In order to perform a detailed study of the stellar kinematics in the vertical axis of bars, we obtained high signal-to-noise spectra along the major and minor axes of the bars in a sample of 14 face-on galaxies, and used them to determine the line of sight stellar velocity distribution, parameterized as Gauss-Hermite series. With these data, we developed a diagnostic tool that allows one to distinguish between recently formed and evolved bars, as well as estimate their ages, assuming that bars form in vertically thin disks, recognizable by low values for the vertical velocity dispersion sigma_z. Through N-body realizations of bar unstable disk galaxies we could also check the time scales involved in the processes which give bars an important vertical structure. We show that sigma_z in evolved bars is roughly around 100 Km/s, which translates to a height scale of about 1.4 Kpc, giving support to scenarios in which bulges form through disk material. Furthermore, the bars in our numerical simulations have values for sigma_z generally smaller than 50 Km/s even after evolving for 2 Gyr, suggesting that a slow process is responsible for making bars as vertically thick as we observe. We verify theoretically that the Spitzer-Schwarzschild mechanism is quantitatively able to explain these observations if we assume that giant molecular clouds are twice as much concentrated along the bar as in the remaining of the disk.
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