Structural behaviour of concrete columns under natural fires

Abstract

Structured abstract.Purpose -The paper aims to give an insight into the behavior of reinforced concrete columns during and after the cooling phase of a fire. The study is based on numerical simulations as these tools are frequently used in structural engineering. As the reliability of numerical analysis largely depends on the validity of the constitutive models, the development of a concrete model suitable for natural fire analysis is addressed in the study. Design/methodology/approach -The paper proposes theoretical considerations supported by numerical examples to discuss the capabilities and limitations of different classes of concrete models and eventually to develop a new concrete model that meets the requirements in case of natural fire analysis. Then, the study performs numerical simulations of concrete columns subjected to natural fire using the new concrete model. A parametric analysis allows for determining the main factors that affect the structural behavior in cooling. Findings -Failure of concrete columns during and after the cooling phase of a fire is a possible event. The most critical situations with respect to delayed failure arise for short fires and for columns with low slenderness or massive sections. The concrete model used in the simulations is of prime importance and the use of the Eurocode model would lead to unsafe results. Practical implications -The paper includes implications for the assessment of the fire resistance of concrete elements in a performance-based environment. Originality/value -The paper provides original information about the risk of structural collapse during cooling.

A performance indicator for structures under natural fire

“…Also, the DHP appears to be approximately a linear function of R. Finally, considering a required Fire Resistance for the three columns, the tendency to exhibit failure during the cooling phase will be more pronounced for case 3 column than for the others and for case 1 column than for case 2. This confirms that delayed failures are more prone to arise in columns with low slenderness and massive sections, as mentioned in [10].…”

“…In a previous research [10], it was shown that for concrete columns the situations of delayed failure were more likely to arise for short-duration fires and columns with low slenderness or massive sections. It is shown here how these results can be interpreted in terms of the new indicator.…”

“…This delayed decrease in load-bearing capacity may be caused by the combination of various phenomena among which the delayed temperature increase in the sections due to thermal inertia and the non-recovery or additional loss of material mechanical properties during cooling. A previous research carried on the behavior of reinforced concrete columns under natural fires has indicated that there was a possibility of structural failure during and even after the cooling phase of a fire [10]. http Natural fire scenarios are thus associated with a threat that was entirely disregarded as long as standardized fires were considered: the possibility of a structural failure arising after the time of maximum temperature in a compartment, referred to as delayed failure hereafter.…”

Mechanical properties of lightweight concrete after fire exposure

“…The mixes were produced by varying cement content, w/c ratio and type of admixture. The same type of Portland cement (CEM I 42.5 R conforming to EN 197-1:2011) and to the delayed temperature increase in the section core, and due to an additional loss of strength after cooling [6].…”

“…However, these studies have not answered the question about the influence of the additional reduction on the possible delayed failures of concrete members, which may occur long after the fire has been extinguished. The importance of exploring concrete behaviour after reaching the maximum temperatures has already been illustrated in recent scientific research [6], [7], where a new performance parameter was proposed to take into account the cooling phase in the assessment of fire resistance. The new parameter pointed to a large difference in the fire resistance of concrete members if the cooling phase of the fire curve is included in the analysis.…”