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PurposeThe main purpose of this study was to propose theoretical calculation models to evaluate the theoretical bending strengths of welded wide-flange section steel beams with local buckling at elevated temperatures.Design/methodology/approachSteady-state tests using various test parameters, including width-thickness ratios (Class 2–4) and specimen temperatures (ambient temperature, 400, 500, 600, 700, and 800°C), were performed on 18 steel beam specimens using roller supports to examine the maximum bending moment and bending strength after local buckling. A detailed calculation model (DCM) based on the equilibrium of the axial force in the cross-section and a simple calculation model (SCM) for a practical fire-resistant design were proposed. The validity of the calculation models was verified using the bending test results.FindingsThe strain concentration at the local buckling cross-section was mitigated in the elevated-temperature region, resulting in a small bending moment degradation after local buckling. The theoretical bending strengths after local buckling, evaluated from the calculation models, were in good agreement with the test results at elevated temperatures.Originality/valueThe effect of local buckling on the bending behaviour after the maximum bending strength in high-temperature regions was quantified. Two types of calculation models were proposed to evaluate the theoretical bending strength after local buckling.

PurposeIn this study, the effects of strain rate on the bending strength of full-scale wide-flange steel beams have been examined at elevated temperatures. Both full-scale loaded heating tests under steady-state conditions and in-plane numerical analysis using a beam element have been employed.Design/methodology/approachThe load–deformation relationships in 385 N/mm2-class steel beam specimens was examined using steady-state tests at two loading rate values (0.05 and 1.00 kN/s) and at two constant member temperatures (600 and 700 °C). Furthermore, the stress–strain relationships considering the strain rate effects were proposed based on tensile coupon test results under various strain rate values. The in-plane elastoplastic numerical analysis was conducted considering the strain rate effect.FindingsThe experimental test results of the full-scale steel beam specimens confirmed that the bending strength increased with increase in strain rate. In addition, the analytical results agreed relatively well with the test results, and both strain and strain rate behaviours of a heated steel member, which were difficult to evaluate from the test results, could be quantified numerically.Originality/valueThe novelty of this study is the quantification of the strain rate effect on the bending strength of steel beams at elevated temperatures. The results clarify that the load–deformation relationship of steel beams could be evaluated by using in-plane analysis using the tensile coupon test results. The numerical simulation method can increase the accuracy of evaluation of the actual behaviour of steel members in case of fire.

PurposeIn this study, engineering stress-strain relationships considering an effect of strain rate on steel materials at elevated temperatures were formulated and a simplified analytical model using a two-dimensional beam element to analytically examine the effect of strain rate on the load-bearing capacity and collapse temperature was proposed.Design/methodology/approachThe stress-strain relationships taking into account temperature, strain, and strain rate were established based on the past coupon test results with strain rate as the test parameter. Furthermore, an elasto-plastic analysis using a two-dimensional beam element, which considered the effect on strain rate, was conducted for both transient- and steady-state conditions.FindingsThe analytical results agreed relatively well with the test results, which used small steel beam specimens with a rectangular cross-section under various heating rates (transient-state condition) and deformation rates (steady-state condition). It was found that the bending strength and collapse temperature obtained from the parametric analyses agreed relatively well with those evaluated using the effective strength obtained from the coupon tests with strain equal to 0.01 or 0.02 under the fast strain rates.Originality/valueThe effect of stress degradation, including the stress-strain relationships at elevated temperature, was mitigated by considering the effect of strain rate on the analytical model. This is an important point to consider when considering the effect of strain rate on steel structural analysis at elevated temperatures to maintain analytical stability unaccompanied by the stress degradation.

PurposeThe purpose of this study is to clarify both tensile and shear strength for self-drilling screws, which are manufactured from high-strength, martensitic-stainless and austenitic stainless-steel bars, and the load-bearing capacity of single overlapped screwed connections using steel sheets and self-drilling screws at elevated temperatures.Design/methodology/approachTensile/shear loading tests for the self-drilling screw were conducted to obtain basic information on the tensile and shear strengths at elevated temperatures and examine the relationships between both. Shear loading tests for the screwed connections at elevated temperatures were conducted to examine the shear strength and transition of failure modes depending on the test temperature.FindingsThe tensile and shear strengths as well as the reduction factors at the elevated temperature for each steel grade of the self-drilling screw were quantified. Furthermore, either screw shear or sheet bearing failure mode depending on the test temperature was observed for the screwed connection.Originality/valueThe transition of the failure modes for the screwed connection could be explained using the calculation formulae for the shear strengths at elevated temperatures, which were proposed in this study.

