Robustness of Steel Joints in Fire - PowerPoint PPT Presentation

1 / 1
About This Presentation
Title:

Robustness of Steel Joints in Fire

Description:

Robustness of Simple Steel Connections in Fire Ying Hu & Dr. Buick Davison and Prof. Ian Burgess (Supervisors), Department of Civil and Structural Engineering ... – PowerPoint PPT presentation

Number of Views:52
Avg rating:3.0/5.0
Slides: 2
Provided by: fp1
Category:

less

Transcript and Presenter's Notes

Title: 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.
Write a Comment
User Comments (0)
About PowerShow.com