Development and application of a thermal analysis framework in OpenSees for structures in fire
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Date
01/07/2013Author
Jiang, Ya-Qiang
Metadata
Abstract
The last two decades have witnessed the shift of structural fire design from
prescriptive approaches to performance-based approaches in order to build more
advanced structures while reducing costs. However, it is recognised that the
implementation of performance-based approaches requires several key elements
that are currently not fully developed or understood. This research set out to
address some of these issues by focusing on the development, validation and
application of methodologies for accurate predictions of thermal responses of
structures in fire using numerical methods.
This research firstly proposed a numerical approach with the finite element and
the discrete ordinates method to quantify the fire imposed radiative heat fluxes
to structural members with cavity geometry. With satisfactory results from the verification and validation tests, it is used to simulate heat transfer to unprotected
steel I-sections with symmetrical cavities exposed to post-flashover fires. Results
show that the cavity geometry could strongly attenuate the radiative energy,
while the presence of hot smoke enhances radiative transfer by emission. Average
radiative fluxes for the inner surfaces of the I-sections are seen to increase with
smoke opacity. In addition, the net radiative fluxes are observed to decrease faster for I-sections with higher section factors. This work also shows that the self-radiating
mechanism of I-sections is important in the optically thin region, and
existing methodologies neglecting these physics could significantly underpredict
steel temperatures.
The next focus of this work is to develop a thermal analysis framework dedicated
to structures-in-fire modelling in the OpenSees (Open System for Earthquake
Engineering Simulation) platform which has been developed towards a highly
robust, extensible and flexible numerical analysis framework for the structural
fire engineering community. The thermal analysis framework, which is developed
with object-oriented programming paradigm, consists of a fire module which has
incorporated a range of conventional empirical models as well as the travelling
fire model recently developed elsewhere to quantify the fire imposed boundary
conditions, and a heat transfer module which addresses non-linear heat conduction
in structural members with the finite element method. The developed work
has demonstrated good performance from benchmark problems where analytical
solutions are available and from full scale tests with measured data.
With the thermal analysis capability developed in this work together with
the work by other colleagues to quantify the mechanical response at elevated
temperatures, the extended OpenSees framework can be used to predict structural
performances subjected to a wide range of re scenarios. This work uses OpenSees
for a case study of a generic composite structure subjected to travelling fires.
The latest work on travelling fire methodology for structural fire design has been
implemented in the OpenSees framework. The work presented in this thesis is the
first effort to examine both the thermal and structural responses of a composite
tall building in travelling fires using OpenSees. Results from the thermal analysis show that travelling fires of larger sizes (e.g. burning area equal to 50% of the
floor area) are more detrimental to steel beams in terms of more rapid heating
rate, while those of smaller sizes (e.g. burning area equal to 4% of the floor
area) burn for longer duration and thus are more detrimental to concrete slabs
in light of higher peak temperatures. The results also show that fires of large
sizes tends to produce higher through-depth thermal gradients in the steel beam
sections particularly in neighbouring regions with the concrete slab. Due to less
rapid heating rates but prolonged burning durations, smaller fires produce lower
thermal gradients but with higher temperatures in the concrete slab particularly
at locations far from the fire origin. The subsequent structural analysis suggests
that travelling fires produce higher deflections and higher plastic deformations
in comparison with the uniform parametric fires, particularly with smaller fire
sizes producing more onerous results. The results seem to be more physically
convincing and they challenge the conventional assumption that the post-flashover fires are always more conservative for structural performance.