Michal Pazdanowski

An alternative computational approach to the critical temperature specification for a steel column, related to the fire conditions and associated with the loss of the capacity to safely resist the loads applied to the said column is presented and discussed in detail. The algorithm recommended by the authors has been derived from the empirical Rankine–Merchant rule, with parameters calibrated so as to obtain quantitative agreement with critical temperature estimates arrived at after application of the conventional code-based approach. It has been indicated, that this approach results in an iterative calculation. Knowledge of the critical temperature determined for given structural component or a substructure under assumed fire development scenario allows for identification of its fire resistance interpreted as the forecast time of reliable service when subjected to the influence of high temperature. Numerical example presented in this paper pertains to the column devoid of any restrictions in elongation, not fire protected and heated uniformly around the whole perimeter and along the whole length. To simplify the example it has been assumed, that at any given moment of fire steel temperature is constant in the column cross-section and increases with increasing temperature of surrounding fire plume. The approach proposed by the authors allows for verification, in both qualitative and quantitative terms, of the influences exerted by geometrical imperfections of various origins on the final critical temperature specifications, in particular those related to a potential eccentricity of load application point as well as those induced by the longitudinal axis of column deviating from straightness.

Initial deviations from plumb exhibited by the columns of the multistorey steel frame, forecast a priori as an imperfection applied for design purposes, are after the erection of the said frame measured and verified during the final inspection. In particular, it is verified whether the appropriate tolerances assumed by the designer and based on the regulations prescribed by the standards have been observed during erection and the displacements and deformations remain within allowable limits. Verification of this type should be based on the integrated formal approach combining an appropriate measurement program with statistical analysis of the results. An innovative model for the mathematical verification of the acceptability of initial column tilts registered on‐site, developed by the authors, and based on the rules of statistical quality control, is presented in this paper. It is subsequently applied to estimate a posteriori the parameters of global geometrical imperfection specific for a given frame and to update afterwards the column plumb criteria assumed prior to erection.

The experimental quantification of retention factors related to the post-fire strength as well as the post-fire ductility of intentionally selected stainless steel grades applied in construction is the objective of the research presented here. These steel grades are characterized by a two-phase austenitic–ferritic microstructure of the duplex type. In this context, two mutually corresponding chromium–nickel–molybdenum steel grades are subjected to analysis, namely X2CrNiMoN22-5-3 steel belonging to the standard duplex group (DSS 22% Cr) and X2CrMnNiN21-5-1 steel belonging to the lean duplex group (LDSS). The similarities and differences in the mechanical properties exhibited by these steel grades after effective cooling, following more or less prolonged simulated fire action conforming to several development scenarios, are identified and indicated. The resistance of a given steel grade to permanent structural changes induced by the heating program proved to be the critical factor determining these properties and resulting in many cases in increased susceptibility to brittle fracture. The results obtained experimentally seem to confirm the quantitative estimates of post-fire retention factors forecast by Molkens and his team, specified for the steels exhibiting a duplex-type structure and tested by us. However, several of these estimates might be considered somewhat risky. Nevertheless, our results do not confirm the significant post-fire strengthening of steel grades belonging to the LDSS group following prior heating at a sufficiently high temperature, as reported earlier by Huang Yuner and B. Young.

A numerical example allowing for effective forecasting of the remaining service time for corroded steel shell of a tank used to store liquid petroleum fuels is presented. This time is interpreted here as a period of time counted from the moment of an obligatory tank shell inspection to the moment of the anticipated shell failure understood as the loss of the capacity to safely resist the loads applied to it. Reaching the limit value of failure probability, i.e. the highest probability of failure acceptable to the tank user, is in this approach a determinant of such failure. The detailed considerations pertain to a typical on-the-ground storage tank equipped with a floating roof, located in one of fuel depots in the south of Poland. The forecast has been prepared based on measurements of the random thickness of tank shell weakened by corrosion and measured after 27 years of service time. In the recommended analysis fully probabilistic computational procedures have been applied. This led to a more credible and less conservative service time assessment than the one usually determined via the traditional standard approach. For comparative purposes qualitatively different but formally corresponding safety measures have been applied to describe the obtained results. Zaprezentowano przykład numeryczny pozwalający na efektywną prognozę pozostającego czasu zdatności skorodowanej stalowej powłoki zbiornika używanego do magazynowania ropopochodnych paliw płynnych. Czas ten jest interpretowany jako okres liczony od momentu obligatoryjnej oceny stanu technicznego obiektu do antycypowanej w przyszłości chwili jego awarii, rozumianej jako wyczerpanie możliwości bezpiecznego przenoszenia obciążeń. Osiągnięcie granicznej wartości prawdopodobieństwa zawodu, maksymalnej możliwej do zaakceptowania przez użytkownika obiektu, jest w tym podejściu wyznacznikiem chwili awarii. Szczegółowe rozważania dotyczą typowego, naziemnego zbiornika magazynowego z dachem pływającym, zlokalizowanego w jednej z baz paliwowych Polski południowej. Prognoza została opracowana na podstawie pomiarów losowej grubości powłoki, osłabionej przez oddziaływanie korozji, wykonanych po 27 latach użytkowania zbiornika. W rekomendowanym podejściu wykorzystano w pełni probabilistyczne procedury obliczeniowe. Takie ujęcie prowadzi do lepiej uzasadnionych i mniej konserwatywnych ocen pozostającego czasu zdatności w zestawieniu z analogicznymi ocenami otrzymanymi na podstawie tradycyjnego podejścia normowego. W celach porównawczych w opisie proponowanego algorytmu zastosowano jakościowo różne jakkolwiek odpowiadające sobie miary bezpieczeństwa.

