Experimental And Theoretical Study On Web Opening Of Steel And Concrete Composite Beams In Negative Moment Zon

As a structural form of reasonable stress,steel-concrete composite beam is widely used in the multi-storey and high-rise buildings.In the design of high-rise structure,through the web opening in composite beam,all kinds of pipe-line equipments(water supply and drainage,electric,etc)can be allowed to pass through the opening,which can reduce the occupation of these pipe-line equipments to space,lower the storey height.Thus,the project cost can be lowered and better economic benefit can be achieved.In some practical projects,the negative moment of continuous composite beams needs to be opened in order to satisfy usable demands.At present,the research on the composite beams of web opening under the negative moment is relatively limited in China.And there is few corresponding method of theoretical analysis and standard to follow.Only if the mechanical property of the composite beams of web opening in the region of negative moment is comprehended comprehensively and thoroughly and the negative influences exerted by opening are reduced can the technical and economic advantages be expressed better.Therefore,the paper studies the stress of the composite beams of web opening under the negative moment through methods of related experimental study,finite element value calculation and theoretical analysis.The finished works and conclusions are listed as follows.1.Through the experiment of the composite beams of web opening under concentrated load,the paper studied the failure mode and mechanical property of the composite beams of web openings in the region of negative moment.The results of experiment show:the stiffness and carrying capacity of the composite beams of web reduce a lot after they are opened under negative moment and the section in the opening area does not conform to the plane cross-section assumption;the destruction forms are that the shearing of concrete slab in the opening area is destroyed,meanwhile,partial stud connectors appears pull-out phenomenon in the opening area;in addition,increasing the thickness of concrete flange plate can improve the carrying capacity of the composite beams of web opening in the negative moment region and increasing the reinforcement ratio of longitudinal rebar can improve its deformability;the concrete flange plate makes a great contribution to the shearing capacity of the composite beams of web opening in the negative moment region and the shearing properties are more prominent than that in positive moment,strengthening concrete flange plate can have an obvious reinforced effect.2.The paper analyzed the test specimen used in experiment in the nonlinear finite element method,in which the destruction form and deformation feature similar with the experiment were obtained and load deflection curves of various test specimens conformed to experimental results.Therefore,the accuracy of finite element method was verified and the finite element model,which could simulate well the carrying property of the composite beams of web opening in the negative moment region was obtained,which provided the foundation for parameter analysis.3.The paper analyzed the influence of different influential parameters to the carrying property of the composite beams of web opening in the negative moment region.The selective influential parameters are the thickness of concrete slab,the reinforcement ratio of longitudinal rebar,the opening position,the opening height,the opening width,the opening shape and the opening eccentricity.In the analysis results,positive and negative influences caused by diversified parameter variations provide some references for practical application of engineering.4.On the basis of experimental study,according to plastic stress distribution of four angular cross sections in the limiting condition,the paper deduced secondary moment functions and built the computing method of ultimate bearing capacity of the composite beams of web opening,using vierendeel truss as a mechanical model.The computing method takes the shearing function of concrete slab above the opening into account,which is close to physical truth.Through the formula,the correlation of internal force in the opening section is calculated and anti-bending and shearing capacity are worked out in the opening.Results of various specimens obtained from the theoretical method coincide well with the experimental results.5.The paper put forward several reinforcement measures of the composite beams of web opening in the negative moment region and made a comparison among carrying capacity,deformability and shearing property.Results show that choosing the reasonable reinforcement measure can reduce the negative influences caused by opening to a certain degree.On the premise of easy construction,the combined use of several reinforcement measures can have a better reinforcement effect.6.The paper deduced computational formula of ultimate bearing capacity of composite beams of web opening with reinforced measures in the negative moment region.In the process of deduction,the positive influences of stiffening plates to carrying capacity in the opening area are considered.The composite beams of web opening with different stiffening plates in the negative moment region are calculated and analyzed.Results show that setting stiffening plates in the opening can improve the shearing capacity of the composite beams of web opening in the negative moment region effectively,and it will improve obviously with the increase of the area of stiffening plates.The theoretical results coincide well with finite element results,which verifies the accuracy of the method of theoretical calculation.