Plenary lectures
Abstract of the presentation: The utilisation of high-strength materials in concrete-filled steel tubular members represents an efficacious methodology for enhancing the ultimate bearing capacity of the member while concomitantly reducing cross-sectional dimensions and dead weight, which exhibits considerable promise for prospective applications. However, the extant codes do not adequately address the use of high-strength materials. It is therefore essential to conduct a comprehensive investigation into the working mechanism and loading performance of high-strength concrete-filled square high-strength steel tubular (HCFHSST) members, followed by a further revision of the design codes and an expansion the application scope these codes. In this study, an experimental investigation was conducted to analyse the damage modes of the HCFHSST members. A total of 146 HCFHSST specimens were tested, and the buckling development mechanism of the steel tube under different loads was determined. On this basis, a refined numerical model was established. The application scope of the conversion equation between yield strength and tensile strength of high-strength steel was modified, and a constitutive relationship for composite materials in HCFHSST members was formulated. Furthermore, the deformation evolution law, strain development law, balance point motion law, and steel-concrete contact pressure of the members were analysed throughout the entire loading process, thereby revealing the working mechanism and stress performance of HCFHSST members. Finally, the bearing capacity calculation equations and the simplified bearing capacity calculation formula for various basic members have been established, thereby satisfying the criteria for reliable indexes for ductility failure structural members as outlined in GB 50068-2018.
Abstract of the presentation: In common seismic design applications, response spectra are a common means of specifying seismic action. According to Eurocode 8, every country may adopt its own elastic response spectrum form after defining it in the National Annex. Since such country-specific spectra should, if possible, be derived through analysis of the strong motion data recorded in the same seismo-tectonic region, the strong motion records currently available for the North-Western Balkan region are examined for several strong-motion recording sites. The results show that the shallow and deep geology conditions have a significant influence on the severity of surface strong ground motion. Moreover, it is demonstrated that the parameter that modifies the spectral amplitudes in relation to viscous damping is also dependent on the deep and shallow geology, and that it alters with vibration periods in a way that is comparable to the amplitudes of the pseudo-velocity spectra. Therefore, this parameter is neither independent of vibration periods nor amenable to being averaged over a large range of vibration periods. Lastly, it is shown that the uniform hazard spectra should be used directly to define the Eurocode 8 spectra, instead of scaling the spectra by just two spectral amplitudes (as has been recently proposed for the upcoming version of Eurocode 8) or scaling the normalized spectra by the peak acceleration values (which is the method still used).
Abstract of the presentation: In order to study the static and dynamic mechanical properties of polyvinyl alcohol (PVA)-basalt fiber concrete after freeze-thaw damage under appropriate mixing ratios, as well as the enhancement mechanism of mixed fibers on the freeze-resistant properties of concrete, the quasi-static compression test and separated Hopkinson compression bar (SHPB) test were carried out for the PVA-basalt fiber concrete and ordinary C35 concrete treated with freezing and thawing cycles, respectively, for a controlled investigation. The degradation of mass loss rate and compressive strength of PVA-basalt fiber concrete under freeze-thaw cycle was investigated by quasi-static compressive test and separated Hopkinson compression bar (SHPB) test, to analyze the degradation of mass loss rate and compressive strength of PVA-basalt fiber concrete under freeze-thaw cycle, and to introduce "freeze-thaw degradation function C(n)" and dynamic increase factor (DIF), to establish a dynamic equation of compressive strength for the concrete by considering the double factors of strain rate effect and freeze-thaw damage. The results show that PVA-basalt fiber can well limit the freeze-thaw damage of concrete and the development of internal microcracks, the quality loss rate of PVA-basalt fiber concrete is 0.22% at 100 times of freezing and thawing, and only a slight spalling of the surface cement mortar and aggregate and a small amount of fiber exposure occurred; the static-dynamic mechanical test data were processed and analysed, and it was found that the mixed fiber of PVA-basalt has a significant influence on the Concrete compressive mechanical properties after freezing and thawing damage has a significant improvement, slowing down the decay rate of static and dynamic compressive strength; PVA-basalt fiber concrete after freezing and thawing still has a significant strain rate effect, the use of dynamic compressive strength formula obtained from the theoretical value and the experimental value of the good degree of agreement, which verifies the reliability of the calculation model, the formula to a certain extent for the preliminary estimation of PVA-basalt fiber concrete in any freezing and thawing damage. To a certain extent, this formula provides a convenient means to preliminarily predict the dynamic compressive strength of PVA-basalt fiber reinforced concrete under freeze and thaw damage at high strain rate.