Fire represents one of the main dangers for single-storey industrial structures. In this context, the numerical investigation of fire effects and fire induced collapse mechanism becomes of interest .
According to many studies on the behaviour of a frame with a span in fire condition the collapse of single-span frames occurs in two ways conventionally named “sway” and “no sway” collapses. The collapse is caused by the outward movement of the columns due to thermal expansion of the rafter. This initial mechanism may lead to the collapse of the whole frame (sway-collapse), or the columns may be pulled back towards the upright position (no sway-collapse) due to the collapse of the rafters.
A critical aspect of the collapse analysis concerns the determination of a proper parameter, which identifies the design configuration of the frame and, at the same time, is strictly related to the mode of collapse. For this purpose a parametric study was conducted to assess the role of some parameters (such as the section and the span of the elements and their ratio) in determining the collapse mode. A set of nonlinear transient analyses have been performed, which account for thermo-plastic material behaviour and geometrical nonlinearities and provide not only for the verification of structural elements under fire, but also allow for a deeper understanding of structural system behaviour as a whole.
In this study, constant reference will be made to the structure shown in figure.

All the elements are connected with moment and shear resisting connection. The connection to the ground is pinned. The steel is idealized as an elasto-plastic material with a bi-linear constitutive law and a plastic hardening of 5% is assumed. To define the stress-strain curve at elevated temperatures, the reduction factor indicated in the Annex C of Eurocode 3 [8] is utilized. The reduction factor has been used in order to evaluate both the effective yielding stress and the slope of linear elastic range as the temperature increases (see Figure 3).
The fire scenario considered involves the left column and the beam. This assumption relies on the fact that it is realistic to think that some material might burn near the column and the hot gasses gather under the roof by heating also the beam. All the steel section are considered as unprotected, this lead to the assumption thet the temperature in the element is considered the same of the gasses. The thermal action is modeled with the nominal fire curve ISO 834 [9]. In the Figure 3 the ISO 834 (European standard curve) is reported.
In order to consider the shadow effect, and the over-strength that is required in design guidelines, the beam-column connections are modeled as perfectly rigid.


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