Shunichi Nakamura

Forced and ambient vibration tests and vibration monitoring of Hakucho suspension bridge

Forced and ambient dynamic tests of the Hakucho Bridge were carried out to study the dynamic characteristics of this suspension bridge. Dense-array measurement was employed in order to capture not only natural frequencies and damping, but also the mode shapes of the bridge. The natural frequencies and mode shapes obtained from the forced and ambient vibration tests agreed well with those calculated by a three-dimensional finite element model. A new method that combines the random decrement method with the Ibrahim time domain method is proposed to systematically identify the natural frequencies, damping, and mode shapes. This method is successfully applied to ambient vibration data. It is shown that the natural frequency of the first vertical bending mode decreases noticeably as the wind speed increases. It is also shown that the shape of the first vertical bending mode changes slightly near the towers, depending on the wind velocity; this finding indicates that the change may be associated with friction in the bearings at the towers. Finally, application of the Global Positioning System to measure static displacement of the girder is explained.

Effectiveness of Repair Methods of Corroded Bridge Cables

It is important to repair the corroded bridge cables by proper methods so that corrosion does not progress further. Six repair methods were proposed and applied to cable specimens. Then, the specimens were exposed to the severe corrosion environments and the effectiveness of the proposed repair methods was compared. Two different types of test cables were used in this study: parallel wire strands and spiral strands. The parallel wire strand cables consist of 19 non-galvanized steel wires. This aimed at the main cables of suspension bridges. Six repair methods were applied to these cable specimens: coating with zinc or epoxy resin paint or zinc powder paste, filling with epoxy resin or oil, and dehumidification method. Then the specimens were wrapped with wet gauze and kept at 40°C for 15 months to accelerate corrosion. By investigating mass loss due to corrosion and appearance during this period, effectiveness of six repair methods was compared. As for the surface wires, the dehumidification method was the most effective followed by the epoxy resin paint and filling, the zinc powder paste, and the zinc and epoxy resin paint on the surface. The oil filling was not very effective compared with other repair methods. The corrosion of the inside wires was much less than those of the surface wires. The spiral strand cables consist of seven galvanized steel wires. This test aimed at hangers of suspension bridges and stays of cable-stayed bridges. By investigating mass loss due to corrosion and appearance of both inside and surface wires during the 16 month period, the proposed six repair methods were all very effective compared with those of unrepaired strands. This study proves that, even if a cable is corroded, proper repair works are effective in preventing further corrosion.

Strength of Corroded Bridge Wires and Repair Methods

Suspension bridge cables need to perform under severe corrosive environments. To simulate this environment, galvanized steel wires were wrapped with wet gauze and kept in an enclosed box at a temperature of 40° C. This paper investigates corroded galvanized steel wires on different corrosion levels and their mechanical properties and remaining strength. Laboratory studies found that actual tensile strength of corroded wires did not decrease with corrosion levels, whereas elongation decreased sharply after the zinc layer was partly depleted and the steel started to corrode. The accumulated amount of diffusive hydrogen of corroded wires was less than 0.2 ppm, which was well below the critical concentration of 0.7 ppm to cause brittleness. However, fatigue strength significantly decreased after steel corrosion below the galvanized layer progressed. Fatigue strength further lowered when the steel wire was cyclically stressed under wet environments. It was estimated that the wires were fractured by the mixed effects of corrosion, cyclic stresses and hydrogen. Various repair methods were applied to the corroded wires: zinc rich paint, epoxy resin paint, zinc powder paste, filling with oil and dehumidification method. The effectiveness of these repair methods were evaluated by the corrosion simulation tests. The paper shows that the dehumidification and epoxy resin paint ant filling methods are most effective repair methods.