Experimental Research On Joints Of Ribbed Beam In New Tubular Roof Structure

New tubular roof (NTR) structure construction technology is a new construction method for underground excavation structural engineering. This construction method has some more incomparable advantages than conventional construction in the fields of construction under the existing structure, construction of adjacent subway structure, long span and ultra-shallow underground structures and so on. From the point of engineering view, the technology of the NTR structure construction method was experienced by a large number of tests, and the well-known products that were the combination of both technologies and innovation. The combination of the technology makes the NTR structure a very different structure type from the conventional structures. There are many uncertainties for both the related design method and the construction theory of the NTR structure in the implementation process. Research on joints of ribbed beam for the NTR structures is one of the key issues on this kind of structure.This paper consists of two parts of experimental research and theoretical analysis. The experimental researches on both behaviors of inner forces, failure modes and mechanism of the joints with various migration rates of cross-section and shear spans were firstly carried out to develop the static performance of the joints through the static test of the four joints of ribbed beam in NTR structure. Then the main influencing factors for the mechanical performance of the joints were also studied by the experimental data processing. The four main aspects were described as following: (1) the influence of the migration rate of the cross-section of the ribbed beam on shear strength and ductility of the joints; (2) the influence of shear span ratio of the concrete filled steel tube beam on the performance of shear strength of the joints; (3) the influence of the reinforcement stirrup on the performance of shear strength of the joint; (4) establishment of equations for the bearing capacity calculation of the joints.The experimental phenomenon and data analysis showed that when the shear span was relatively small, the joints were prone to being shear failure; when the shear span was large enough and the migration rate of ribbed beam cross-section was high, the joints were easy to generate shear failure; when the migration rate were relatively low, the joints trended to occur combination failure of shear and flexure. Experimental data showed that: (1) the shear capacity and ductility of the joints may be improved if the migration rate of the cross-section of the ribbed beam was appropriately increased to make the core concrete of the joint in the state of the biaxial compression. (2) The moving scope of the neutral axial of the cross-section for the concrete filled steel tube beam may be reduced if the ratio of the beam was properly reduced, thereby, in reducing the scope for the state of concrete to transfer from a two-way compression state to a combination state of tension and compression to improve the shear capacity of the joint. (3) Stirrup on both sides of steel tube ribs beam had a larger strain. Therefore, the shear capacity of the joint can be increased by adding the stirrup on both sides of steel tube ribs beam by increasing the stirrup ratio or increasing the level of the stirrup.The calculation equation for the bearing capacity of the joint was established according to the failure mode and properties in the ultimate state and combination of the main influencing factors on the bearing capacity. The results for both the calculation using the developed equations and experiment were compared and they matched well.