Study On The Shear Performance Of New Bolted Shear Connectors For Prefabricated Steel-UHPC Composite Beams

Steel-concrete composite beams have the advantages of large bearing capacity,stable mechanical properties,easy prefabrication and installation,and can give full play to the respective advantages of steel and concrete materials.As an important component connecting steel and concrete,high-strength bolted connections have the advantages of good mechanical properties and convenient installation,and are the preferred type of connection for assembled composite beams.UHPC,as a material in the frontier field of civil engineering,has much better performance than normal coagulation material in all aspects.Using UHPC plate instead of normal concrete plate in steel-concrete composite beam can not only effectively reduce the dead weight of bridge,but also greatly improve its mechanical properties.To this end,a new type of embedded bolted connection with inner nut is proposed in this paper,and the mechanical properties of assembled steel-UHPC composite beam are studied based on the new connection.The main research contents include:(1)The test specimens and loading system of bolt connection of assembled composite beam are designed.In order to explore the mechanical properties of bolted connections of assembled composite beams under different loading modes,18 groups of test specimens with different bolt strength,inner nut type and concrete slab type were designed.At the same time,in order to meet the loading requirements of different test specimens,an appropriate test loading and connecting system is designed,and a detailed loading scheme is formulated,including monotone loading and cyclic loading.(2)The mechanical property and failure mode of the push-out test piece of bolted joint of composite beam are investigated.The push-out test of bolted connection of assembled composite beam under different loading modes was carried out.It was found that the increase of bolt strength had a significant impact on the increase of mechanical properties(ultimate bearing capacity P_u,initial stiffness k,energy consumption capacity)(for example,Pu increased by 38.3%and 44.7%respectively with the increase of bolt strength from G4.8→G8.8→G12.9),but the failure degree of the test piece would be aggravated;The mechanical property of the test piece was significantly improved by T-shaped inner nut compared with flange inner nut(compared with flange nut,P_u increased by 2.3%,k increased by 7.9%and friction resistance P_fincreased by 20.3%);UHPC materials have better mechanical performance in thin concrete slab composite beams.In addition,the loading failure process of the specimen is demonstrated by the finite element calculation software,and the mechanical property advantage of the inner nut is verified.(3)The ultimate bearing capacity design formulas of embedded bolted connections with internal nuts in assembled combinations are proposed.Based on the test and finite element results,combined with the specifications and previous research results,the ultimate shear capacity and load-slip curve formulas of embedded high-strength bolted connections with internal nuts in assembled composite beams are proposed.The accuracy and feasibility of the two formulas are verified by comparing the results of the formulas with the test results.