Effects of Axial Force Variation in the Seismic Response of Bridges Isolated With Friction Pendulum Systems

Issues in the Upgrade of Italian Highway Structures

“…As is shown in Calvi et al [2004], the overturning moment net effect on the pier is null since while the shear force in one device increases the force in the other one decreases, hence this effect can be disregarded. The effect of the vertical excitation, on the other hand, increases or decreases the shear force in both devices at the same time, hence this effect should be considered when there is reason to believe that the vertical component can be significant for the site under consideration (near-fault).…”

Seismic isolation of buildings using devices based on sliding between surfaces with variable friction coefficient

“…Among those, the Friction Pendulum TM (FP) is a sliding bearing that uses gravity as the restoring force and consists of an articulated friction slider that travels on a spherical sliding surface, characterized by a constant radius of curvature and by a single nominal value of friction coefficient. FP systems have been extensively studied analytically and experimentally by a number of authors ( [2,8,9,15,16,22,25], among many others). Alternatives to standard FP systems have been proposed, with the intent of improving the response and possibly reducing the size of the bearings.…”

“…Upon activation, the tangent stiffness of the system decreases to a value equal to the ratio between the borne weight and radius of curvature of the sliding surface. In an extreme simplification, and neglecting a number of effects, such as the variation of the axial force [2], the dependency of the friction coefficient on pressure and velocity, etc., the shear force that is transmitted by an FP system is merely a function of the friction coefficient (typically assumed constant in value), the borne weight, the radius of curvature and the lateral displacement.…”

“…Four objectives may be pursued: (1) to protect all or part of the structural system, forcing it to remain in a quasi-elastic range; (2) to increase the main period of vibration, reducing the acceleration of the construction floors; (3) to reduce the relative displacement demand introducing additional dissipation sources; (4) to regularize the response, modifying the local stiffness and strength of parts of the structure.…”

Nonlinear Dynamic Behaviour of Base-Isolated Buildings With the Friction Pendulum System Subjected to Near-Fault Earthquakes

“…However, the (low) constant isolation frequency, proportional to the curvature of the sliding surface, may induce resonance of base-isolated structures if it is close to the predominant vibration period of near-fault ground motions [3]. Specifically, fling-step and forwarddirectivity in near-fault areas can produce long duration pulses of intense velocity in the horizontal direction [4,5], amplifying the displacement and inducing torsional and re-centring problems [6,7]. On the other hand, since the response of the FP system during the sliding phase is strongly influenced by the axial load, amplification of torsional demand and residual displacement and uplift of the FP bearings may also be induced by the high frequency vertical component of near-fault earthquakes [8][9][10].…”