Sealed tuned liquid column dampers, with a gas-spring effect taken into account are ideally suited to increase the effective structural damping of bridges vibrating in the low-frequency band, up to about 5 Hz. Modal tuning in analogy to the classical mechanical damper followed by fine-tuning in state space, render the parameter optimal and the control more robust. Sufficient conditions to prevent parametric resonance of vertical motions of the TLCD and to avoid the synchronization effect of densely walking pedestrians on the lateral bridge vibrations are presented. Numerical simulations are experimentally checked for a novel laboratory model. Effective structural damping of the Millennium Bridge by five TLCD and of the Toda Park Bridge by a single TLCD are worked out in detail. Copyright © 2005 John Wiley & Sons, Ltd.