Research On Mechanical Behaviour And Design Method Study Of Small Span-to-depth Ratio Coupling Beams Based On Statically Indeterminate Strut-tie Model

Because of the high shear-moment ratio，fragile shear destruction often happenedinside Small span-to-depth ratio coupling beams under the influence of horizontalseismic action It is difficult that coupling beams with conventional reinforcement havehigh ductility under the high shear-compression ratio. Thus,our group presented a set ofcomposite diagonally reinforced program that add the diagonal reinforcement and Lreinforcement on the basis of conventional reinforcement.A series of experimentsconfirmed that the coupling beam adopted this scheme can get no less than4.0and4.5chord rotation ductility under the condition of0.25-0.3shear-compression ratio.According to crack distribution, damage type and all kinds of steel straindistribution characteristics in compositely reinforced coupling beams(CRCB) withspan-to-depth ratio1.5＜L/h≤2.5, a statically indeterminate strut-and-tie model isproposed based on the analysis for the special transfer mechanism of CRCB withspan-to-depth ratio1.5＜L/h≤2.5. The strut-tie model consists of three types of modelswhich are diagonally reinforced truss model, L reinforcement truss model and stirruptruss model and work together.Base on reasonable assumptions for CRCB(1.5＜L/h≤2.5), the geometric deformation，compatibility equations, constitutive equationsand equilibrium equations about Steel rod and concrete compressive strut areestablished and a new program named LL-N which can simulate the mechanicalperformance of CRCB(1.5＜L/h≤2.5) is written. The example analysis illustrates thatthe analysis results of strut-and-tie model and testing are in good agreement on macro（P-curve） and micro（steel strain） level.Moreover，the program LL-N not onlydistinguish the failed state of CRCB(1.5＜L/h≤2.5) by obtaining the compressive strainof the concrete strut，but also reveals shear contribution of various models and itschange rule at different loading stages of coupling beams.Design clauses about CRCB(1.5<L/h≤2.5) in the code for design of concretestructures(2010) are based on the few experiment resluts.This paper calculates a seriesof CRCB(1.5<L/h≤2.5) with different shear-compression ratio and reinforcementthrough the program LL-N.According to the calculation resluts of the program LL-Nand experiment,a shear capacity design formula and corresponding structural measuresis proposed to provides the foundation for finer design of CRCB.