Keywords: earthquake engineering, structural control, seismic design, damping, earthquake resistant structures, ground motion, impulsive loads, response spectra.

Passive tuned mass dampers (TMDs) are widely used in civil engineering to mitigate vibrations induced by quasi-stationary dynamic loads, but their seismic performance is known to depend on ground shaking properties. Requiring the motion of the primary structure to react with, TMDs proves effective against long-duration, narrow-band ground motions, but may fail in reducing the peak response to pulse-like earthquakes.

One typical example of pulse-like earthquakes is given by near-field (NF) ground motions. Ground shaking near a fault rupture may be characterised by a short-duration impulsive motion that exposes structures to high input energy at the beginning of the record. This pulse-type motion can be explained through the concepts of "forward-directivity" and "fling-step" effects. Both effects may result in large-amplitude, long-period pulses in the velocity and displacement time histories, which are particularly challenging for the structural safety of long-period structures.

Many studies have been devoted to improving the performance of structures exposed to NF ground motion. The use of simplified analytical models of ground motion pulses may prove a valid tool for the systematic design and assessment of seismo-protective systems. Several models are available for this purpose [1,2]. Most of these models, based on discontinuous functions, are not ideal for extensive parametric studies. Continuous models have been proposed recently which appear more promising for that purpose. One of them is the velocity pulse model suggested by He and Agrawal to study the performance of supplemental viscous dampers on a base-isolated building [3]. Based on the Belarge model and essentially consisting of an amplitude modulated sinusoid, this model can successfully depict both buildup and decaying phases of recorded ground motions, beyond possessing two valuable properties: (a) existence of a closed-form solution for a single-degree-of-freedom (SDOF) structure subject to the ground model; (b) existence of a counterpart frequency domain and state-space domain expression that can be used in the design of seismic protective systems.

In this paper, in order to show the possible advantages of a pulse-oriented TMD design, this model is applied to the optimization and evaluation of TMDs against impulsive earthquakes. A new "pulse design" is introduced as an alternative to the classical approach, and the two strategies are tested on SDOF and multi-degree-of-freedom (MDOF) linear structures subject to both analytical pulses and NF real records. The resulting statistical assessment, expressed by percentile response spectra, shows the advantages and disadvantages of a pulse-oriented TMD design and improves the general understanding of TMD performance subject to impulsive ground motions.

1

N. Makris, S.P. Chang, "Response of damped oscillators to cycloidal pulses", Journal of Engineering Mechanics, 126(2), 123-131, 2000. doi:10.1061/(ASCE)0733-9399(2000)126:2(123)

2

B. Alavi, H. Krawinkler, "Effects of near-fault ground motions on frame structures", The John A. Blume Earthquake Engineering Center Research Rep. No. 138, Stanford, California, 2001.

3

W.L. He, A.K. Agrawal, "Analytical model of ground motion pulses for the design and assessment of seismic protective systems", Journal of Structural Engineering, 134(7), 1177-1188, 2008. doi:10.1061/(ASCE)0733-9445(2008)134:7(1177)

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