[精品论文]Full-Range Analysis on Concrete-filled Steel Tubular Column to Steel Beam Joint Subjected to Full-Range Fire HAN Linhai, SONG Tianyi.doc

精品论文full-range analysis on concrete-filled steel tubular column to steel beam joint subjected to full-range fire han linhai, song tianyi5(department of civil engineering, tsinghua univeristy, beijing 100084)abstract: this paper performs a full-range analysis on concrete filled steel tube (cfst) column to restrained steel beam joints subjected to combined initial load and heating and cooling fire. a finite element analysis (fea) model is established to simulate the cfst column to restrained steel beam joint with considering the influence of the slippage between steel beam and rc slab and the10high-temperature creep of steel. based on the fea model, failure modes, deformations, internal forces and moments of this type of joint in the entire phase, including ambient tempearature loading, heating,cooling and post-fire loading, are analyzed. finally, joint moment versus relative rotation angle full-range relation curve of the composite joints is calculated, and parameter analysis is performed to investigate the influence of various factors on post-fire residual load bearing capacity index of the joint.15keywords: concrete filled steel tube (cfst); steel beam; joint; full-range fire; full-range analysis0introductionconcrete filled steel tube (cfst) has been found increasingly wide applications in the construction of new buildings due to its excellent structural performance, such as superior strength,20ductility and fire resistance. in the past, plenty of research works have been done to investigate the behavior of cfst members and structures under and after fire 1. for real structures subjected to real fire, the structural components experience an entire time (t) - temperature (t) - load (n) path,as shown in fig. 1, which can be divided into four phases, which are:nncrdppost-fire loading phase- 14 -noinitial loading phase ac dbtheating phase ooacooling phasethctp tdtdthtb25fig. 1 time (t) - temperature (t) - load (n) path(1) initial loading phase (aa). apply initial load (no) on the structural component at ambient temperature;(2) heating phase (ab). increase environmental temperature and keep no constant;30(3) cooling phase (bcd). after environmental temperature achieves th corresponding tofoundations: specialized research fund for the doctoral program of higher education (no. 20090002110043) brief author introduction:han linhai, (1967-), male, professor, research direction: performance of composite structures. e-mail: heating time (th), the temperature starts to decrease with constant load no, where, tp and td represent the time that environmental and structural temperature drop to ambient temperature, respectively;(4) post-fire loading phase (ddp). after the structural temperature drops to ambient35temperature, increase external load until the structural component fails at critical load (ncr).cfst column to steel beam joint, which is a type of typical beam-column connection, has been adopted in several high-rise buildings in recent years. previous researchers have performed some investigations on cfst column to steel beam joints under fire and after fire1, such as, han et al.2 studied the post-fire performance of cfst column to steel beam joints under the path40a-b-c-d-dp in fig. 1. however, the research based on a full-range fire path is limited. the significant influence of full-range fire path, including initial loading at ambient temperature, heating, cooling and post-fire loading phases, on the performance of structures has been proved by the researches on cfst columns34. therefore, for the cfst column to steel beam joint, the life-time performance research by adopting an entire t-t-n path just like the a-a-b-c-d-dp45path in fig. 1 is necessary.cfst column to steel beam joint specimens under the entire t-t-n path were tested by song et al.5, and a finite element analysis (fea) model was established to simulate the joint tests. here, based on the established fea model by song et al.5, a numerical model of cfst column to restrained steel beam joint isolated from a cfst planar frame is proposed, and some key issues,50including failure modes, deformations, internal forces and moments, are investigated to understand the behavior of this type of joint in the entire loading and fire phases. finally, joint moment versus relative rotation angle relationship is calculated, and parameter analysis is performed to study the influence of key factors on residual load bearing capacity index of the composite joint.551finite element analysis modelbased on the decision of concrete and steel properties in ambient temperature, heating, cooling and post-fire phases, a finite element analysis (fea) model is proposed by song et al. to simulate the behavior of cfst column to steel beam joint subjected to combined fire and load5. in this paper, the fea model is modified to simulate the behavior of cfst column to restrained60steel beam joint. the influence of slippage between steel beam and rc slab and high-temperature creep of steel are considered in the model by using subroutines in abaqus software.1.1general descriptioncross-shaped beam-column joint isolated from a planar frame, as shown in fig. 2, is chosen as the calculation model. the joint is composed of a circular cfst column, two steel beams65connected with the column via external ring and a reinforced concrete (rc) solid slab connected with steel beam via