Seismic Reliability-Based Design of Softening Structures Equipped With Double Sliding Devices

Abstract

This study deals with seismic reliability-based design (SRBD) relationships in terms of behavior factors and displacement demands for softening structures equipped with double friction pendulum system (DFPS) bearings. An equivalent 3dof system having a softening post-yield slope is adopted to describe the superstructure behavior, whereas velocity-dependent laws are assumed to model the responses of the two surfaces of the DFPS. The yielding characteristics of the superstructures are defined for increasing behavior factors in compliance with the seismic hazard of L'Aquila site (Italy) and with NTC18 assuming a lifetime of 50 years. Considering several natural seismic records and building properties under the hypothesis of modelling the friction coefficients of the two surfaces of the DFPS as random variables, incremental dynamic analyses are performed to evaluate the seismic fragility and the seismic reliability of these systems. Finally, seismic reliability is assessed and seismic reliabilitybased design (SRBD) curves for the two surfaces of the double sliding devices are described.

Influence of the breakaway friction on the seismic response of buildings isolated with curved surface sliders: Parametric study and design recommendations

“…Indeed, underestimating the trigger acceleration implies that, in practical cases, the designer would calculate the peak floor accelerations at low-intensity earthquakes as if the building were isolated, whereas in reality it would behave as a fixed-base structure with the isolators persisting in the sticking phase. The average value of the / ratio for the overall group of data (i.e., considering all the numerical cases of this parametric study) is 0.83, which signifies that a mean underestimation of the trigger acceleration of about 20% is obtained with the analytical formula (7). A closer look at the distribution of the / ratio reveals that underestimation cases are more frequent for relatively low breakaway friction coefficients (see Figure 12-bottom), which are also pertinent to the common sliding materials adopted by manufacturing companies.…”

“…The sliding motion is triggered when the shear force through the CSS, which results from the inertial forces acting on the superstructure mass and on the base mass , exceeds the resisting breakaway frictional force g, g being the acceleration of gravity. This condition is achieved when the ground-motion acceleration is larger than the trigger value [21] ⋅ g (7) where is the ratio between the spectral acceleration evaluated at the superstructure fundamental period and the PGA. It is worth noting that according to Eq.…”

“…By eliminating the 439 no-activation cases from the overall group of 9072 NLTHAs, it is possible to assess the reliability of the analytical formula (7) to estimate the trigger acceleration. On the other hand, the actual value of the trigger acceleration computed from the NLTHAs is identified as the ground-motion acceleration corresponding to the first time instant in which the isolator displacement exceeds the yield displacement / , where ⋅ denotes the breakaway frictional resistance (cf.…”

“…Figure 12 Comparison between analytical and numerical trigger acceleration (top left), PDF of the relative error measurements given by Eq. (8) (top right), and distribution of the / ratio depending on the breakaway fricton coefficient (bottom) Due to the inability of the analytical formula (7) to predict the trigger acceleration with satisfactory accuracy, regression analysis relations are derived by processing the numerical data. To this aim, we construct the cumulative distribution function (CDF) of the discrete variable (which attains values , , … with probability P ), which is defined as follows…”

“…Seismic isolation is an effective technology for the enhancing the seismic protection of structures and their contents. Typical isolation devices include elastomeric bearings (rubber, recycled rubber or fiber-reinforced rubber [1]- [3]) and sliding supports with curved surfaces [4]- [7]. The Curved Surface Slider (CSS), introduced in the '80s in the version called Friction Pendulum System® (FPS) [8]- [10], is one of the most popular isolation hardware worldwide owing to its inherent simplicity.…”