### New research projects

**Investigation of Progressive Collapse in Seismically Designed Steel Structures**

A progressive collapse is a situation where local failure of a primary structural component leads to the collapse of adjoining members which, in turn, leads to additional collapse. Global system collapse will occur if the damaged system is unable to reach a new static equilibrium configuration.
During their lifetime, civil engineering structures could be subjected to natural hazards like earthquakes, hurricanes, tornadoes and fires, and man-made hazards such as blast and impact. Structures are usually designed for credible events that can happen during their lifespan, but extreme events for which they were not adequately designed for can result in catastrophic failure. Potential abnormal load hazards that can trigger progressive collapse are categorized as: aircraft impact, design/construction error, fire, gas explosions, accidental overload, hazardous materials, vehicular collision, explosions, etc. As these hazards have low probability of occurrence, they are either not considered in structural design or addressed indirectly by passive protective measures. Nowadays, extreme events are considered to be credible events, with a finite probability of occurrence. Most of them have characteristics of acting over a relatively short period of time and result in dynamic responses. Lack of knowledge about structural behavior under collapse conditions reveals the importance of this topic.
The objective of this research is to investigate important issues related to progressive collapse of seismically designed steel structures using numerical and analytical methods.

**Active Vibration Control**

Slender structures tend to oscillate. These oscillations can reduce the lifetime or even lead to failure of the structure. To reduce the oscillations damping devices can be used. An active damping device, the so-called twin rotor damper, was developed at the Hamburg University of Technology at the Structural Analysis and Steel Structures Institute. The basic unit consists of two eccentric masses which rotate with the same rotational velocity about two parallel axes creating centrifugal forces. Depending on the sense of rotation and on the relative rotational positioning between the two rotors, the resultant can be a harmonic force, a harmonic moment, or a combination of both. The resultant can be used for structural damping. To generate the desired control action it is essential to develop appropriate control strategies. The purpose of this study is to create and test control strategies for the twin rotor damper, which will be developed with help of simulations and experiments. In addition the effectiveness of the damping system for real structures, such as wind turbines and long-span bridges, will be studied with help of numerical simulations.

M.Sc. Richard Bäumer

**Progressive collapse of high-rise buildings**

Since the collapse of the World Trade Center Twin Towers in New York questions regarding the safety against global collapse of high-rise buildings under extreme loading scenarios have come to the fore. However, very few scientific studies have dealt specifically with the collapse type observed in the Twin Towersthe so called pancake-type collapse. Such collapses are triggered by the failure of all columns in one or several neighboring stories, and are observed mostly after big earthquakes. The possible triggering events can varyfor example fire or aircraft impact. The failure of the columns in one story causes the building section above to start gaining downward momentum. Once set in motion, such a collapse often ends with total destruction of the buildingthe floors pile up, one on top of the other, like the layers of a pancake. This is how the term "pancake-type collapse" came about.

The purpose of this study is to follow the recommendation at the end of the FEMA-report on the collapse of WTC 1 and 2, and to "determine, given the great size and weight of the two towers, whether there are feasible design and construction features that would permit such buildings to arrest or limit a collapse, once it began".

**Vibrations at steel bowstring bridges with free standing arches**

Lateral arch vibrations, resulting from crossing vehicles, are frequently observed at existing steel bowstring bridges. The long post oscillation induces a large fatigue load for the arch and the adjacent construction components. Also, it feels uncomfortable to persons on the bridge. This problem always occurs at the same, widespread steel bowstring bridge type.

The subject of this research project is the analysis and prevention of arch vibrations. Therefore, the arch is extracted from the bridge system and its differential equation is examined. This reveals key parameters, on which the structure can be optimized. Also, tuned fatigue load models are tested for the vibration susceptible arch to avoid vibration problems already in the design phase.

**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.

Editor:

**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.

Editor:

Dipl.-Ing. Eur.-Ing. CEng MICE Hasan Aslan

**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.

Im Rahmen des Forschungsvorhabens werden daher alternative Verfahren untersucht und im Hinblick
auf den Einsatz im Hafenbau weiterentwickelt.

Editor:

Schenk