Study On Torsional Behavior Of Single-box Multi-cell Composite Box Girders With Corrugated Steel Webs

Composite box girders with corrugated steel webs are a kind of steel-concrete structure which replaces concrete webs in the traditional concrete box girders with corrugated steel webs.Since the corrugated steel webs are thinner than the concrete webs,the torsional stiffness of composite box girders with corrugated steel webs is much smaller than the concrete box girders.Especially for the single-box multi-cell composite box girders with corrugated steel webs which have large width-height ratio,they need to carry large torque when subjected to the eccentric loading,and the torsional response should not be neglected.To capture the mechanical behaviors under external pure torque and to reveal the torsional mechanism of single-box multi-cell composite box girders with corrugated steel webs,the method of combination among the scaled model test,finite element analysis(FEA)and theoretical analysis is applied to systematically investigate the mechanical behavior of single-box multi-cell composite box girders with corrugated steel webs under pure torsion.The main research work and results are shown as follows:(1)After introducing the new shear strain relationship between corrugated steel webs and concrete slabs,the models of TS-STMT and SMMT for the torsional analysis of reinforced concrete members are extended to the torsional analysis of single-box single-cell composite box girders with corrugated steel webs based on the existing models of RA-STMT and FA-STMT for the torsional analysis of single-box single-cell composite box girders with corrugated steel webs.By contrast,the predictions of SMMT are better than those of TS-STMT.The SMMT can more reasonably describe the biaxial strain and stress in the reinforced concrete members and can accurately predict the mechanical behavior of composite box-girder under torsion.This proposed model can provide the theoretical basis for the analysis of single-box multi-cell composite box girders with corrugated steel webs.(2)The pure torsional tests of double-cell and triple-cell box girders with corrugated steel webs are conducted with the newly designed apparatus for the torsional test and the torsional mechanism is preliminary revealed.Specifically,the whole torque-twist curves,the distribution pattern of concrete crack,the failure mode of corrugated steel webs,the variation law of shear strain in concrete slabs and corrugated steel webs and the variation law of axial strain in steel bars are obtained in the model test.This work can provide the experimental data for the validation of finite element model and the theoretical analysis model developed for the single-box multi-cell composite box girders with corrugated steel webs.(3)The FEA software is used to build the finite element model of the single-box multi-cell composite box girders with corrugated steel webs.In the constitutive model of concrete,the damage factors are determined.The mechanical behaviors of composite box girders with corrugated steel webs are successively simulated to further reveal the torsional mechanism of this kind of structure.The parameter analysis shows that the torsional capacity is almost linearly proportional to the transverse steel bars,as well as the thickness,yield stress and transverse position of corrugated steel webs in the given ranges.Moreover,it is approximately considered that the torsional capacity linearly increases with the increase of compressive strength of concrete and the number of cells(2-10 cells).The results of FEA also demonstrate that the farther the cells are from the torsional center,the smaller shear strain on the outer surface of upper and lower slabs they will have,while the farther the corrugated steel webs are from the torsional center,the larger shear strain they will have.The relations of average shear strains between the different corrugated steel webs and the relations of average shear strains on the concrete slabs of different cells can be respectively expressed as unified fitting formulas.These formulas can provide research foundation for the development of theoretical analysis model for the single-box multi-cell composite box girders with corrugated steel webs.(4)The unified SMMT is proposed for the single-box multi-cell composite box girders with corrugated steel webs.It can accurately predict the torsional behavior of composite box girders with corrugated steel webs in which the numbers of cells are no more than 10.Moreover,the practical formulas for the calculation of torsional capacity are proposed on the basis of the theoretical analysis and the existing code for concrete design.The practical formula of torsional capacity derived from the Design Code for Concrete Structure(GB50010-2010)can be used for the torsional design and check for the single-box multi-cell composite box girders with corrugated steel webs.(5)The single-box multi-cell composite box girders with corrugated steel webs with/without CFRP strengthening are tested under the secondary loading.The whole torque-twist curves,the failure mode of corrugated steel webs and concrete slabs,the variation law of shear strain in corrugated steel web and the variation law of axial strain in CFRP strips are obtained in the model test.Then the torsional mechanism of single-box multi-cell composite box girders with corrugated steel webs with/without CFRP strengthening subjected to secondary loading is preliminary revealed.Incorporating the characteristics of cyclic constitutive relationships of concrete,steel bar and corrugated steel webs and the effect of CFRP strengthening on the stress state and constitutive relationships of reinforced concrete member,the torsional analysis models for single-box multi-cell composite box girders with corrugated steel webs with/without CFRP strengthening subjected to secondary loading are developed on the basis of unified model SMMT.The model is validated by the experimental results and hence can predict the torsional behavior of single-box multi-cell composite box girders with corrugated steel webs with/without CFRP strengthening subjected to secondary loading.The experimental and theoretical results show that the whole cross section of damaged composite box girders should be strengthened to give a full play to the torsional resistance of concrete slabs and corrugated steel webs,and to ensure that the whole section can resist the external torques together.