Bending and flexural-buckling strength of steel members considering strain-rate-effects at elevated temperatures

Abstract

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.

Proposal of steel stress-strain relationships and simple analytical models of beams considering strain-rate effects at elevated temperatures

“…Each effective strength under the various strain rates at elevated temperature is shown in Figure 17 (Ozaki and Umemura, 2021).The analytical results indicated by the marks in Figure 16 show the bending strength and collapse temperature under the transient-state condition at the collapse deflection δ b p . All analytical results exhibited the same trend as the past test results, which used smallbeam specimens (Ozaki and Umemura, 2021). The evaluated values (lines in Figure 16) when factor k y ðθÞ on the 0.002 offset yield strength f y were smaller than both the steady and transient test results (Figure 16).…”

“…Figure 16 shows the analytical results of the collapse temperature and bending strength, which have been summarised by the same methods used in the previous research (Ozaki and Umemura, 2021). The non-dimensional bending strength mðθÞ shown by the three lines in Figure 16 was obtained using equation (33).…”

“…Based on the test result of the T700F specimen under the fast-loading condition (Figure 9), the elastic stiffness is smaller than that of the analytical results. This was because the measurement interval for the T700F test was set as one second (Ozaki and Umemura, 2021). An accurate measurement could not be obtained using the above-mentioned measurement interval owing to the very rapid deformation rate (55 [mm/min]) of the T700F specimen.…”

“…The authors have examined effects of steel strain rate on effective yield strengths, load-bearing capacities, and collapse temperatures for steel members at elevated temperatures, by conducting the coupon tests, the bending and buckling tests using small simple-supported steel beam and pin-supported steel column specimens (Ozaki and Umemura, 2021). From those test results, it was clarified that the steel strain-rate at the elevated temperatures had an influence on the structural fire resistance for the steel beam and column specimens under the steady-state condition (load-increasing condition under a constant temperature) and the transient-state condition (temperature-increasing condition under a constant load).…”

“…From those test results, it was clarified that the steel strain-rate at the elevated temperatures had an influence on the structural fire resistance for the steel beam and column specimens under the steady-state condition (load-increasing condition under a constant temperature) and the transient-state condition (temperature-increasing condition under a constant load). In this study, engineering stress-strain relationships considering the effect of strain rate on the steel materials at elevated temperatures were formulated based on the coupon test results under the various strain rates, which has been obtained by the authors (Ozaki and Umemura, 2021). The formulated stress-strain relationships were applied to a simplified analysis using a two-dimensional beam element proposed in this study to analytically examine the effect of strain rate on the load-bearing capacity and collapse temperature of a cantilever steel beam at elevated temperatures.…”