Effects of membrane action on the ultimate strength of reinforced concrete slabs.
Thesis DisciplineCivil Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This thesis describes an investigation of the effects of membrane action on reinforced concrete slabs, particularly the implications of allowing for compressive membrane action in the design of slab and beam floors. An examination of the minimum reinforcement requirements of a rectangular slab reveals that high design live loads are required before the benefits of membrane action can be fully exploited. Studies of the effect of compression on the flexural capacity of a reinforced concrete section and of the effect of membrane action on a clamped circular slab with elastic lateral restraint at the circumference are undertaken. These show that lightly reinforced, thick slabs with high concrete strength will benefit most from compressive membrane action in practical situations, and that if the surround is flexible, tensile membrane action will be evident at the stage when the ultimate load of the slab is reached. The effects of compressive forces in the panels on the design of the supporting beams is studied. It is shown that some beams are required to resist considerable tension and that membrane action may have considerable effect on the torsion induced in the edge beams. A design method is derived to deal with beams subject to tension. An investigation is then made of the lateral restraint provided at the edges of an interior panel by the surrounding panels, considered to be of elastic, homogeneous material. An experimental study of a quarter-scale, nine-panel slab and beam floor was conducted. The equations derived by Park for the ultimate strength of slabs with compressive membrane action were used to design the floor. The membrane action was assessed as sufficient to double the Johansen ,ultimate load of the centre panel. A smaller enhancement was allowed for in the centre-edge panels and none was allowed for in the corner panels. The centre spans of all beams were designed to carry the tension induced by the compressive membrane forces in the panels. Results of fourteen load tests on this model floor are analysed with particular reference to the effects of membrane action. Satisfactory behaviour at service load was observed and the floor sustained the predicted ultimate load before failure of the centre panel. The measurement of concrete and steel strains at critical sections revealed the presence of compressive membrane forces in the centre panel and tensions in the beams that were of the order expected . .A comparison of the volumes of steel reinforcement required in the model floor indicated that design including compressive membrane action brings no advantage except when the additional steel that is required to resist the tensile forces induced in the beams can also be used to resist moments due to earthquake or other lateral loading of the structure. It is concluded that allowance for membrane action in design would be of small benefit for normal slab and beam floors and would be of greatest use when very high loads are imposed on slabs with high lateral restraint at the edges.