The Fire - Resistance Properties Of Welded Square Steel Nodes

For square tubular joint with hollow section, local buckling failure usually occurs around thechord/brace intersection on chord surface due to inadequate radial stiffness of chord which isgenerally lower than the axial stiffness of brace when axial load is applied on the brace end of tubularjoint. Additionally, that yield strength and elastic modulus of steel degrade considerably at elevatedtemperature makes unprotected tubular joints with hollow section destroyed easily in the condition offire. Hence, it is necessary to study the fire resistance of square tubular joint so that somerecommendations could be provided for engineering design and norm.In this thesis, the fire resistant performances of two square tubular T-joints with hollow sectionare tested. During tests, a constant axial load is applied at brace end firstly, and then the T-joint isheated in accordance with standard heating curve until failure of the joint. The temperatures on tubesurface and displacements of joint are monitored during test, and the heating process, failure modeand failure procedure of T-joint under fire are also recorded. Then, general finite element softwareABAQUS is used to simulate the fire resistant experiments of the T-joints. By comparing the resultsof finite element simulation and tests, the accuracy and reliability of temperature model andforce/displacement model is proved. The results of experiments and finite element analysis show that(1) the whole failure procedure of square tubular T-joint with hollow section under fire could bedivided into three stages comprised of expanding and deformation decreasing of joint at thebeginning of heating, deformation increasing caused by deteriorated mechanical properties of steelwith the temperature increasing and failure stage in the end.(2) The failure mode of square tubularjoint with hollow section at elevated temperature, which is similar with the one at ambienttemperature, is local buckling around the brace/chord intersection on the chord.Based on the verified finite element models, analysis on the fire resistance of24square tubularjoints with different geometrics is conducted. The failure mode and duration of fire resistance ofjoints are obtained, and effects of geometrical parameters including α, β, γ and τ on fire resistance ofjoint are also concluded. Additionally, another21T-joint models are also calculated, and the impactof load ratio n_band axial load ration of chord ncon fire resistance of T-joints is also obtained. It can beconcluded that (1) in all dimensionless geometrics, β and γ affect the duration of fire resistance greatly, and parameters α and τ have little effect on the duration of fire resistance. When otherparameters are unchanged, the critical temperature of T-joint with a bigger β will increase and thecorresponding fire resistant period will be longer. On the contrary, Critical temperature of T-jointdecreases and fire resistant period is shortened with the increase of γ.(2) Fire load ratio nbhassignificantly effect on the duration of fire resistance of joint. When the geometrical parameters areconstant, critical temperature of T-joint decreases and fire resistant period is shortened with theincrease of nb.(3) Axial load on chord has influence on both duration of fire resistance and failuremode of joint. When the axial tension load is big enough, the failure mode of joint is changed fromlocal buckling around intersection on chord surface to plastic failure of chord. When the geometricalparameters are unchanged, critical temperature decreases and duration of fire resistance is shortenedwith the increase of compressive load which is applied along the axial direction of chord, and axialtensile load has relatively limited influence on duration of fire resistance of joint.