The influence of semi-rigid connections on the performance of steel framed structures in fire.
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Observation of
fire damaged
structures and recent
fire
tests at the Cardington LBTF have
suggested that even nominally
`simple'
connections are capable of providing significant
restraint at elevated-temperatures. As most
frames
are designed
assuming pinned response
at ambient-temperature, with no account being
taken of the reduction
in mid-span
moments, this
is
an aspect of connectivity which may be
utilised
in the assessment of the
fire
resistance of steel
framed buildings, without necessitating changes
in
the approach
adopted
in
ambient-temperature design
or construction. To date
the assessment of the
influence
of connection response on
frame behaviour has been limited by
the quantity of
available test data, although
initial
studies based
on postulated moment-rotation-
temperature characteristics concluded that the failure
temperatures for beams
are
increased
due
to the rigidity of
`simple' connections.
Moment-rotation
relationships have been measured
for
a
flush
end-plate connection, both
as bare-steel
and as composite with a concrete slab across a range of temperatures. To
define accurately the full moment-rotation-temperature response a series of tests have been
conducted
for
each arrangement, where specimens were subject to varying constant
levels
of
load
and
increasing
temperatures. Observed failure mechanisms have been
compared
with those for
a nominally
identical
specimen
tested at ambient-temperature, and
initial
recommendations presented
for
the degradation of ambient-temperature connection
characteristics. A mathematical expression
is proposed
in order to represent the test data
at
a number of temperatures.
It is
clearly unrealistic to expect that many such
tests can be anticipated
in
the future,
and as
such a spring-stiffness model has been presented
for both bare-steel
and composite
flush
end-plate connections. The use of a spring-stiffness model compares
favourably with other
forms of modelling due to the combination of efficient solution and the ability to follow
accurately the full
non-linear range of connection response,
based on an understanding of
the response of the component parts. A multi-linear representation of response has been
adopted, where the stiffness of the connection
is
revised as elements enter the plastic range
of response. Comparison has been made between the response predicted and that recorded
experimentally.
Experimentally derived
connection characteristics
have been incorporated within analysis of
typical sub-frames, with parameters
including connection stiffness, capacity and
temperature being
varied. Further studies are presented considering the sensitivity of
overall
frame behaviour to inaccuracies in
the representation of connection response and
the use of simplified models to generate elevated-temperature connection characteristics.
Based on postulated elevated-temperature moment-rotation characteristics
for
the
connections contained within the Cardington
test
frame,
predictions have been
presented
for the response of the structure subject to a series of
full
scale
fire
tests, with semi-rigid
behaviour being compared with the common assumptions of pinned and rigid
characteristics.