Concept of both short voussoir beam and step beam in shallow seam and voussoir beam theory

Based on geometry condition of rock column balance of the voussoir structure,concept of both short voussoir beam and step beam in shallow seam part of the teaching material,i.e.Rock pression and ground control was discussed.It is considered that the roof of the shallow seam can not form the voussoir structure because of the balance condition that the length of ruptured rock column must be twice more than its thickness.Short voussoir beamis the voussoir structure the ratio of length to thickness of which is adjacent to 1,short voussoir beam structure is contrary to the voussoir theory.Step beam structure is based on erroneous suppose by incorrect use of voussoir theory: rock column N collapsed is still analyzed with hinge condition of voussoir beam,and step beamis nonexistent.In fact,it is feasible that rock column M(i.e.control rock column)is counted as a to set up model.