectrum Intensity. Plastic conditions are assessed at the potential critical sections of the beams (i.e. end sections of the sub-elements in which a beam is discretized) and columns (i.e. end sections), whose reduced mechanical properties are evaluated in accordance with a thermal mapping and considering the 500 °C isotherm method proposed by Eurocode 2. Maximum ductility demand under the horizontal and vertical components of near-fault ground motions depends on the position of the fire compartment and increases moving from rock-site to soil-site.