1. Rigid finite elements are used to construct a kinematically admissible velocity field.

2. Kinematically admissible velocity discontinuities are permitted to occur at all inter-element boundaries.

3. Simple criteria are given providing a possibility for the static solution of a kinematically admissible velocity field.

4. On the outer side, they are laterally bounded by a curved and kinematically admissible hyperbolic surface, which is defined by enveloping the hyperbolical half cones and part of the case surface of the leading half cone.

5. The solution to the lower bound linear programming problem defines a statically admissible stress field and a "safe" estimate of the collapse load, whereas the solution to the upper bound linear programming problem defines a kinematically admissible velocity field and an "unsafe" estimate of the collapse load.