The interactions between 60 degrees dislocation pile-ups with grain boundaries (GBs) are studied using multiscale modeling. Careful quantitative analyses of complex processes associated with 60 degrees dislocation absorption and transmission phenomena at Sigma 3, Sigma 9 and Sigma 11 symmetric tilt boundaries in Al are interpreted in terms of a set of modified Lee-Robertson-Birnbaum (MLRB) criteria. Our results and the MLRB criteria (i) explain experimental observations, (ii) rationalize new mechanisms such as deformation twinning and formation of extended stacking faults, (iii) show that reactions can be controlled more strongly by the leading partial of an incoming dislocation rather than the full Burgers vector and (iv) demonstrate that non-Schmid stresses, e. g. shear and compressive stresses along the GB, GB dislocation processes and step-height changes on the GB all influence the critical nucleation stress, but to differing degrees among different tilt boundaries. The MLRB criteria do not capture the effects of local GB structure that can also influence behavior. Quantitative metrics based on the MLRB criteria are formulated, using the simulation results, for various absorption and transmission phenomena. These metrics can be used as input into mesoscale models such as discrete dislocation plasticity, so that atomic-scale observations can inform higher-scale predictions plasticity.