Three-dimensional effects in turbulent duct flows, i.e., side-wall boundary layers and secondary motions, are studied by means of direct numerical simulations (DNS). The spectral element code Nek5000 is used to compute turbulent duct flows with aspect ratios 1 to 7 (at Reb, c = 2800, Reτ 180) and 1 (at Reb, c = 5600, Reτ 330) in streamwiseperiodic boxes of length 25h. The total number of grid points ranges from 28 to 145 million, and the fluid kinematic viscosity n was adjusted iteratively in order to keep the same bulk Reynolds number at the centerplane with changing aspect ratio. Spanwise variations in wall shear, mean-flow profiles and turbulence statistics are analyzed with aspect ratio, and also compared with the 2D channel. These computations show good agreement with experimental measurements carried out at IIT in parallel, and reinforces one important conclusion: the conditions obtained in the core region of a high-aspect-ratio duct cannot exactly be reproduced by spanwise-periodic DNSs of turbulent channel flows.