Deformation measurements as part of monitoring activities can provide information about the behavior and structural condition of bridge structures. Geodetic measurement methods are preferred for these deflection measurements. These methods allow reflectorless measurements at sampling rates of several hundred kilohertz. But the used laser scanners are very expensive, by this they are losing attractiveness for measurement periods of several weeks or years and in case of moderate accuracy requirements. In contrast, a relatively inexpensive method to measure the deformations of box girder bridges is presented in this paper. In this method, a cable or wire is clamped through the bridge structure. This acts as measurement base ("base-line) and allows deformation measurements by optical transducers. This measurement method is currently being tested as part of a monitoring program at a semi-integral viaduct in Norway. Preliminary studies of the measurement method, conducted measurements on a box girder bridge near of Hanover and first results of test measurements at the Norwegian viaduct are presented.