Static And Fatigue Behavior Of Q690 High-Strength-Steel-UHPC Composite Beams And Design Methods

Steel-concrete composite beam makes full use of the properties of steel beam and concrete in tension and compression,respectively.It has many advantages such as light weight,high flexural capacity,high stiffness,and fast construction.Composite beam has been widely used in building and bridge structures.In the past few years,as the height of buildings and the span of bridges increasing,the cross-section and weight of the composite beam increased.In order to make the structure lighter,the strength of concrete and steel is increasing.Among all the high-strength concrete,ultra high performance concrete has the most excellent properties.In engineering,high strength steel-UHPC composite beams begun to be used.In addition,in order to improve the quality and speed of construction,assembly construction has been developing rapidly in recent years.Under the assembly construction,the studs are arranged in group.The precast concrete slab is pre-drilled with holes at the location of the group studs,and it only needs to be pouring at the area of group studs on-site.However,the materials in the domestic and foreign national codes are normal steel and normal concrete,without involving high-strength steel and UHPC.These codes rarely involved the group stud arrangement as well.Furthermore,there are few studies on the static and fatigue performance of high-strength steel-UHPC composite beams.The static and fatigue failure modes and design methods have not been fully understood,which undoubtedly limits the application of high-strength steel-UHPC composite beams in engineering.Therefore,this paper presents an in-depth and systematic study on the static and fatigue performance of high strength steel-UHPC composite beams with experiments,numerical analysis and theoretical analysis.The main research work and the results obtained in this paper are as follows:(1)Six Q690 high-strength steel-UHPC pushout specimens were designed into three group.The experimental parameters included diameter and arrangement form of studs.The test results showed that the failure mode of Q690 high-strength steel-UHPC pushout specimens was stud fracture.There were no obvious cracks in the UHPC slab except for crushing at the local area near the studs,which showed that UHPC has good compressive properties.The increase of stud diameter could significantly improve its shear capacity and shear stiffness,while the stud arrangement pattern has little effect on the shear bearing capacity of the studs.The stud arrangement had effect on the shear stiffness,and the shear stiffness of the studs would decrease after the group stud arrangement pattern was adopted.In comparison with other studies,it was found that UHPC could improve the shear capacity and stiffness of the studs compared with normal concrete,while the ductility would be reduced.Q690 high-strength steel does not have a significant effect on the static shear performance of the studs.(2)Finite element analysis software ABAQUS and its scripting interface program Python were used to analyze the effects of different parameters on the static shear performance of studs under the combination of high-strength steel and UHPC.The results showed that,for the single stud specimens,shear capacity of the studs would be affected by stud diameter,tensile strength of studs and compressive strength of UHPC.Among these parameters,stud diameter has the most significant influence.Besides,shear stiffness of the studs would be affected by stud diameter and elastic modulus of UHPC.The stud diameter has the most significant influence as well.For the group stud specimen,when the longitudinal and transversal interval of studs was less than 6 times and 4 times stud diameter,the interval has less effect on shear capacity and stiffness of the studs.(3)Six Q690 high-strength steel-UHPC composite beams were designed,the test parameters included type of bending moment(positive or negative moments),stud arrangement(single or group stud arrangement),and the degree of shear connection(full or partial connected).The test results showed that the failure mode of the full shear connected composite beam was overall bending failure,while the failure mode of the partially shear connected composite beam under positive moment was fracture of studs in shear span.The stud arrangement did not have a significant effect on the failure mode of Q690 high-strength steel-UHPC composite beam,but only affected the crack distribution on the UHPC slab.The stud arrangement had not significant effect on the flexural capacity of the composite beam.However,when the stud arranged in group,shear stiffness of the studs would reduce.(4)The finite element model of high-strength steel-UHPC composite beam was established to analyze the bending performance difference between UHPC and normal concrete,Q690 high-strength steel and normal steel in composite beam.Then,the effect of different parameters on the flexural performance of high-strength steel-UHPC composite beam were analyzed,including degree of shear connection,yield strength of steel,compressive strength of UHPC,etc.The results showed that UHPC was better than normal concrete C60 on flexural performance in composite beam,as the flexural capacity and stiffness improved.Q690 high-strength steel could significantly improve the flexural capacity of the composite beam compared with normal steel,but did not improve the flexural stiffness.As the shear connection degree increased,the flexural capacity and flexural stiffness of composite beam would improve,and the improvement of flexural stiffness was more obvious.The compressive strength of UHPC and the strength of steel would improve the flexural capacity of the composite beam,and the strength of steel would improve the flexural capacity more significantly.The influence of UHPC slab width,thickness and reinforcement ratio on the flexural capacity and flexural stiffness of the composite beam was small.(5)Based on the previous experimental research and finite element analysis,the static design method of high-strength steel-UHPC composite beams was investigated.The shear capacity of studs was calculated by equation fitting.The energy equivalent model was established to derive the shear stiffness of the studs under single and group stud arrangement.With reference to the chinese steel design standard and using the finite element model,the equation of flexural capacity of full and partial shear connected composite beam was proposed,and the deflection of high-strength steel-UHPC composite beam under positive and negative bending moment was proposed by the micro-sectional unit analysis of the composite beam,respectively.(6)12 Q690 high-strength steel-UHPC pushout specimens were designed into three groups,which launched for fatigue tests.The parameters included stud diameter and arrangement pattern.The test results showed that the fatigue failure mode of all Q690 high-strength steel-UHPC pushout specimens was fracture of the studs.The increase of stud diameter and the use of group stud arrangement pattern would reduce the fatigue shear performance of studs.Compared with the studs under the combination of normal steel and normal concrete,the fatigue shear performance of studs under the combination of Q690 high-strength steel and UHPC was more excellent.The fatigue strength in the national codes was obviously conservative.(7)17 Q690 high-strength steel-UHPC composite beams were designed into five groups,which launched for fatigue tests.The parameters included the form of bending moment,the stud arrangement pattern and the degree of shear connection.The test results showed that the fatigue failure mode of fully shear connected composite beam was fracture of the steel beam,while failure mode of one partially shear connected composite beam was the stud fracture in shear span.The UHPC slab was cracked under the negative bending momen,but the crack width develops slowly.Only after the fatigue fracture of the longitudinal reinforcement in the UHPC plate,the crack width develop rapidly.Thus,UHPC has good fatigue properties in tension.The fatigue life of the steel beam was counted,and the fatigue S-N curves of the steel beam were obtained under different fatigue details.For the fatigue details of the steel beam flange wielded by stud,the fatigue strength of the group stud arrangement pattern was lower than that of the single stud arrangement pattern.(8)The fatigue design method of Q690 high strength steel-UHPC composite beam was studied,and the corresponding fatigue strength S-N curves were proposed for three fatigue details,which including the studs in Q690 steel beam flange subjected to shear stress,Q690 steel beam flange welded by studs subjected to positive stress,and Q690 steel beam flange and web fusion penetration weld subjected to positive stress.This thesis clarified the static and fatigue flexural properties of Q690high-strength steel-UHPC composite beams,established a fine numerical analysis model,and proposed a reasonable and reliable static and fatigue design method,which provided a guide for the design of high-strength steel-UHPC composite beams and promoted the application.