Model Parameter Sensitivity for Structural Analysis of Composite Slab Structures in Fire

The behavior of buildings during fires has recently become a significant issue. Analyzing structures at elevated temperatures is complex and challenging in structural engineering as engineers must take into consideration factors that may not be included at ambient temperatures, namely material and geometric non-linearity as well as time-temperature-varying strength. In this study, the finite element software ABAQUS was applied to model and simulate the behavior of structures in fire events. Steel beams and columns were modeled using two-node linear beam elements, while concrete slabs were discretized using shell elements. A series of verification analyses were conducted to ensure that the analysis produced an acceptable level of accuracy. Furthermore, an extensive sensitivity study was carried out to obtain the appropriate modeling parameters to be used in subsequent numerical analyses.

Dynamic analysis; Fire engineering; Heat transfer; Steel composite; Steel structures

However, because steel is a sensitive material, its material properties, particularly strength and modulus of elasticity, are significantly reduced at high temperature. Fire insulation, such as spray fire-resistive material (SFRM), is commonly applied to the surface of the steel structure to maintain the stability of the structures during a fire. The main aim of fire insulation is to delay the temperature rise of the steel at elevated temperatures.

    In standard fire design, the fire resistance of structures has been evaluated based on the behavior of isolated structures under standard fire tests (BS EN, 1999). It is believed that this approach does not represent the actual behavior of a building in a fire. The behavior of a composite building in a real fire could be seen during the Cardington Tests (Bailey et al., 1999) in the United Kingdom (UK), which indicated that the whole building structure had higher fire resistance than the associated single elements. Significant research has been conducted on the modeling of composite buildings in a fire (Gillie et al., 2001; Nguyen et al., 2015; Jiang et al., 2017). Most previous studies have highlighted and discussed the complexities of modeling structures in a fire. However, there is a lack of detailed research into the influence of parameters on the composite steel frame at high temperature. The present study focuses on this gap. The main objective of this study is to develop and validate numerical models capable of predicting the 3-D behavior of composite steel frames in a fire. A series of sensitivity studies were also performed to investigate the parameters that affect the behavior of composite buildings during a fire.

This study has described a numerical model generated using ABAQUS. Steel beams and columns were modeled using 1-D line elements, and concrete slabs were modeled using shell elements. A tie constraint between the steel beam and concrete slab was applied to accommodate the fully composite action between the two. The beam-to-column and secondary beam-to-primary beam connections were assumed to be rigid and pinned, respectively.

Because no new experimental study was conducted, a series of validations were carried out to confirm that the results of the analysis provide an acceptable level of accuracy. The results were compared to those of existing experiments. Overall, the results obtained from the analysis demonstrate a good level of agreement with those obtained by others. Therefore, the modeling approach has been validated for important specific aspects of structural behavior of composite buildings in fires. 

The results of the sensitivity study indicate that it is appropriate to use a default energy dissipation factor of 0.2´10^-4. It can also be seen that the mesh sizes have little influence on the deflection, in which even a mesh size of 0.5 m produces reasonable results. The results also show that the deflection is not sensitive to the compressive strength of the concrete slab. Thus, it seems that the reduction in compressive strength has a minor effect on the results. Furthermore, the presence of the composite slab increases the fire resistance of the frame by 50% compared to the frame without the concrete slab.

        However, it should be noted that the beam element used in this study cannot capture all possible failure modes, such as local buckling. An experimental study by Wang and Li (2009) demonstrated the possibility that a steel column can fail prematurely due to localized buckling. Detailed elements, such as solid or shell elements, can be used if detailed beam behavior is needed. In addition, the connections were assumed as rigid and pinned, so connection failure was not taken into account.

    This work is supported by the Research and Technology Transfer Office, Bina Nusantara University as a part of Bina Nusantara University’s International Research Grant, entitled “Sustainable infrastructure and transportation development in rural and coastal area,” with contract number: No.080/VR.RTT/Vlll/2020 and contract date: 21 August 2020

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