29 research outputs found
Analysis of the structural response of tall buildings under multifloor and travelling fires
The last decades have seen a surge in the construction of tall buildings all over
the world. Due to their, often, innovative and complex layouts, tall buildings can
pose unique challenges to architects and engineers. Previous tall building failures
raised significant concerns on the applicability of prescriptive fire design for these
structures. The use of structural fire engineering can enhance the safety of a tall
building under fire by strengthening any vulnerable areas in the structure and at
the same time reduce the costs of fire protection by removing it when unnecessary.
Commercial finite element and specialist structural fire engineering software
have their advantages and disadvantages. In this thesis, the object-oriented
and open-source finite element software OpenSees is presented along with its
development with structural fire capabilities by the author and other researchers
at the University of Edinburgh. Specifically, new pattern, element, section
and material classes have been introduced. All the developed code follows
the object-oriented paradigm and is consistent with the ethos of the existing
framework. Verification and validation studies of the developed code are also
presented. Several procedures including that for dynamic analysis of structures
in fire for the collapse assessment of structures are discussed. The development of OpenSees with structural fire capabilities allows the collaboration of engineers
across geographical boundaries and disciplines using a community tool.
In this work, the behaviour of tall buildings under different fire scenarios has been
modelled using the developed OpenSees code. Firstly, the collapse mechanisms
of generic tall buildings are investigated, namely the strong and weak floor
mechanisms are demonstrated, and criteria are established on when each of these
mechanisms occurs. The parametric study performed demonstrated that the weak floor collapse is less probable for generic composite buildings however this type
of failure can become easier to appear as the number of floors in fire increase.
The effect of vertically travelling fires on these mechanisms is also examined.
The results of the study show that slower travelling rates delay or avoid the
global failure of a tall building compared to quicker travelling rates allowing for
the time required for steel members to regain their strength during cooling to
ambient temperature. However, it was seen that higher tensile membrane forces
were observed in the floors as the travelling rates increased which could result in
possible connection failure.
Most of the research and design codes, such as Eurocode, typically assume a
uniform thermal environment across the floor area of a structure when defining
the design fire. However, in reality fires are more likely to travel in large
enclosures, hence there is a need to understand how tall buildings behave under
more realistic fire conditions such as travelling fires. A methodology for defining
the thermal environment of large enclosures using travelling fires has been recently
developed at the University of Edinburgh. Taking into account OpenSees'
programmable architecture and its recent inclusion with heat transfer capabilities
by other researchers, there was a collaborative effort in order to understand the thermal and structural response of a generic composite tall building under
horizontally travelling fires. The findings of the study showed that larger travelling
fire sizes produce quicker heating to the steel beams while smaller fire sizes
produce higher peak temperatures in the concrete slab. The structural analysis
also demonstrated that travelling fires produced higher midspan deflections in
comparison to Eurocode parametric fires and higher plastic deformations which
is an indication of higher damage.
Further work focused on looking at the behaviour of tall buildings under the
combined scenario of horizontally and vertically travelling fires. The results of
the study showed that the travelling fires produce lower maximum compressive
and tensile membrane forces in the composite floor compared to the Eurocode
parametric fires for the building examined and thus in a multi-floor scenario the
columns are pulling-in less after large deflections develop in the floor. More
specifically, the short-hot fire produced the most demanding response. This
suggests that in long floors where uniform heating is really impossible, the time of
failure predicted by parametric fires in a multi-floor scenario can be more onerous.
The outcomes of this work can aid designers when considering the structural
fire response of tall buildings in a performance based design context. It was
demonstrated that multi-floor fires could be a threat for tall buildings, and thus
this possibility should be considered in design. The use of more realistic fire
definition for large enclosures, such as travelling fires, should also be considered.
The travelling fire methodology can provide an enhanced level of confidence for
the safety of a building since it can represent a range of similar fires to those that
may occur in a real fire scenario
Modelling thermal performance of unloaded spiral strand and locked coil cables subject to pool fires
Structural cables are used to design critical bridge structures. These are not typically redundant; a loss or compromise of a few cables can lead to the progressive collapse. Previous experimental research has shown that the degradation of material properties and thermal expansion of structural cables is more onerous than the standard carbon prestressing steel. Despite its importance for design no well-validated methodology exists to aid in the thermal performance of structural cables in the event of a fire, particularly for spiral and locked-coil cables which inherently are complex due to their cross-sectional geometry. A state-of-the-art methodology for modelling structural cables' thermal response is presented. The methodology will enable the development of an understanding of the temperature distribution and thermal deformation in a cable cross-section and allow estimation of post-fire resilience. Validation is performed against experiments of locked-coil and spiral cables, subjected to realistic pool fires. The cables range from 22 to 100 mm in diameter and are constructed of galvanized or stainless steel. The cables are modelled undergoing non-linear thermal analysis in LS-DYNA. 2D models are found to provide conservative estimates for critical values such as peak temperature with 90% accuracy, while 3D models provide slightly more conservative estimates