  We study elliptic fibrations for F-theory compactifications realizing 4d and
6d supersymmetric gauge theories with abelian gauge factors. In the fibration
these U(1) symmetries are realized in terms of additional rational sections. We
obtain a universal characterization of all the possible U(1) charges of matter
fields by determining the corresponding codimension two fibers with rational
sections. In view of modelling supersymmetric Grand Unified Theories, one of
the main examples that we analyze are U(1) symmetries for SU(5) gauge theories
with \bar{5} and 10 matter. We use a combination of constraints on the normal
bundle of rational curves in Calabi-Yau three- and four-folds, as well as the
splitting of rational curves in the fibers in codimension two, to determine the
possible configurations of smooth rational sections. This analysis
straightforwardly generalizes to multiple U(1)s. We study the flops of such
fibers, as well as some of the Yukawa couplings in codimension three.
Furthermore, we carry out a universal study of the U(1)-charged GUT singlets,
including their KK-charges, and determine all realizations of singlet fibers.
By giving vacuum expectation values to these singlets, we propose a systematic
way to analyze the Higgsing of U(1)s to discrete gauge symmetries in F-theory.
