The present thesis work studies the parametrically excited vibrations on bridge stay cables when the natural frequency of the bridge tower is half of the resonance frequency of the cable. This condition is found at the longest stay cables of the Øresund Bridge. Experimental and numerical analysis is carried out based on a simplified scaled model (1:200) of this bridge, replacing the cables by strings made of steel. When parametrically excited vibrations are studied, certain relations between resonance frequencies of bridge tower and cable lead to high vibration levels. The most critical relation is when the tower frequency is double the cable frequency. The case studied here, where the tower frequency is half of the cable frequency, shows to be less severe.