As already explained in the previous paper describing the structure, the bridge crossing River Tagus at Alconétar is an arch bridge with an upper deck and a 220m long span. Each half arch was built in vertical position on each river bank to be rotated in a vertical plan and finally connected at mid span. During the rotation phase some vertical vibrations were observed but once the two half arches were connected at the crown and built into their foundations the vertical vibrations became very important under certain wind conditions. The purpose of this paper is to describe the phenomenon as well as the studies which were made and the solution which was put in place to solve the problem.