Robustness of Steel Joints in Fire

1. Robustness of Simple Steel Connections in Fire Ying Hu & Dr. Buick Davison and Prof. Ian Burgess (Supervisors), Department of Civil and Structural Engineering Prof. Roger Plank (Supervisor), School of Architecture The University of Sheffield From experimental results, the minimum tying resistance (75 kN) cannot be assured for partial depth endplate connections at high temperatures. The tying capacities calculated in accordance with the industry standard Green book are likely to overestimate the real resistance of simple steel connections in fire conditions. The finite element model, embedded with cohesive elements, was presented and verified with experimental results. The quasi-static analysis strategy was proved to be a reliable and suitable tool to effectively simulate the performance of bolted connections in fire. Further Research Currently, the research in connection already completed experimental tests and validation of numerical models. To improve understanding, a series of parametric studies will be conducted on a range of steel connections by using the finite element simulation. Background Robustness of Steel Joints in Fire What is the robustness in steel structures? • Robustness is the ability of a structure to withstand events like fire, explosions, impact or the consequences of human error, without being damaged to an extent disproportionate to the original cause. • Two main approaches are recommended for structural robustness: Tying force approach: tying a steel frame horizontally and vertically to increase its structural continuity and create a structure with a high level of robustness. Alternate load path method: if part of a structure had been removed by an accidental action, the remaining members were still well connected to develop an alternative load path which transfers the load of the collapsed members to the surrounding stiffer members. • In structural fire engineering, it is implicitly assumed that simple steel connections are capable of maintaining structural integrity to resist progressive collapse. But evidence from full scale fire tests (at the BRE Large Building Test Facility at Cardington) demonstrates that steel connections may be the weakest and most vulnerable components in fire conditions. Relying solely on standard design details, the failure of steel connections may arise from rupture of endplates, fracture of bolts or bearing in the beam web. • Issues concerning robustness of simple steel joints in fire conditions variation of tying resistance of simple connections in a fire situation. alteration of ductility in fire, (recent research, completed at The University of Sheffield proved that the ductility needed to be taken into account in a fire situation, as an indicator of structural robustness) modes of failure for simple steel connections in a fire situation (failures caused by brittle components or ductile failures) • Rupture of endplates for partial depth endplates • Web cleats • Fin plates • Flexible end plates • To investigate the robustness of joints in fire conditions, a research group at the University of Sheffield developed a series of tests for simple steel connections. From test results, it was clearly proved that simple connections, except web cleats, do not possess sufficient rotation capacity to permit the deformation required to catenary action in fire conditions. Damage in brittle components (bolts and welds) may provoke the failure of connections; therefore, part of the research effort in this project has been put into the investigation of the performance of brittle components in fire conditions. • Simple steel connections include fin plates, flexible end plates and web cleats. • Assumptions for simple steel connections: to be ductile, possess large rotation capacity and nominally pin the beams and columns. to resist shear forces only • Bearing failure and bolt fracture for fin plates Flexible End Plates in Fire Standard 8.8 Bolts in Fire Why is research needed for structural 8.8 bolts? a) reduction factors for 8.8 bolts in fire Catenary action and tying force approach Background Experimental arrangement and objectives • Modes of failure for standard 8.8 bolts in fire • A comparative study was performed for bolts ordered to British standards (BS 4190: 2001) and European standards (BS EN ISO 4014) at the University of Sheffield. • The first objective was to identify an approach to eliminate premature failure due to thread stripping. • The second was to observe the performance of these two classes of bolts in fire conditions • Catenary action is the behaviour of a steel beam acting as a cable hanging from the surrounding cold structure, which is observed in a fire situation. Note that, to develop catenary action in fire, steel connections are required to experience a large rotation and support a tensile load (tying force). At ambient temperatures, the minimum tying force is taken to be 75 kN. • In the tying force approach, the tying force is the action which is generated within steel beams and passed on to steel connections. Note that the tying forces applied in a structure could be horizontal and vertical, even inclined in catenary action. • However, the tying resistance is defined as the ability of steel connections to resist a horizontal force in accordance with an industry standard design manual. This definition implicitly suggests that engineers should determine the tying resistance of simple steel connections in the absence of beam rotations (without considering the moment). • To develop catenary action in fire, steel connections are required to be ductile enough to accommodate the induced rotation in a fire situation. Therefore, ductility (rotational capacity) may be regarded as an indicator of robustness of steel connections in fire conditions. a) Catenary action furnace a) Bolt shank failure b) Threads stripping • Standards for grade 8.8 bolts b) bolts’ performance in fire Experimental results Withdrawn standards BS 3692:1967 and BS 4190:1967 Current bolt standards British standards BS 3692:2001 and BS 4190:2001 European standards BS EN ISO 4014 and BS EN ISO 4032 BS EN ISO 