Udo Peil

Assessment of bridges via monitoring

The prediction of a realistic life cycle and the extension of the service life is an important task in reducing costs of civil engineering structures in the future. The paper first gives an overview of the necessary steps of the overall procedure of a precise life cycle assessment. The knowledge of the existing, inherent damage of a structure is an essential prerequisite for life cycle prediction. A theoretical and an experimental approach to determine the inherent damage is described. The paper concludes with the presentation of a fairly precise method to assess the fatigue life cycle of steel bridges. The methods are developed besides others at the Collaborative-Research-Center “Monitoring of Structures” at the Technical University at Braunschweig, Germany.

Guyed masts under wind load

Abstract The essential requirement for a safe and economic design of guyed masts is an exact knowledge about wind loading and the corresponding response of the structure. In thisresearch project windprofiles and statistical parameters of the turbulence structure are determined by measuring the wind speeds on 17 levels with relatively small vertical distances (18m) up to the height of 341m. The dynamic response of the mast is determined by measuring the strains of the corner legs and the rope forces as well as the accelerations of the mast shaft. Based on the experimental results a system identification is performed, taking account of the nonlinearities of the ropes. Calculated results are compared with measured ones.

Dynamic behaviour of guys under turbulent wind load

The dynamic behaviour of guyed masts depends on the dynamic behaviour of the guys. The dynamic behaviour of guys under wind load is studied. The main excitation is the mast shaft movement, which moves the upper guy connection points. Due to the nonlinear behaviour of the guys the dynamic amplitudes are reduced. In a linearized calculation, this effect can be taken into account by using an equivalent damping. Near the resonance chaotic motions occur. The additional influence of gusty wind which hits the guys directly is investigated. It is shown that the influence on the overall behaviour of a guyed mast is negligibly small.

Unsteady buffeting wind loads in the time domain and their effect on the life-cycle prediction of guyed masts

For predicting the life-cycle of guyed structures under turbulent wind loads, a model-chain has to be applied including models for: wind-loading; mechanical behaviour of the structure; and fatigue analysis. Each model affects the predicted lifetime. Unsteady wind load models in the frequency domain require a linearisation of the structures nonlinearities due to the sagging cables. Quasi-steady models for the wind loading in the time domain lead to forces that correspond to measurements only in high winds. This paper gives an overview of the identification of complex aerodynamic admittances in full-scale measurements. Aerodynamic impulse response functions in combination with nonlinear numerical simulations of guyed masts in the time domain under turbulent wind are used for a life-cycle analysis. The results indicate a reduction of the predicted damage compared with common quasi-steady simulations of buffeting wind loads.

A New Aerodynamic Phenomenon and Its Effects on the Design of Ultra-High Cylindrical Towers

The paper presents the discovery – by means of wind tunnel tests and with the support of CFD simulations – of a new aerodynamic phenomenon around circular cylinders. It is a new type of bistable flow, which is induced by the presence of ring beams along the height of a cylinder with a free-end. This investigation was performed within a research project on Solar Updraft Towers, i.e. ultra-high structures (1 km or even more) to produce renewable energy. They convert solar radiation into electric power by using the natural updraft of heated air in a ultra-high chimney. Bistable flows around isolated circular cylinders are known in the critical range of Reynolds number. This new type of bistable flow presents some similarities, but the conditions of occurrence make it original and physically unique. The dependency on the Reynolds number has been investigated in the wind tunnel and in URANS numerical simulations. So far, no reason has been found, which could suggest the disappearance of the phenomenon induced by the spanwise rings in full-scale condition. Therefore, stiffening rings along the height of towers – which are usually regarded as a strategy for reduction of structural vulnerability to the wind action – might become a double-edged sword, especially when the distance between rings is lower than the tower diameter.

