In the present study, an experimental investigation was conducted to characterize the unsteady vortices and turbulent flow structures around twin-box-girder (TBG) bridge deck models with and without cross beams. While the oncoming wind speed was fixed at U-infinity=8.0 m/s (i.e., the corresponding Reynolds number, Re = 1.01 x 10(4), based on the height of the box girder) during the experiments, the gap width between the TBG was varied to have four different gap ratios (i.e., the ratio of the gap width between the TBG to the deck height). The corresponding test cases were classified into two categories: the cases with relatively small gap ratios (i.e., gap ratio=0.85 and 1.70) and the cases with relatively large gap ratios (i.e., gap ratio=2.55 and 3.40). In addition to measuring the surface pressure distributions around the TBG bridge deck models using an array of digital pressure transducers, a high-resolution particle image velocimetry (PIV) system was utilized to perform detailed flow field measurements to quantify the evolution of the unsteady vortex structures around the TBG bridge deck models. The measurements reveal that as the gap ratio increases, the vortex shedding moves from the trailing edge of the leeward box to the rear edge of the windward box, which simultaneously increases the turbulent kinetic energy in the gap region and the fluctuating pressures on the leeward box. The vortex dimensions and the core-to-core distances between two neighboring vortices are also affected by the gap ratio. Combining with the estimation of the pressure field and the measured fluctuating pressure coefficient distributions on the TBG models, it is found that the TBG bridges with larger gap ratios will dramatically strengthen the fluctuating pressure coefficients on the leeward box which are greatly higher than those for the small gap ratio cases. Moreover, for large gap ratio cases, the test model with cross beams also has higher fluctuating pressure coefficients on the leeward box which just decrease a little comparing with the test model without cross beams. (C) 2014 Elsevier Ltd. All rights reserved.