  We propose a simple and efficient scheme based on adaptive finite elements
over conforming quadtree meshes for collapse plastic analysis of structures.
Our main interest in kinematic limit analysis is concerned with both purely
cohesive-frictional and cohesive materials. It is shown that the most
computational efficiency for collapse plastic problems is to employ an adaptive
mesh strategy on quadtree meshes. However, a major difficulty in finite element
formulations is the appearance of hanging nodes during adaptive process. This
can be resolved by a definition of conforming quadtree meshes in the context of
polygonal elements. Piecewise-linear shape functions enhanced with generalized
bubble functions in barycentric coordinates are used to approximate the
velocity field. Numerical results prove the reliability and benefit of the
present approach.
