Mike Schlaich

Carbon Fibre Reinforced Polymer for Orthogonally Loaded Cable Net Structures

Abstract With the advantages of high strength, light weight, no corrosion and high fatigue resistance, carbon fibre reinforced polymer (CFRP) cables have the potential to replace steel cables in a broad range of applications. The ideal structures for such cables are highly pre-tensioned cable systems that are loaded orthogonally to their cable axes. This type of high performance structure with CFRP cables, such as CFRP cable roofs and facades, can be built economically with large or small spans. To illustrate this point, after introducing the history of cable structures and their structural behaviours, a cable net facade, which is a typical orthogonally loaded cable structure, is investigated in a case study. The mechanical properties and economic efficiency of the cable nets with CFRP cables with different Young’s moduli and different tensile strengths are compared with those of the corresponding steel cable net. The results show that CFRP cables can effectively improve the mechanical and economical performances of orthogonally loaded cable net structures.

Active vibration control of a light and flexible stress ribbon footbridge using pneumatic muscles

Abstract This paper describes the development of an active vibration control system for a light and flexible stress ribbon footbridge. The 13 m span Carbon Fiber Reinforced Plastics (CFRP) stress ribbon bridge was built in the laboratory of the Department of Civil and Structural Engineering, Berlin Institute of Technology. Its lightness and flexibility result in high vibration sensitivity. To reduce pedestrian-induced vibrations, very light pneumatic muscle actuators are placed at handrail level introducing control forces. First, a reduced discretized analytical model is derived for the stress ribbon bridge. To verify the analytical prediction, experiments without feedback control are conducted. Based on this model, a velocity feedback control strategy is designed to actively control first mode vibrations. To handle the nonlinearities of the muscle actuator a subsidiary nonlinear force controller is implemented based on exact linearisation methods. The stability of the entire closed-loop system with actuator saturation is investigated by the Popov Criterion. Control performance is verified by experiments. It is demonstrated that handrail introduced forces can efficiently control the first mode response.

Accelerometer-based estimation and modal velocity feedback vibration control of a stress-ribbon bridge with pneumatic muscles

Lightweight footbridges are very elegant but also prone to vibration. By employing active vibration control, smart footbridges could accomplish not only the architectural concept but also the required serviceability and comfort. Inertial sensors such as accelerometers allow the estimation of nodal velocities and displacements. A Kalman filter together with a band-limited multiple Fourier linear combiner (BMFLC) is applied to enable a drift-free estimation of these signals for the quasi-periodic motion under pedestrian excitation without extra information from other kinds of auxiliary sensors. The modal velocities of the structure are determined by using a second Kalman filter with the known applied actuator forces as inputs and the estimated nodal displacement and velocities as measurements. The obtained multi-modal velocities are then used for feedback control. An ultra-lightweight stress-ribbon footbridge built in the Peter-Behrens- Halle at the Technische Universitat Berlin served as the research object. Using two inertial sensors in optimal points we can estimate the dominant modal characteristics of this bridge. Real-time implementation and evaluation results of the proposed estimator will be presented in comparison to signals derived from classical displacement encoders. The real-time estimated modal velocities were applied in a multi-modal velocity feedback vibration control scheme with lightweight pneumatic muscle actuators. Experimental results demonstrate the feasibility of using inertial sensors for active vibration control of lightweight footbridges.

Stress Ribbon Bridges: Mechanics of the Stress Ribbon on the Saddle

AbstractThe most slender and flexible footbridges are stress ribbon bridges. One kind of these elegant and simple bridges consists of two hanging steel ribbons, usually made of steel, which carry concrete deck panels and are anchored between two heavy abutments. At the anchorage and on intermediate supports the ribbons are often supported by saddles that are curved so that the additional bending stresses in the ribbon, which occur there, stay within allowable limits. The magnitude and the distribution of the bending stresses, the transverse pressure the ribbons exert on the saddle, and stress changes in the region of the detachment point caused by live loads are crucial for the dimensioning of these ribbons. A clear understanding of these issues via parametric studies, analytical solutions, and confirmation by finite-element (FE) analyses are provided in this paper. The findings presented here can also be used for other saddle systems, such as those used in cable-stayed and extradosed bridges.

