Effects of near-fault vertical earthquakes on the nonlinear incremental response of r.c. base-isolated structures exposed to fire

Abstract

High values of the ratio between the peak of the vertical and horizontal ground accelerations of near-fault ground motions can become critical once the strength level of a fire-weakened base-isolated structure is reduced especially as the ductility demand can increase much more rapidly for an ever-lower strength level. In the present work, the incremental dynamic analysis (IDA) of base-isolated structures exposed to fire is carried out considering the horizontal and vertical components of seven near-fault ground motions with different values of the peak acceleration ratio. To this end, six five-storey reinforced concrete office buildings, base-isolated with high-damping-laminated-rubber bearings (HDLRBs), are designed assuming different values of the ratio between the vertical and horizontal stiffnesses of the HDLRBs. The design of the test structures is carried out in a high-risk region considering (besides the gravity loads) the horizontal seismic loads acting in combination with the vertical ones. Five fire scenarios are considered at 45 (i.e. R45) and 60 (i.e. R60) min of resistance, with the parametric temperature–time fire curve evaluated in accordance with Eurocode 1: i.e. fire compartment at the fifth level (F5), the upper two (F4/5) and the other (Fi, i = 1–3) levels. The nonlinear IDA analysis is carried out in a step-by-step procedure, which is based on a two-parameter implicit integration scheme and an initial-stress-like iterative procedure. At each step of the analysis, plastic conditions are checked 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), where a bilinear moment–curvature law is adopted; the effect of the axial load on the ultimate bending moment (M–N interaction) of the columns is also taken into account. Finally, reduced strength and ultimate ductility of the cross-sections are evaluated in line with the 500 °C isotherm method proposed by Eurocode 2.