4017 and BS EN ISO 4032 • From test results, the premature failure may be prevented by using standard 8.8 bolts with grade 10 nuts, and closer tolerance in threads may help bolt assemblies to achieve better performance in fire conditions. • It is not suggested to use zinc plated nuts, which may impair the performance of bolts in fire • The avoidance of thread stripping between bolt and nut threads is beneficial for robustness of steel connections in fire conditions. b) Furnace for tests Experimental arrangement for flexible end plates in fire c) Sketch of loading system • A series of experimental tests were carried out at the University of Sheffield for investigation of robustness of steel connections in fire conditions. Simulation of Endplate Connections in Fire • Steel sections (254UC89 and 305x165UB40) were supplied by Corus and fabricated by Billington Structures Ltd. All the bolts were M20 Grade 8.8, used in 22 mm clearance holes, and standardized fitting end plates were used for simple connections. All these tests were performed in an electric furnace and the load was applied through three linked Φ26.5 mm Macalloy bars. Why do engineers need finite element modelling? • For realistic simulations, actual material properties must be required in the solution procedure. The material properties for the various components of steel connections may be determined from the engineering stress-strain relationships recommended in Eurocode 3. • Conducting experimental tests is always time consuming and expensive. • Furthermore, carrying out tests at high temperatures has extra difficulties in recording displacements and strains. • Thus, using experimental data for validation, but simulating the connection performance with finite element modelling, provides an opportunity for wider parametric investigations and eliminates the limitations associated with experiments. d) Location of a flexible endplate connection Experimental results for flexible end plates • It is clearly observed, from the load-versus-rotation curves, that the resistance and rotation capacity of steel connections are both decreased at high temperatures. The reduced rotation capacity of flexible end plates at high temperatures is caused by the rupture of end plates occurring before the beam flange contacts with the column flange, which occurs at ambient temperatures as evidenced by the kink in the curve at about 6o rotation. • From experimental results, only one mode of failure has been observed for flexible end plates in fire conditions: the rupture of endplates around heat affected zone. • For a real structure in fire, simple steel connections are capable of resisting some moment and the inclined tying force may be produced within a steel beam. As a consequence, the tying resistance calculated in accordance with the industry standard Green book and EC3 are likely to overestimate the real resistance of these connections. Comparison between experimental results and calculated values proved this point clearly, as shown in the left table. • The data in this table demonstrates that the minimum tying force (75 kN) for simple steel connections cannot be assured at high temperatures (over 450oC). • The ductility of flexible end plates is a crucial issue concerning robustness of steel structures in fire. Verifying numerical model with experimental results How to create a finite element model for endplate connections? • The above FE model started with creation of individual components such as bolts, endplates, beams and columns. These components were then assembled into a numerical model in the global coordinate. • A small number of cohesive elements has been embedded into this model, as indicators of the failure of endplates. The contact interactions between bolts, endplates and column flanges were simulated by surface-to-surface formulations in ABAQUS. e) Test results for flexible endplate connections and modes of failure in fire • In comparison with experimental results, the numerical model, embedded with cohesive elements, is capable of estimating the resistance and ductility of flexible endplate connections in fire conditions. • Furthermore, through this comparison, the quasi-static analysis technique is proved to be a reliable and suitable tool to effectively simulate the performance of bolted connections. • Therefore, the simulation strategies employed in this part may be reliable for the further parametric studies of flexible endplate connections. f) Test results for flexible endplate connections Conclusions and Recommendations • To present a simplified model is the eventual target in this research work. This (mechanical) model is based on fully understanding of the performance of individual components in fire conditions, and simplifies these components with bi-linear or tri-linear load-deformation characteristics. As a consequence, the mechanical model should represent the performance of steel connections, which is now the most simple and popular approach to simulate the performance of simple connections in fire. The premature failure of bolts, owing to thread stripping, has been reported in the connection tests (not in flexible endplate tests), and the avoidance of this failure mode is beneficial for robustness of steel connections in fire conditions. Zinc plated (zinc finished) nuts are not suggested to be used in steel construction, and selecting black finished nuts (Grade 10) assembled with 8.8 bolts is an effective measure in achieving improved performance of bolts at both ambient and elevated temperatures. The connection tests demonstrate that both the resistance and rotation capacity of flexible end plate connections were reduced at high temperatures, and the mode of failure observed for these connections is the rupture of endplate around heat affected zone. Acknowledgement The research work described in this poster is part of a project funded under Grant EP/C510984/1 by the Engineering and Physical Sciences Research Council of the United Kingdom. This support is gratefully acknowledged by the authors. In this project, the authors would also like to thank technical staff for their assistance and excellent work.