Alternative structures for long-span briges
Over the previous years bridges with very long spans have been built in increasing numbers. Scientists and engineers study the liability to vibrations of long-span bridges. Wind-induced flutter vibrations can cause sudden stability failure whereas vibrations induced by wind gusts or vortex shedding can impair the serviceability of the structure.
The topic of this research project is wind-induced flutter vibrations. The global bridge design is optimized in terms of geometry, topology and connectivity to create inherent aero-elastic resistance. The resistance of different systems against flutter vibrations is assessed through non-affinity of bending and torsional vacuum-eigenmodes and critical wind speed. These properties are confirmed by subsequent aero-elastic analyses based on the finite element method.
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Progressive collapse
Progressive collapse is characterised by a distinct disproportion between the triggering,
spatially limited failure and the resulting widespread collapse. The cause
for initial failure - be it a local load or a local lack of resistance - is
irrelevant for progressive collapse defined in this way. In a structural design,
resistance is merely regarded on a local level. Additionally, low probability
events are disregarded. For this reason, additional global considerations are
necessary to ensure structural safety after an initial local failure. This
necessity is supported by recently collapsed structures (Terminal Charles-de-Gaulle,
WTC, Alfred P. Murrah Building, Oklahoma City et. al.) where single elements failed,
though they seemed to be designed properly, and huge parts of the structure collapsed.
Resistance against progressive collapse is referred to as structural robustness.
The demand for robustness has not yet been taken consistently into account in
guidelines and codes.
The aim of this research project is to develop design rules to ensure structural robustness.
For this purpose, different kinds of structures are considered and assessed towards
their robustness. Initially, this is done by finite element analyses, but simpler
procedures for practical use shall be identified.
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Progressive Collapse and Robustness
Progressive collapse moved into the focus of current research on structural design due to catastrophic
building collapses (Murrah Building, Oklahoma City, USA, 1995; World Trade Center, New York, USA, 2001; etc).
A progressive collapse is characterised by a disproportion between the magnitude of an abnormal event and
the resulting widespread collapse of large parts or the entire structure; it develops in the kind of a
chain reaction triggered by an initial local failure.
The susceptibility of different structures to progressive collapse is not considered within the
context of classical design of structures, in which resistance is merely regarded on a local level,
and low probability events are disregarded. For this reason, additional global considerations are
necessary to ensure structural safety after an initial local failure, and thus adequate safety against
progressive collapse. This necessity is supported by recent structural collapses where single elements
failed, although they seemed to be designed properly, whereupon huge parts of the structure collapsed.
The feature of a structure to resist local damages without progressive collapse is called structural
robustness. Some standards and design guidelines demand the consideration of the failure type
progressive collapse, thus ensuring robustness. However, so far, these requirements are drafted
only qualitatively. A quantitative description of structural robustness seems useful and necessary
for the optimisation of the robustness of a structure as well as for the comparison of different
structures. The aim of this research project is the development of measures for the quantitative
description of structural robustness.
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Aerodynamic Flutter Control of Bridges
With ever increasing span to depth ratios, the aero-elastic behaviour of bridges becomes
the governing design criteria. In particular, the flutter stability of the structure becomes critical.
The common approach to increase the critical wind speed by increasing the structural stiffness
through larger section dimensions becomes unfeasible after a certain point as the weight and cost
increase disproportionately with growing span lengths.
In the current research project, new flutter control mechanisms, whereby the bridge vibration is controlled
by externally-applied forces by means of passive devices (e.g. tuned mass damper or gyroscope driven control surfaces), are developed and investigated. The mechanisms are analysed experimentally
in the wind tunnel, numerically with the Computational Fluid Dynamics (CFD) method, and theoretically
and optimised if necessary in order to create the basis for a practical application.
Hasan Aslan
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Realistic Computation of Reinforced Concrete Frames
Many of the common structures consist of one dimensional members.
New construction methods and progress in concrete technology render
possible bolder and more slender concrete structures. Their safe and
economical realisation calls for nonlinear and powerful computational
methods. Currently used structural analysis software is suitable only to
a limited extent for these highly stressed structures. Particularly with
regard to spatial frames, the geometrical and material nonlinearities are
not accurately considered. In the framework of beam theory the material
nonlinearity can be captured on cross-sectional level. Existing approaches
can be classified into resultant models, truss models, uniaxial models,
wall models as well as models based on finite element analyses.
So far, there is no generally accepted model for arbitrary cross sections
considering all six section forces and the associated interactions.
A new hybrid approach combines a uniaxial fiber model with a wall model.
The proposed approach allows for a new, uniform design and checking
procedure for concrete structures.
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The interaction of the steel tube and the concrete core in
conrete filed steel tube column
The concrete filled steel tube (CFT) column system has many
advantages compared with the ordinary steel or the reinforced concrete
system. The enhancement in structural properties is due to the interaction between the steel
tube and concrete core. The confinement created by steel casing enhances the material properties
of concrete due to the triaxial state of stress. Conversely, the inward bucking of the steel tube
in prevented by the concrete, thus increasing the stability and strength of the column as a
system.