PurposeIn this study, the bending strength, flexural buckling strength and collapse temperature of small steel specimens with rectangular cross-sections were examined by steady and transient state tests with various heating and deformation rates.Design/methodology/approachThe engineering stress and strain relationships for Japan industrial standard (JIS) SN400 B mild steels at elevated temperatures were obtained by coupon tests under three strain rates. A bending test using a simple supported small beam specimen was conducted to examine the effects of the deformation rates on the centre deflection under steady-state conditions and the heating rates under transient state conditions. Flexural buckling tests using the same cross-section specimen as that used in the bending test were conducted under steady-state and transient-state conditions.FindingsIt was clarified that the bending strength and collapse temperature are evaluated by the full plastic moment using the effective strength when the strain is equal to 0.01 or 0.02 under fast strain rates (0.03 and 0.07 min–1). In contrast, the flexural buckling strength and collapse temperature are approximately evaluated by the buckling strength using the 0.002 offset yield strength under a slow strain rate (0.003 min–1).Originality/valueRegarding both bending and flexural buckling strengths and collapse temperatures of steel members subjected to fire, the relationships among effects of steel strain rate for coupon test results, heating and deformation rates for the heated steel members were minutely investigated by the steady and transient-state tests at elevated temperatures.

The mechanical properties and buckling strength of cold-formed steel square hollow section columns after heating and cooling processes were examined by coupon tensile tests and axially loaded compressive and stub-column tests at ambient temperature. The coupon tensile test specimens were made from the steel columns with the small sectional shapes (□-50–50-1.6 and 3.2 mm, respectively). Axially loaded compressive and stub-column tests were performed on these specimens. The specimens were treated by two heating and cooling processes: air cooling in a furnace for the specimens heated from 200 to 800 °C, and quenching with water for those heated from 600 to 800 °C for the coupon tensile test specimens. The test results of the axially loaded compressive and stub-column tests were compared with Eurocode and AS/NZS 4600 design equations. It was confirmed that the yield, tensile strength, and flexural and local buckling strength of cold-formed steel square hollow section columns decrease after the heating and cooling processes. Comparing with the test and calculation results of the Eurocode design equations, it was confirmed that the residual flexural and local buckling strength could be evaluated after the fire by the Eurocode design equations using the yield strength.

Charpy impact tests were performed on a cold-formed steel square hollow section at high temperatures and the ambient temperature after heating and cooling processes. The specimens were quarried from the flat and corner parts of the cross-section, respectively. High-temperature tests were conducted at 100–600 °C to verify the Charpy impact energies, including the blue-brittleness temperatures region (100–300 °C). In the ambient-temperature tests after the heating and cooling processes, the specimens were subjected to three heating and cooling methods: air cooling in a furnace from 600 to 900 °C (slow cooling process from the high temperature), quenching with water from 600 to 900 °C (fast cooling process from the high temperature), and air cooling outside of the furnace from 200 to 400 °C (slow cooling process from the medium temperature). The test results confirmed that the Charpy impact energy was not reduced after the slow cooling from the high temperature. However, the specimens subjected to either the quenching process from the high temperature or the slow cooling process from the medium temperature exhibited low Charpy impact energies.

Charpy impact tests were conducted on steel welded connections at elevated temperatures and at the ambient temperature after heating and cooling processes. The elevated temperature tests were conducted to examine the Charpy impact energies under the blue brittleness temperature (100–300 °C) of steel. V-notch specimens quarried from steel base metal, weld metal, and heat-affected zone of the welded connection were heated and subjected to Charpy impact tests. In the ambient temperature tests after the heating and cooling processes, the heated specimens were either air cooled in a furnace or quenched with water. The test results show that the Charpy impact energies do not decrease in the blue brittleness temperature, and the quenched welded connections demonstrated very low Charpy impact energies.

This paper presents a relationship between strain hardening of steel and plastic region length in a steel beam subjected to fire. A theoretical simple fire response analysis is proposed to investigate elasto-plastic behavior of the statically indeterminate heated beam. From observation of parametric calculations, it is clarified that large plastic region length in the beam, which is induced by the large strain hardening, is effective to avoiding strain concentration. This contributes to the robust attenuation of the thermal stress at the fire. Theoretische Beziehung zwischen Verfestigung von Stahl und Lange der plastischen Zone von brandbeanspruchten Stahltragern. Die Veroffentlichung stellt die Beziehung zwischen der Verfestigung von Stahl und der Lange der plastischen Zone von Stahltragern im Brandfall dar. Mit Hilfe eines analytischen Modells wird das elastisch-plastische Verhalten statisch unbestimmter Trager bei Erwarmung untersucht. Eine Parameterstudie zeigt, dass Dehnungskonzentrationen und hohe thermische Spannungen durch ausgedehnte plastische Zonen vermieden werden.

Fire resistance of structures depends on the structural stability during a fire.If a fire occurs in a building after an earthquake, structural members would lose their stability earlier than the nominal fire resistance time because the fire protection would be damaged by the earthquake. This paper shows the deformation behavior of the fire protections which comprise covering with sprayed rock wool, calcium silicate board, heat-resisting rock wool blanket, and heat-resistance coating sheet in shaking table tests on a full-scale steel structure. As a result of the tests, the fire protections were little damaged about efficiency of fire resistive performance.