A generic fire performance assessment framework for steel plate girder bridges with specific focus on Wildland Urban Interface fires
Creators: name: Dissanayake, Amila Prasanna
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400513
subject: Water resources engineering

description: Bridges are critical pieces of infrastructure that are essential for the socio-economic sustenance of a country. During the lifetime of a bridge, it may be subjected to various extreme loading events that may cause significant damages or total loss of the asset. Most research works in the literature have focussed on earthquakes, floods, impacts and hydrocarbon fires. However, the hazards due to Wildland Urban Interface (WUI) fires that include bushfires has received little research attention owing to practical difficulties associated with real-time testing and modelling. Given the socio-economic importance of bridge structures and the increasing urbanisation, there is an imperative need for WUI based fire research on bridge safety. This thesis presents a modelling framework for bridge structures subjected to WUI fires using steel and concrete composite bridges as case studies. In particular, the flexural and shear behaviour of bridges under elevated temperatures inclusive of varied fuel and spatial conditions were investigated using validated finite element and physics-based Fire Dynamic Simulation (FDS).

A significant requirement of the research arises from the need to develop fire loading curves since most of the bridge fire models are based on standard fire curves that were originally developed for building fire safety evaluation purposes.  Based on the fuel characteristics, the Wildland Urban Interface (WUI) fires were studied in two forms, namely the fine-fuel (&lt;6mm in diameter) based fires inclusive of bush and grass fires and coarser-fuel based fires inclusive of fires that can be generated by the accumulation of heavy vegetation fuel beds such as debris with longer burning duration. Results show that the extended burnout time of fuels around a bridge structure can have a direct effect on the fire structure interaction. Furthermore, the service life of operating bridges that are ageing can add towards the risk of failures at elevated temperatures. This effect was investigated on sample steel plate girders with low fire resistance with high thermal conductivity and thermal expansion. The analysis shows that the web panels with local corrosion damage can reduce the elevated temperature shear buckling strength of the bridge girders. 

A validated Finite Element (FE) modelling using ABAQUS was used to evaluate the shear behaviour of steel plate girders. Corrosion patterns commonly reported in the literature were simulated to represent the actual deterioration.  Results revealed that both the position and severity of the damage is important for the fire resistance degradation, based on the simulation of thin-walled steel girders. These results when compared with the EN 1993-1-5 design guidelines revealed that the web contribution to the shear buckling resistance of a steel plate girder can go well below the Eurocode design guidelines. Furthermore, this research proposed a modified lower bound equation covering a range of corrosion geometries to calculate the factor of web contribution to shear buckling resistance of a plate girder with rigid end conditions. This result when applied to WUI fires revealed a significant reduction of the factor of web co shear buckling resistance in each case study.  A further examination of the elevated temperature shear buckling resistance behaviours of steel plate girder with corroded transverse web stiffeners revealed a significant reduction in standard fire resistance and a rotation of the tension diagonal field.

A numerical modelling approach for WUI fire structure interaction on healthy and deteriorated structures involving fine vegetative fuels was accomplished utilizing the general framework developed above. Physics-based Fire Dynamic Simulation (FDS) methodology was used to model the Adiabatic Surface Temperature (AST) development of the structure for effective heat transfer analysis.  Parametric variations include the varying grass height, vegetation load, vegetation moisture (related to vegetation curing and relative humidity according to Grass Fire Danger Index (GFDI)) and domain wind velocity to understand the effect of temperature development of different girder geometries. Relevant thermophysical, pyrolysis and combustion parameters were used to model the vegetation burning. A linear pyrolysis model and a "simplified" Arrhenius degradation model were used to represent the thermal degradation of boundary and fuel element vegetation models.  A concept based on critical temperature was developed to study the presence of corrosion on web deteriorations of steel girders. It was identified that the shear capacity of the girders was below the Eurocode design capacity for varied fire scenarios.

Further investigation includes the study of bridge structures under prolonged burning of heavy vegetation fuel beds. A 25m long 11m wide composite steel plate girder bridge that comprised of 5 girders was used as a case study. Fire development under the bridge was modelled using FDS to capture the AST development of the structure. The resulted ASTs were coupled with thermomechanical analysis using sequentially coupled thermomechanical analysis procedure in ABAQUS. In the later stage of the analysis, a fully coupled dynamic explicit thermomechanical analysis was used as a method of numerical cross-validation and as a novel method applied to analyse WUI fires. The numerical model was then used to estimate the temperature and displacement development of the bridge at axially free and axially restrained ends of the girders to understand the alternate load transferring mechanisms. Results show that debris fire under a bridge structure can cause significant flexural damage to a structure.  

The proposed modelling frameworks and the outcomes of this research along with comparisons of the Eurocode will enable practising engineers and the asset managers to conduct necessary fire risk assessments on bridges incorporating the aspects of bridge deterioration with a particular focus on the WUI fires.