Theoretical Analysis And Experimental Research On Behavior And Crack Control Of Negative Moment Zone In Steel-Concrete Composite Beams

Continuous composite beams have some disadvantages when the steel girder is in compression while the concrete slab is in tension at the negative moment zone,and the crack of concrete slab will decrease the stiffness and durability,so it is very important to understand the behaviors of negative moment zone deeply,and control or prevent concrete crack at the negative moment zone of continuous composite beams.Combined with the project granted by national natural science foundation of China,the behaviors of negative moment zone,concrete crack mechanics and control method under short-term loads are studied in service stage for composite beams,the main contents and conclusions are as follows:(1) Considering multiple effects of slip,shear lag,and Timoshenko shear deformation,a theoretical method for analyzing load effects under vertical loads is established for steel-composite beams(short for SCBs).With the inner and cantilever concrete top slab and the steel bottom slab set by different shear lag warping shape functions,the more accurate elastic closed-form solutions of stress and flexibility under uniform load,concentrated load,and two-point symmetry loads are deduced in service stage by energy-variational principle.These solutions are used in subsequent study of SCBs' behaviors at negative moment zone.(2) Considering double effects of slip and shear lag,a theoretical method for analyzing load effects under axial load is established for prestressed SCBs.According to the characteristics of external prestressed SCBs,analytical stress solutions of prestressed SCBs under prestressing axial load and eccentric bending moment are decuced by energy-variational principle.The slip law between concrete slab and steel girder is analyzed,which shows that the stress concentrate and slip are rather greater in prestressing stage.The calculation method of longitudinal shear force is suggested, which can provide a reference for local strengthening design of connectors.(3) Static model tests are conducted on three simply-supported SCBs under negative moment and three continuous two-span SCBs under concentrated loads.The study focuses on the development law of concrete crack and improvement effect due to external prestressing on crack,stiffness,and bearing capacity of composite beams.The law of stress distribution,slip,internal force redistribution,and crack development under negative moment are obtained in the tests.The investigation results show that under short-term loads,applying prestress in concrete slab is an effective measure for crack control under negative moment.At the same time,the theoretical results and test results are verified each other.(4) The crack widths and stiffness of SCBs at negative moment zone are studied. The force ratio and transverse reinforcement spacing are considered in crack spacing, the formula of crack width is modified.Based on the concept of reduced stiffness due to slip effect and the tension stiffening effect,the calculation precision will increase if calculating the deflection of continuous composite beams in service stage by effective stiffness.(5) The theoretical load effect solutions of variable stiffness SCBs which cracked in partial range along the beam are obtained by energy-variational principle.The influence of concrete crack on shear lag effect and effective flange width are investigated,and the verified formula of effective flange width at negative moment zone is suggested.The study results suggest that the concrete crack would lead to smaller shear lag coefficient and larger effective flange width,the effective flange width could be calculated by using a completed cracked beam with a uniform cross-section in the range of equivalent span length corresponding to the negative moment zone.It is also important that the actual position of inflection point should be carefully considered for the continuous composite beams.(6) According to the super tension strength and lower shrinkage and creep of Reactive Powder Concrete(short for RPC),the concept of steel-RPC composite beam is introduced.The full-range analysis shows that adopting RPC as the material of slab is also an effective measure which can increases the crack resistance of composite beams at negative moment zone,especially when the steel-RPC composite beam is applied prestressing load.There are totally 121 graphs,13 tables and 184 references in this paper.