Coupling beams are well recognised as deep beams and shear-critical structural elements.High shear stresses are induced in the coupling beams when the structure is subjected to lateral loads caused by wind or earthquake.In general,conventionally designed,reinforced concrete coupling beams are found to be inadequate in resisting high shear stresses and not able to achieve desirable seismic performance.Possible solutions were proposed by the researchers,including the use of diagonal reinforcement,rhombic reinforcement,structural steel and steel fibre reinforced concrete,etc.Indeed,the technology of vertical steel plates encased in concrete coupling beams as shear reinforcement is considered as one of the most efficient and practical ways to improve both the shear strength and ductility of traditional reinforced concrete coupling beams.In this paper,reversed-cyclic load tests of three large-scale reinforced concrete coupling beams,in which steel plate is encased, are reported.The coupling beam specimens have different span-to-depth ratios,ranging from 1.0 to 2.0.The effect of span-to-depth ratio on the seismic performance of these composite coupling beams is particularly addressed.The experimental results show that all specimens have the similar hysteretic responses,while their failure mechanisms are geometry dependent and greatly influenced byspan-to-depth ratio.It is shown that when the span-to-depth ratio increases the failure mode ofthe couplingbeams is changed from diagonal shear to flexure-shear with bond-slip and the crack angle becomes gentler.On the contrary,the shear strength and energy dissipation capacity of the coupling beams decrease with theincrease in the span-to-depth ratio.