Effect Of Web Reinforcement And Opening On Shear Performance Of Lightweight Aggregate Concrete Deep Beams

Lightweight aggregate concrete(LWAC)deep beams is characterized by the complicated stress state and obvious brittle features,especially the deep beams with openings,the shear mechanism still need further investigation.Therefore,systematically research on the shear performance of LWAC deep beams,revealing the shear failure mechanism,the quantifying the effect of design parameters on a macro level,has important theoretical significance for promoting the shear design innovation of LWAC deep beams.An experimental study of eight LWAC deep beams and two normal weight aggregate concrete deep beams based on web reinforcement ratio,and nine LWAC deep beams with web opening based on opening parameters were carried out.The shear analysis framework including design parameters analysis,shear transfer nechanism,shear capacity,macro-element model and shear force-deflection reaponse,and design mehod is established.The major conclusions of this dissertation are as follows:1.Effect of web reinforcement on shear behavior and size effect of LWAC deep beam.Eight LWAC deep beams and two naormal weight aggregate concrete deep beams were test with main variables of web reinforcement ratio and beam depth.The action mechanism of web reinforcement on the shear capacity and size effect of deep beam is emphasized.The test results indicate all ten beams failed on the shear-compressiion mode,which is indendent on size.The existence of web reinforcement can limit the spalling of concrete.The normalized diagonal cracking and ultimate shear load will increase with the web reinforcement ratio and a significant reduction in maximum diagonal cracking width can be observed.The web reinforcement can resist the internal tensile stress of the inclined section and form effective lateral restraint on the concrete strut,thus mitigating the influence of size effect.2.Effect of web opening on the shear behavior of LWAC deep beam.Nine LWAC deep beams with web opening were test with test variables of opening dimension,location,extra reinforcement and opening numbers.The experimental results in terms of failure process and model,load-deflection behavior,characteristic load,and strain in reinforcement were thoroughly investigated.The test results indicate that the failure of LWAC deep beams with web opening is always controlled by the development of diagonal cracks above and below the openings.Shear strength and stiffness are primarily dependent on the degree of the interruption of the natural load path,the deviation of web opening lead to a 12%～40.8%increase on shear capacity.Inclined additional reinforcement around the openings can restrain the propagation of diagonal crack,thus improving the shear strength.3.Shear capacity model of deep beams based on strut-and-tie model(STM).To quantify the contribution of web reinforcement in the shear mechanism,a modified strut-and-tie model(MSTM)is established combining with the evaluation on typical strut efficiency factor models,in which the web reinforcements were considered as vertical ties.The MSTM is evaluated by the shear test databases of deep beams and was compared with original STM,showing MSTM can better predict the contribution of vertical web reinforcement to the shear strength and reflect the effects of the main desigh parameters.A method to construct the STM which reflect the real transfer mechanism of deep beams with openings.Predictions obtained from the proposed STM approach has a consistent agreement with experimental results.4.Shear capacity model of deep beams based on kinematic model.Two-parameter kinematic theory(2PKT)is used to investigate the shear behavior of LWAC deep beam,showing that 2PKT accurately consider the contribution of vertical web reinforcement,but not consider the effect of horizontal web reinforcement.As for deep beams with web opening,a principle to distribute the shear force among the transfer paths is established based on shear stiffness ratio.Axial force is introduce to the shear demand equation to modified the 2PKT,and a kinematic model for the deep beam with web openings was established.The evaluation based on the shear test databases of deep beams with web opening showing that the proposed model accurately predict the failure mode as well as shear capacity,and reflect the effect of opening size and location.5.Macro-element model and shear force-deformation response of deep beams.Macro-element models for shear span in terms of STM and kinematic model theory was established,thus the proposed model is applied to the shear force-deformation response.In macro-element model based on STM,the vertical deformation is simplified as the sum of deformation caused by compression of diagonal strut between loading and support points and the deformation caused by the tensile of longitudinal bars.Combined with the shear capacity model,it is shown that simplified shear force-deflection curves are in good agreement with the test curves.As for macro-element model based on kinematic model,shear deformation and rotational deformation are represented by the shear spring and rotational spring,respectively,then the constitutive relationship of V-(35)_c of shear spring and M-(?)of rotational spring were established.It is shown that the predicted shear force-deflection curves are in good agreement with the test curves,which can reflect the nonlinear response of the specimen after diagonal cracking.6.STM shear method for deep beams considering the contribution of web reinforcement.The shear desigh method of deep beams in Chinese code was evaluated,showing the method did not consider the influences of size effect and longitudinal bars,but overestimate the shear contribution of horizontal web reinforcement.To avoid the iterative process,a practical simplified MSTM for the purpose of practical design was proposed,and a shear design method based on the proposed model was established.In this dissertation,the influence of web reinforcement and web opening on the shear behavior of LWAC deep beam was determined.Then,the shear capacity model based on strut-and-tie model and kinematic model was established,and acro-element models for shear span was developed.Finally,the STM-based design method foe LWAC deep beam was proposed.The research provides a theoretical reference for the promotion and application of LWAC specimens and the innovation in shear resistance design.