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.
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.
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.
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.
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.
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.
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 zurück
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"
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.
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.
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.