Design of Footbridges

Guidelines for the design of footbridges is the title of a fib-report that was just published in December 2005. This paper highlights some of the aspects that are treated in the guidelines and elaborates on the conceptual design of footbridges using three examples.

Monoleg towers with transverse stabilising cables : Towers for cable-stayed bridges

Three monoleg towers with some unusual features have been built for the Ting Tau bridge in Hong Kong, a multispan cable-stayed bridge with 1177 m of cable-stayed deck. The tower legs are very slender because they are stabilised by transverse cables and thus behave structurally much like the masts of a sailboat. The layout and shape of these towers have been conceived for minimum wind-resistance, optimum aerodynamic behaviour, speed of erection and other considerations that will be described here.

Effect of Relative Displacement of Strands Bent Over Circular Saddles on Fatigue Life Under Fretting Conditions

High strength steel strands are widely used as main supporting elements in cable-stayed and extradosed bridges. Saddle systems have recently been introduced as an alternative to attaching the stays using anchor heads to pylons. As a drawback, strands installed in such systems are subjected, in addition to plain fatigue, to the fretting phenomenon, which accelerates fatigue damage and shortens the fatigue life of the strands.

Active Vibration Control with Artificial Pneumatic Muscles for Carbon Fibre Stress-Ribbon Bridge

This paper describes how stress-ribbon bridges are among the lightest and smartest bridges. The lightweight materials used in such structures results in low damping properties and at the same time in high vibration sensitivity and these effects increase when stiffness and mass decrease. This is the case when light high-strength carbon fiber reinforced plastic (CFRP) is used for the stress-ribbon and when the weight of the surfacing is small. System properties such as natural mode change as the mass of the bridge varies due to changing pedestrian traffic. The paper shows how passive dampers to reduce vibrations, which cannot adjust to such changing system properties, lose effectiveness. Consequently, there is a demand for more intelligent solutions that control a wider range of frequencies and modes. Therefore, an active control system can be installed that consists of sensors, controller and actuators. The actuators can produce specific forces to influence the structural oscillation. The paper shows how the effectiveness of extremely light actuators like artificial pneumatic muscles is presently being tested on a carbon fiber stress-ribbon bridge at the Technical University of Berlin. This paper describes the results of the tests.

Great Heavens Gate, Berlin, Germany

The symbol of the Ecumenical Church Day 2003 in Berlin was the halo. In keeping with this motif, a halo in the form of an air-supported tube with an outer diameter of 25 m was placed at a height of 13 m directly in front of the Brandenburg Gate, creating the socalled Great Heavens Gate (Fig. 1). The image of this temporary structure would be in the centre of all TVand press-reports on Church Day. Therefore, a very attractive and impressive structure was required. However, the structure was temporary, needed only for five days, with a very limited budget and tight construction schedule.

Hanging glass roof in Heilbronn, Germany

A suspended glass roof was unveiled in the spring of 2001 in Heilbronn in the South of Germany. The roof consists of more than 200 individual glass panes. It appears, however, to be one piece of glass floating above the spectator, because its supporting elements are all arranged above the glass surface and therefore are hardly visible. The panes are held by specially designed adjustable stainless-steel nodes which are suspended from stainless steel cables. These cables span between a tubular steel frame which itself is joined by cast-steel nodes. The hanging glass roof of approx. 1000 m2 is only stabilised by its dead weight. For horizontal loads it is stiffened by the glass itself. This roof is probably the largest of its kind to date. An extensive test programme was necessary for official approval. This included wind tunnel testing to obtain realistic wind loads as well as real size prototype testing of the glass panes with their steel nodes.