shear connectors. vertical load (nf) and uniform load (q) are applied on the column and rc slab, respectively. the ends of beam and slab are restrained against rotations and translation in axial direction, the bottom end of column is fixed, and the top end of column is restrained against rotations. the parts of the joint under rc slab expose to heating and cooling70fire.based on the calculation model, a typical cfst to restrained steel beam joint with rc slab is designed according to standards dbj 13-51-20036 and gb 50017-20037. to reach the fire resistance for column and beam required in standard gb 50016-2006 8, fire protection layers are applied on the surfaces of cfst column and steel beam. details of joint are following:nfq75fig. 2 calculation model of joint in frame structurecircular cfst column: outside diameter of cfst column section (dc) is 600mm; thickness of steel tube (ts) is 12 mm; height of column (h) is 6000 mm.80h-shaped steel beam: height of steel beam (h) is 400 mm; width of steel beam (bf) is 200mm;thickness of web (tw) is 15 mm; thickness of flange (tf) is 15 mm; length of steel beam (l) is 8000 mm.859095100105110rc slab: width of slab (bslab) is 3000 mm; thickness of slab (tslab) is 120 mm; length of slab(lslab) is 8000 mm; longitudinal bar: 10150mm; distribution bar: 10250mmexternal ring: width of external ring is 120mm, and the same thickness with the flange of steel beam.shear connectors: 16100mm, two rows along the beam with the spacing of 200mm.fire protection thickness: thickness of column fire protection (ac) is 15 mm corresponding to cfst column fire resistance of 3h; thickness of beam fire protection (ab) is 20 mm corresponding to steel beam fire resistance of 2h.material properties: strength of core concrete in cfst column (fcuc) is 60 mpa; strength of concrete in rc slab (fcus) is 40 mpa; yield strength of steel tube (fyc) is 345 mpa; yield strength of steel beam (fyb) is 345 mpa; yield strength of steel bars in rc slab (fybs) is 335 mpa; tensile strength of shear connector is 400 mpa.load and fire conditions: column load ratio (n) is 0.6, n is defined as nf/nu, where nu is the axial compressive capacity of column at ambient temperature; beam load ratio (m) is 0.4, m is defined as q/qu, where qu is the ultimate capacity of steel beam with rc slab under uniform load; heating time ratio (to) is 0.5, to is defined as th/tr, where th is the heating time, and tr is the fire resistance of joint.based on the decided calculation conditions, finite element analysis model is established to simulate the temperature distributions and mechanical action of cfst column to restrained steel beam joint with initial loads subjected to iso-834 9 heating and cooling fire.for the temperature field analysis model, the modeling method proposed by song et al.5 areadopted. for the mechanics behavior analysis model, the material properties corresponding to different temperature phases, element types and methods for simulating core concrete - steel tube interface and steel bars - concrete interface suggested by song et al.5 are adopted. the boundary conditions as shown in fig. 2 is adopted. slippage between steel beam and slab and the high-temperature creep of steel are considering in the modified fea model. details can be foundin sections 1.2 and 1.3. fig. 3 illustrates the element division, thermal and mechanics boundary conditions of cfst column to restrained steel beam joint.cfst columnnf top endplate(constraint uy, rx, ry, rz)constraintux, ry, rzqrc slabbars inrc slabconstraintux, ry, rz zxysteel beamheat radiationbottom endplate(fixed end)heat fluentcoolingheatingfig. 3 element division and boundary conditions of cfst column to restrained steel beam joint1151201251301351.2slippage between steel beam and rc slabtwo-node spring elements are used to simulate the relative slippage between steel beam and rc column due to shear deformation of shear connectors induced by the horizontal shear. currently, plenty of research have been done to investigate the horizontal shear versus relative slippage relationship of shear connectors at ambient temperature, but for the shear connectors at elevated temperature, the related research is limited. such as, kruupa and zhao10 presented shear force ratio versus slip relationship based on push-out tests of shear connectors at elevatedtemperature. mirza and uy11 adopted solid element to simulate the behavior of shear connectors at elevated temperature. however, there is no shear force (q) - relative slippage () relation equation corresponding to different temperatures has been published. in this paper, a q - equation for shear connector at ambient temperature proposed by ollgaard et al. 12 is tentatively adopted with modified ultimate shear bearing capacity corresponding to different temperatureaccording to en 1994-1-2: 200513. details of modified q - equation can be found in 14.1.3high-temperature creep of steeltotal strain of steel under fire includes stress-related strain, thermal strain and high-temperature creep strain. in the previous fea model proposed by song et al.5, stress-related strain and thermal strain were considered, but the high-temperature creep strain induced by high temperature was ignored. therefore, in this paper, based on the decision of steel high-temperature creep model at heating and cooling phases, steel strain induced by high temperature creep isconsidered in the fea model by adopting user subroutine uexpan.steel high-temperature creep strain is a function regarding to