Experimental quantification of retention factors related to post-fire strength as well as post-fire ductility of intentionally selected stainless steel grades applied in construction is the objective of research presented here. These steel grades are characterized by a two-phase austenitic-ferritic microstructure of the duplex type. In this context two mutually corresponding chromium-nickel-molybdenum steel grades have been subjected to analysis, namely X2CrNiMoN22-5-3 steel belonging to the standard duplex group (DSS 22% Cr) and X2CrMnNiN21-5-1 steel belonging to the lean duplex group (LDSS). The similarities and differences in the mechanical properties exhibited by these steel grades after effective cooling, following more or less prolonged simulated fire action conforming to several development scenarios, have been identified and indicated. The resistance of given steel grade to permanent structural changes induced by the heating program proved to be the critical factor determining these properties and resulting in many cases in increased susceptibility to brittle fracture.

The results of experimental research on forecasting post-fire resistance to brittle failure of selected steel grades used in construction are presented and discussed in this paper. The conclusions are based on detailed analysis of fracture surfaces obtained in instrumented Charpy tests. It has been shown that the relationships formulated based on these tests agree well with conclusions drawn based on precise analysis of appropriate F–s curves. Furthermore, other relationships between lateral expansion LE and energy W_t required to break the sample constitute an additional verification in both qualitative and quantitative terms. These relationships are accompanied here by values of the SFA_(n) parameter, which are different, depending on the character of the fracture. Steel grades differing in microstructure have been selected for the detailed analysis, including: S355J2+N—representative for materials of ferritic-pearlitic structure, and also stainless steels such as X20Cr13—of martensitic structure, X6CrNiTi18-10—of austenitic structure and X2CrNiMoN22-5-3 duplex steel—of austenitic-ferritic structure.

It is presented in detail how the selection of a structural model describing the behaviour of a steel hall transverse frame when subject to fire exposure in a more or less complex way may affect the fire resistance evaluation for such a frame. In the examples compiled in this paper the same typical one-aisle and single-story steel hall is subjected to simulated fire action, each time following the same fire development scenario.Aresultant fire resistance is identified individually in each case, using various computational models, on an appropriate static equilibrium path obtained numerically. The resulting estimates vary, not only in the quantitative sense, but also in terms of their qualitative interpretation. It is shown that the greater the simplification of the model used, the more overstated the estimated fire resistance is in relation to its real value. Such an overestimation seems to be dangerous to the user, as it gives him an illusory but formally unjustified sense of the guaranteed safety level.

The change in the value of the breaking energy is discussed here for selected steel grades used in building structures after subjecting the samples made of them to episodes of heating in the steady-state heating regime and then cooling in simulated fire conditions. These changes were recorded based on the instrumented Charpy impact tests, in relation to the material initial state. The S355J2+N, 1H18N9T steels and also X2CrNiMoN22-5-3 duplex steel were selected for detailed analysis. The fire conditions were modelled experimentally by heating the samples and then keeping them for a specified time at a constant temperature of: 600 °C (first series) and 800 °C (second series), respectively. Two alternative cooling variants were investigated in the experiment: slow cooling of the samples in the furnace, simulating the natural fire progress, without any external extinguishing action and cooling in water mist simulating an extinguishing action by a fire brigade. The temperature of the tested samples was set at the level of −20 °C and alternatively at the level of +20 °C. The conducted analysis is aimed at assessing the risk of sudden, catastrophic fracture of load-bearing structure made of steel degraded as a result of a fire that occurred previously with different development scenarios.

The change in the value of the breaking energy is discussed here for selected steel grades used in building structures after subjecting the samples made of them to episodes of heating in the steady-state heating regime and then cooling in a simulated fire conditions. These changes were recorded based on the instrumented Charpy impact tests, in relation to the material initial state. The S355J2+N, 1H18N9T steels and also X2CrNiMoN22-5-3 duplex steel were selected for detailed analysis. The fire conditions were modelled experimentally by heating the samples and then keeping them for a specified time at a constant temperature of: 600°C (first series) and 800°C (second series), respectively. Two alternative cooling variants were investigated in the experiment: slow cooling of the samples in the furnace, simulating the natural fire progress, without any external extinguishing action, and cooling them in water mist simulating an extinguishing action by a fire brigade. The temperature of the tested samples was set at the level -20oC and alternatively at the level + 20oC. The conducted analysis is aimed at assessing the risk of sudden, catastrophic fracture of load-bearing structure made of steel degraded as a result of a fire previously occurred with different development scenarios.