In the actual building frames and bridges, vertical dead and live loads on beams are usually transferred to
columns by beam-to-column connections. In case when CFTs are used as continuous columns of an actual
building frame, which has a simple connection, shear forces in the beam - ends are not directly transferred
to the concrete core but directly to the steel tube. In this case, strain compatibility between the two
materials is unlikly to be achieved. Despite the large number of push-out tests available in the
literature, they provide limited information about the real bond behavior in CFT colomns.
The aim of this research is to investigate the stress transfer between the steel tube and concrete
core in short and long concrete filled steel tube columns by a natural bond or mechanical shear
connectors on the inside of steel tube experimentally and theoretically. Also the effect of bond
stress on the long-term behavior of concrete filled steel tubes under central axial loading will be
investigated. In the analytical study the mathematical theory of elasticity and the finite element
analyses will be used to establish a numerical model.
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Improvement of the Aeroelastic Stability of Slender Bridges
The tendency to increase the span length of cable supported bridges the construction
of the bridge over the strait of Messina (3300m) is scheduled to begin in 2006 moves
the problem of their vibration behaviour noticeably into the focus of design engineers and
researchers. Thereby, special emphasis is laid on the flutter stability. Bridge Flutter is
an aeroelastic instability problem which can lead to the collapse of the structure e.g. the
first Tacoma Narrows Bridge (USA) in 1940. To avoid flutter vibrations it is standard practise
to base the girder design either on particularly high torsional stiffness or on aerodynamically
optimised cross sections shapes. Exemplars are the second Tacoma Narrows Bridge (USA) and
the Severn Bridge (GB), respectively. In long span applications these design methods lead,
in addition to high costs, to either enormous section depths with corresponding large wind
loads or unacceptable self-weights.
Alternatively, the vibration behaviour can be favourably influenced by imposed forces. This
can be achieved by utilizing the inertia effects or by movable, aerodynamically effective
control flaps. These force generating systems can be active, semi-active, passive or hybrid.
Though numerous proposals may be found in literature, they are hardly realised. An active
vibration control system has not been implemented in constructed bridges so far.
The research objective is both, the development of such devices and the numerical and
experimental examination of their functional capability and economic efficiency.
Dipl.-Ing. Jörn Scheller
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Comutational Analysis of Jointless Quays
In the construction of port facilities jointless quays have become increasingly
popular in recent years. Examples are the new container terminals in
Hamburg-Altenwerder and Bremerhaven. The key advange of the monolithic design
lies in the reduced costs for construction and maintenance. However, currently
available software packages are suitable to only a limited extent for the
computation of these high-grade statically indeterminant structures, since they
do not model the geometrical and material nonlinearities precisely enough.
Hence design engineers depend on numerous assumptions which are based on experience
gained from past projects. Numerival difficulties in the computational analysis
mainly are caused by the discontinuous stiffness gradient typically to be
observed within concrete structures.
Standard finite element methods with their
continuous shape functions represent these discontinuities only in an inadequate
way. The objective of this research project therefore is to evaluate and enhance
alternative methods for the computational analysis of concrete structures with
regard to their applicability for the design of jointless quays.
Graduiertenkolleg "Seehäfen für Containerschiffe zukünftiger Generationen"
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Active mass dampers for control of wind-induced bridge deck vibrations
Long-span and slender bridge girders are very capable of dangerous flutter vibrations
due to very strong winds. They can lead to the total damage of the bridge, e.g., the
collapse of the original tacoma narrow bridge.
My research deals with the active control of wind-induced flutter vibrations of bridge
girders and the corresponding control algorithms and strategies. Proposed methods should
be
imple-mented numerically.The applicability with respect to the control performance will
have to be considered. Reliability and robustness are important appraisal factors. For implemen-tations
in bridge engineering, practical issues will be disscussed. The main topic of the doc-toral
work is the numerical simulation of the control loop.
Another part of the research is focused
on the active vibration control of stay cables. The nu-merical models should be expanded
with the damping elements. The effectivness of the con-trol have to be evaluated.
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Click here to watch a video of an experiment conducted at our institute to demonstrate the stabilizing effect of an accelerated rotating mass on a bridge deck section under wind load.
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Applying the Structural Reliability Theory to Structural Glass Design
For the development of design concepts for glass structures,
the theory of structural reliability can be applied advantageously.
A stochastic model for the description of glass strengths, which scatter
in several respects, and an adapted procedure for the evaluation of test
results are suggested. A method for assessing the structural reliability,
tailored specially for structural glass design, is proposed. Based on a
nested formulation of the problem, the highly multidimensional structural
system modelled with the finite element method can be captured in an efficient
and reliable manner.
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