A multi-story steel-framed building was subjected to dynamic tests at various stages of construction. In the interval between two test series, a dramatic change occurred in the building's dynamic properties. In the first series, the damping capacities of the two fundamental modes (East-West and North-South) were about 2 per cent of critical; of the other modes, 0.4 per cent to 0.9 per cent of critical. In the second test series, the damping capacities of all modes ranged from 5 per cent to 10 per cent of critical.
This increase in damping has been attributed to the interaction of the building with its service tower. The second test series was conducted after the service tower was structurally complete. The two buildings were found to be dynamically coupled, with connections at only the basement and foundation levels. Non-structural connections, in the form of steel plates providing access between the buildings, existed at every floor level and dissipated energy in relative motion between the buildings. From experimental data it was possible to estimate the amount of energy this source of Coulomb damping could dissipate and to show that it was responsible for the large change noted in damping capacity.