Robustness of flexible endplate connections under fire conditions.

Worldwide interest in how to prevent the progressive collapse for tall and large
buildings under exceptional loading conditions was heightened by the collapse of the
twin towers at the World Trade. The performance of steel-framed structures
subjected to fire loading is heavily reliant on the interaction between structural
members such as columns, slabs and beams. The implicit assumption in fire
engineering design is that bolted connections are able to maintain the structural
integrity for a large and tall building under fire conditions. Unfortunately, evidence
from the collapse of the World Trade Centre towers and full scale fire tests at the
BRE Cardington Laboratory indicates that connections may be particularly
vulnerable during both heating and cooling. Hence, this PhD research is focused on
structural performance of simple steel connections under fire conditions, particularly
the interaction mechanism between non-ductile and ductile components in a
connection and connection failure mechanism in a steel-framed structure subjected to
fire loading.
The research involved experimental testing of simple steel connections and
components (structural 8.8 bolts) at elevated temperatures. High temperature tests on
structural bolts demonstrated two modes of failure at elevated temperatures: bolt
breakage and thread stripping. In order to prevent the thread stripping in a connection,
the manufacturing process of bolts and nuts has been investigated and the 'overtapping'
of nut threads to accommodate the (zinc) coating layer for corrosion
resistance has been indentified as a primary reason resulting in this premature failure
between bolts and nuts. Experimental tests on endplate connections revealed the
ductility of these connections to decrease at high temperatures, which might hinder
the development of catenary actions in fire if plastic hinges are attempted to be
formed within the connection zones. Component-based modelling and finite element
simulation have been utilized for investigation of the performance of these
connections.
An improved component-based model has been developed which includes nonductile
(brittle) components (bolts and welds) into a connection model with a
reasonable assumption of their failure displacements, based on experimental tests.
This model also features vertical components for consideration of shear response of
these connections in fire. The component-based connection model has been used in a
sub-frame structure and a parametric study demonstrates that a connection may fail
due to a lack of rotational capacity (failure of bolts or welds) in a structure exposed
to a fire. Therefore, partial depth endplate connections are recommended to be fireprotected
to prevent the failure of these brittle components. Alternatively, ensuring
the strength of brittle components (bolts and welds) is higher than that of other
components in each bolt row is necessary to achieve the ductile failure mechanism of
simple connections. Based on the experimental tests, component-based connection
modelling and finite element simulation, recommendations to improve the robustness
of simple steel connections in fire have been presented.