A Refined Analysis of Guyed Masts in Turbulent Wind

Frequent wind events with moderate wind velocities are usually crucial for the fatigue of slender structures. In these events, the shape of the wind profiles often differs from common assumptions. In order to examine the effects arising from that, a guyed mast with a circular pole subjected to different realistic wind profiles is analyzed in the time domain. For this, an unsteady transfer model for the buffeting wind loads and, simultaneously, a simulation of the vortex excitation considering lock-in effects are applied. The wind-induced loads are derived from numerically generated, turbulent wind fields, whose properties are verified with full-scale wind data recorded at the 344 m data acquisition tower Gartow. The results of the dynamic analysis show a strong dependency of the stress resultants on the shape of the wind profile. The maximum lateral tip deflection and, hence, the maximum bending moment are obtained for the constant profile, as this profile produces the longest dwell period in the lock-in region. Contact person: A. Willecke, Technische Universität Carolo-Wilhelmina zu Braunschweig Institute of Steel Structures, Beethovenstraße 51, 38106 Braunschweig, Germany Phone +49 (0)531 / 391-3375, Fax +49 (0)531 / 391-4592 E-mail a.willecke@is.tu-braunschweig.de A refined analysis of guyed masts in turbulent wind M. Clobes, A. Willecke, U. Peil Technische Universität Carolo-Wilhelmina zu Braunschweig Institute of Steel Structures, Beethovenstraße 51, 38106 Braunschweig, Germany m.clobes@is.tu-braunschweig.de, a.willecke@is.tu-braunschweig.de, u.peil@is.tu-braunschweig.de

An Inspection System to Identify Fatigue Damage on Steel-Bridge Structures

Crack formation and growth in steel bridge structural elements may be due to loading oscillations. The welded elements are liable to internal discontinuities along welded joints and sensible to stress variations. The evaluation of the remaining life of a bridge is needed to make cost-effective decisions regarding inspection, repair, rehabilitation, and replacement. A steel beam model has been proposed to simulate crack openings due to cyclic loads. Two possible alternatives have been considered to model crack propagation, which the initial phase is based on the linear fracture mechanics. Then, the model is extended to take into account the elastoplastic fracture mechanic concepts. The natural frequency changes are directly related to moment of inertia variation and consequently to a reduction in the flexural stiffness of a steel beam. Thus, it is possible to adopt a nondestructive technique during steel bridge inspection to quantify the structure eigenvalue variation that will be used to localize the grown fracture. A damage detection algorithm is developed for the proposed model and the numerical results are compared with the solutions achieved by using another well know computer code.

Fatigue Prediction of Steel Structures by Means of Monitoring and Testing

The prediction of the life time of steel structures is an important task for the owners. The models that are usually used show only small reliability. The models consist of a chain of, a load model, a system transfer model, and a damage model. The results of these coupled models are usually unreliable, especially when the influence of the unsafe load and damage models control the reliability of the result. The paper shows that monitoring at critical points and a time reduced parallel testing leads to very good results.

A modified non-linear member element allowing large loading and displacement increments

In the paper a modified non-linear member element is derived which can lead to satisfactory computed results even for large loading and displacement increments from pre-instability to post-instability of structures. The high accuracy of this element is proved by numerical examples.

Design of solar towers for extreme storm conditions and for vortex excitation

The structure of Solar Updraft Towers is basically a circular cylinder, which may turn into a hyperboloid at lower levels in order apply benefits of shape strengthening. The height of the tower is up to 1.5 km and it is usually designed as a thin reinforced concrete shell. The wind action is the main natural hazard, which plays a decisive role for the feasibility of the technology. An extensive wind tunnel investigation has been recently performed at WiSt laboratory at Ruhr-University Bochum (Germany) and at Criaciv laboratory at University of Florence (Italy). The tests highlighted in no-efflux conditions (out-of-use of the power plant) a new phenomenon egarding cross-wind loads, induced by a bi-stable and asymmetric flow distribution. It is created by compartments between stiffening rings along the tower and enhanced by a strong interaction with free-end flow structures at the top of a finite length circular cylinder. A proper positioning of the rings should allow to avoid this phenomenon.