  The effect of higher order continuity in the solution field by using NURBS
basis function in isogeometric analysis (IGA) is investigated for an efficient
mixed finite element formulation for elastostatic beams. It is based on the
Hu-Washizu variational principle considering geometrical and material
nonlinearities. Here we present a reduced degree of basis functions for the
additional fields of the stress resultants and strains of the beam, which are
allowed to be discontinuous across elements. This approach turns out to
significantly improve the computational efficiency and the accuracy of the
results. We consider a beam formulation with extensible directors, where
cross-sectional strains are enriched to avoid Poisson locking by an enhanced
assumed strain method. In numerical examples, we show the superior per
degree-of-freedom accuracy of IGA over conventional finite element analysis,
due to the higher order continuity in the displacement field. We further verify
the efficient rotational coupling between beams, as well as the
path-independence of the results.
