Study On Fire Resistance Of Beam And Column In The Overall Steel Frame

Steel structure is not refractory, which is widely accepted by the people; a largenumber of fire hazards due to steel structure show that the members in the overallstructure will be seriously deformed and even damaged; however, the overall structuredoes not lose its carrying capacity. This phenomenon is not consistent with the currentfindings of the fire resistance of steel members, and presently, China has not witnessedany study on the fire resistance of the members in the overall steel structure.Sponsored by the National Key Basic Research Development Program "Impact onstructure steel frame structure from the spreading space fire"(ID:2001CB40960310)and the National Natural Science Foundation’s project titled "Testing study on theacoustic emission and vibration characteristics of structural members in fire"(ID:50878069), a3×3full scale structural steel frame building with3floors is built, wherethe floor height of the1st floor is3,500mm, those of the2nd and3rd floors are both3,000mm, the column spacing is4,500mm, and the cast-in-place concrete slab thicknessis120mm. It is the first time for China to conduct the testing study and finite elementsimulation analysis on the fire resistance of steel beams and steel columns at differentpositions under the dead load, steel columns, and the analysis on acoustic emissioncharacteristics and vibration characteristics of the beams and slabs in fire. The maincontents are as follows:(1) Testing study on steel beam in the steel frame. Testing study on fire resistanceof single span skirt beam, mid-span beam, two-span continuous girder, and three-spancontinuous beam in the steel frame building under the dead load. The test coversmeasurement of air temperature at the scene of fire, sectional temperature fielddistribution law of steel beam and slab at different positions, deflection and deformationof steel beam and floor slab, etc., and investigation on failure forms of steel beam andfloor slab.The testing results show that: there exists significant temperature differencebetween the upper flange of steel beam and the temperature at the scene of fire, andthere also exists large temperature difference between the steel beam and the floor slabconcrete on the same section; shortly after the test, many cracks appeared on the floorslab surface, the steel beam was vertically deformed downward at first, and then beganthe decrease in downward deformation degree in the heating process, and was finally deformed upward after recovery to its initial position before such test; the welds on theupper and lower flanges of the steel beam were cracked during the cooling stage; in thewhole process of the fire, steel beam was not disengaged with the cast-in-place concretefloor slab, and they withstood the force and were deformed together. Under alloperating conditions, the steel beam experiences the temperatures above900degrees,which was deflected and deformed after being heated for around120minutes, but didnot lose its carrying capacity, showing a good fire resistance.(2) Testing study on steel column in the steel frame. Testing study on fire resistanceof side column with one side exposed to the fire, side column with three sides exposedto the fire,2middle columns and corner column in the steel frame building under thedead load. The test covers measurement of air temperature at the scene of fire,temperature field distribution of steel column on different sections, lateral deformationof steel column on different sections, axial deformation of steel column, temperature ofconcrete on the top floor slab of steel column, etc., and investigation on deformation ofthe adjacent steel beams and failure forms of steel column. The testing results show that:the beam column with four sides exposed to the fire and the middle column with itsjoint exposed to the fire suffered no obvious axial expansion and deformation during thetemperature rise for76minutes, but sudden axial deformation afterwards.for axialdeformation under other conditions, expansion occurs at first and then decreases indegree in the temperature rise, and at last, compressive deformation occurs; theexpansion deformation of corner column decreases in degree in the cooling stage;inverse lateral deformation of steel column happens during the test; torsion failurehappens to the steel column with four sizes exposed to the fire; side column with oneside exposed to the fire and corner column suffers no significant damage; under alloperating conditions, the structure does not collapse regardless of damage to the steelcolumn.(3) Nonlinear finite element simulation analysis on beam and column in the steelframe.The testing study research cannot cover all behaviors of the members in theoverall structure in fire, but finite element simulation analysis can make up for thisdeficiency. This paper gives consideration to material nonlinearity and geometricnonlinear, uses incremental method to establish the relationship between stressincrement and strain increment, uses ANSYS finite element software to establish themodel for analysis on the fire resistance of steel beam and steel column taking intoaccount the function and overall effect of the floor slab; the results show that: the temperature field of the member on the section, deflection and deformation of steelbeam, lateral deformation and axial deformation of steel beam in the finite elementanalysis are consistent with the testing results, indicating that the establishment of thefinite element analysis model is reasonable and feasible, and is able to carry outsimulation analysis on the members for beams and columns under other conditions.(4) Study on acoustic emission characteristics of beam and slab in the steel framein fire. Use acoustic emission instrument to monitor the acoustic emission of beam andslab in the fire. Monitor the acoustic emission event rate, energy rate and b valuechanges in the fire, and analyze the relationships between the parametric variations andfloor slab crack, steel beam deformation and other phenomena. The results show that:the changes in event rate and energy rate are closely related to the occurrence anddevelopment of macro-cracks, and b value can better reflect the intensity of acousticemission signal of steel beam in the frame building in the fire and determine theconditions of the beam and slab in the fire; in the process of worsening, the steel beamin deformation will generate a lot of acoustic emission signals that will be weakened inthe deformation recovery process; among the acoustic emission events, those less than80db account for more than90%of the total; energy only accounts for around50%; theevents requiring the main function of energy release only account for10%of the total.(5) Study on vibrate characteristics in the steel frame in fire. Use vibrationequipment for monitoring the whole process of acceleration of lower beam and slab infire. And use Fourier transformation for analysis of the acceleration, and combine thenumerical calculation for analysis of vibration of beam and slab in the overall structure.The results show that: The single-span steel beam frequency in the overall structurefluctuates around20Hz; the two-span continuous beam’s vibration frequency is below20Hz; as the test goes on, the stiffness of the overall structure is reduced, smallfluctuation happens to the vibration frequency; the study on vibration characteristics canprovide a theoretical basis for early warning of structure collapse.