Robustness of Steel Joints in Fire

1
Robustness of Steel Joints in Fire
Background
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

1. variation of tying resistance of simple
   connections in a fire situation.
2. 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)
3. modes of failure for simple steel connections in
   a fire situation (failures caused by brittle
   components or ductile failures)

1. Rupture of endplates for partial depth endplates

1. Web cleats

1. Fin plates

1. 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

1. to be ductile, possess large rotation capacity
   and nominally pin the beams and columns.
2. to resist shear forces only

1. 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

1. Background

1. Experimental arrangement and objectives

1. Catenary action and tying force approach

• 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

1. Experimental results

• Withdrawn standards
• BS 36921967 and BS 41901967
• Current bolt standards
• British standards
• BS 36922001 and BS 41902001
• 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

1. 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 F26.5 mm Macalloy bars.

1. 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

1. 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.

1. Verifying numerical model with experimental
   results

1. 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

1. 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.

1. 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.
2. 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.
3. 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.