the constant stress, temperature and length of time, and extensive researches have been previously performed to calibrate the steel high-temperature creep strain equation, and the bailey-norton law was usually adopted 15. fieldsand fields 16 proposed a steel high-temperature creep strain equation based on bailey-norton law,and it has been used to calculate the steel creep strain during the heating and cooling phases successfully, therefore, the same equation is adopted in this fea model. details regarding the steel high-temperature creep formula and user subroutine can be found in 14.1401451501.4verification of fea modeldue to the absence of experimental data on cfst column to restrained steel beam joint subjected to full-range fire, the experimental results on three cfst column to steel beam joint subjected to heating and cooling fire reported by song et al. 5 are adopted to verify the fea model. the deformations and failure modes of the joint specimens were predicted and compared with the tested results. take joint specimen jcfst2 as an example, the comparison between predicted and measured axial deformation of column (c) versus time (t) curve and beam end deflection (b) versus time (t) curve are shown in fig. 4. it can be found that the joint deformations versus time relation curves can be divided into three phases, which are ambient temperatureloading phase (i),heating and cooling phase (ii), and post-fire loading phase (iii)，and generally,the predicted curves agree with the measured results very well.2i ii iii0 c(mm)-2-4 meas uredpredicted-6300b (mm)-30-60-90-120-150i ii iii meas uredpredict ed0 5 10 15 20 25 30 35t (h)0 5 10 15 20 25 30 35t (h)(a) axial deformation of column (c) versus t curve (b) beam end deflection (b) versus t curvefig. 4 comparison between measured and predicted joint deformations versus time (t) curves155fig. 5 shows the comparison between observed and predicted failure mode for joint specimen jcfst2. it can be found that, after the joint specimen survives from the heating and cooling fire, as the increasing of external beam load, the joint fails due to the local buckling of bottom flange of the steel beam near the joint zone. the same failure modes can be found from the observed andpredicted results.buckling160(a) observed failure mode (b) predicted failure modefig. 5 comparison between observed and predicted joint failure modebased on the verified fea model, the full-range analysis on cfst column to restrained steel beam joint subjected to full-range fire can be done.精品论文1651701751802full-range analysis in the entire loading and fire historybased on the established fea model, the performance of cfst column to restrained steel beam joint with initial load exposed to heating and cooling fire are studied. some issues, such as temperature distributions, failure mode, deformations, internal forces, influence of slippage between steel beam and rc slab and high-temperature creep of steel, are discussed. however, in the paper, only some key issues, including failure mode, deformations and internal forces of the joint, are presented.2.1failure modeduring the entire loading and fire phase, the joint may fail in different phase, such as, failure at ambient temperature loading phase due to the increased external load; failure at heating or cooling phases due to the degraded material properties and changed internal forces, in which, the failure occurred at heating phase is also named as fire resistance of the joint; failure at the post-fire loading phase due to the increased beam load. in this paper, the research will focus on the case of joint failure occurred in the post-fire phase.fig. 6 shows the typical failure mode of cfst column to restrained steel beam joint after exposed to heating and cooling fire. it can be found that the cfst column shows good working performance, no significant deformation or steel tube local buckling is observed. the joint failsdue to the local buckling of bottom flange and web of steel beam in the joint zone.upper columnleft beamlocal bucklinglower columnright beam185190195fig. 6 typical failure mode of cfst column to restrained steel beam jointto investigate the influence of various parameters on the failure mode of cfst column to restrained steel beam joint, two important parameters, including column load ratio and beam load ratio, which directly affect the behaviors of column and beam, are chosen to study the joint failure modes. when beam load ratio is 0.4, column load ratio ranges from 0.2 to 0.8, and when column load ratio is 0.6, beam load ratio ranges from 0.2 to 0.8. the predicted results indicate that the influence of column load ratio is minor. the reason is that fire protection is applied on the cfst column to make sure the fire resistance of column reaches 3h, which leads to the fact that damage of cfst column during the heating and cooling phases is slight. for the beam load ratio, the influence on joint failure mode is moderate. position of local buckling changes as the increasing of beam load ratio, but the failure modes are closed to that in fig. 6. it can be concluded that, for the cfst column to restrained steel beam joint, the failure of joint is controlled by the steel beam. to improve the fire and post-fire performance of this type of joint, additional fire protection or fire resistance structural measures are necessary.2002052.2deformations of jointdeformations of cfst column to restrained steel beam joint in the entire loading and fire phase, including axial deform