Thomas Ummenhofer

Resilience and Sustainability of Civil Infrastructure: Toward a Unified Approach

In recent years, the concepts of resilience and sustainability have become very topical and popular. The concept of sustainability rose to prominence in the late 1980s and became a central issue in world politics, when the construction industry began to generate the first sustainable building assessment systems with more or less equally weighted environmental, economic, and social aspects for office buildings over their life cycles. On the other hand, resilience is usually connected to the occurrence of extreme events during the life cycle of structures and infrastructures. In the last decade, it has been used to minimize specifically direct and indirect losses from hazards through enhanced resistance and robustness to extreme events, as well as more effective recovery strategies. A detailed comparison of the studies dealing with either infrastructure sustainability or resilience presented in this paper leads to the conclusion that they have a vast number of similarities and common characteristics. For instance, they both combine structural analyses with social and economic aspects; they both rely on techniques for the life-cycle analysis and decision making; they both are in an early stage, where the academic world is trying to find the best way to promote the application of the scientific results among professional engineers and the industry. Indeed, both approaches try to optimize a system, such as a civil infrastructure system, with respect to structural design, utilized material, maintenance plans, management strategies, and impacts on the society. However, for the most part, researchers and practitioners focusing on either resilience or sustainability operate without a mutual consideration of the findings, which leads to a severe inefficiency. Therefore, this paper suggests that resilience and sustainability are complementary and should be used in an integrated perspective. In particular, the proposed approach is rooted in the well-established framework of risk assessment. The impact of the infrastructure and its service states on the society in normal operational conditions (assessed by sustainability analysis) and after exceptional events (assessed by resilience analysis) should be weighted by the associated probabilities of occurrence and combined in a global impact assessment. The proposed perspective and assessment technique is applicable to various types of civil infrastructure systems, but the case of transportation networks and bridge systems is emphasized herein. A numerical application dealing with the comparative analysis of two possible bridge layouts is presented to exemplify the approach. The results show that both resilience and sustainability analyses assess a relevant amount of the impact of the bridge on the community where it is built, so neither one can be neglected.

Fatigue Performance of Tension Steel Plates Strengthened with Prestressed CFRP Laminates

An experimental and analytical study was conducted to investigate the fatigue behavior of tension steel plates strengthened with prestressed carbon-fiber-reinforced polymer (CFRP) laminates. A simple fracture mechanics model was proposed to predict the fatigue life of reinforced specimens. Double-edge-notched specimens were precracked by fatigue loading and then strengthened by CFRP laminates at different prestressing levels. The effects of the applied stress range, CFRP stiffness, and prestressing level on the crack growth were investigated. Experimental results show that the increase of the prestressing level extends the fatigue life of a damaged steel plate to a large amount. The CFRP with the highest prestressing level performed best, prolonging fatigue life by as much as four times under 25% higher fatigue loading. Theoretically, predicted results were in a reasonable agreement with the experimental results. A parametric analysis was also performed to investigate the effects of the applied stress ran...

Fatigue Behaviour of Welded High-Strength Steels after High Frequency Mechanical Post-Weld Treatments

Investigations have been carried out regarding the fatigue strength of welded details improved by high frequency treatment methods. These methods increase the fatigue strength by cold forming of the surface, so that the weld toe is smoothened, the surface hardness is increased and compressive residual stresses are present up to a depth of 1 to 1.5 mm. In this paper, the surface residual stresses at the weld toe are investigated before and after different loading conditions and for different steel grades. It is shown that only high tensile fatigue loading can lead to a slight residual stress relaxation for low-strength steels. The fatigue crack behaviour is analysed in more detail. The crack propagation rates with and without surface treatment are investigated, using defined lines of rest. The study shows that crack propagation in the edge layers is reduced. Several cracks may start in the UIT-treated zone but will not propagate further, until one final crack, often close to the edge of the specimens, will lead to failure. The results of fatigue tests for butt welds and longitudinal stiffeners improved by high frequency hammer peening are presented. The fatigue strength is seen to be doubled. For high-strength steels, the improvement at different load levels is identical, but for lower-strength steels, high stress ranges lead to reduced improvement. This fact results in flatter SN-curves and can be explained by the lower maximum of residual stresses and residual stress relaxation.

On the modelling of different types of imperfections in silo shells

The assessment of imperfections is most important for determining the load-bearing capacity of a thin-walled shell structure. Different ways of modelling imperfections are discussed in this paper and steel silo shells are used as an application. Buckling tests were performed on different types of model shell standard quality and high quality with reduced heat input during welding. For the numerical studies two different approaches were used as well: an axisymmetric approach with substitute geometric imperfections and an FEM approach, where the nodal coordinates were derived from surveying the specimen. It was found that there is considerable gain in the buckling strength due to the presence of the granular solid. The larger the initial imperfections in the shell the greater the gain in strength compared to the empty cylinder. The modelling of the uneveness of the edges with uneven dead loading is also discussed.

Detection of localized fatigue damage in steel by thermography

Fatigue damage of unalloyed steels in the high cycle regime is governed by localized cyclic plastic deformations and subsequent crack initiation. The extent of early microplastic deformations depends on the applied stress level, stress concentration at macroscopic notches, surface treatment, residual stresses etc. The onset of a nonlinear material response can be regarded as an early indicator of fatigue damage. During fatigue loading thermoelastic coupling and thermoplastic dissipation cause characteristic temperature variations in tested specimens which have been assessed by a highly sensitive infrared camera. A specialized data processing method in the time domain has been developed which allows to separate the different contributions to the measured temperature signal. In contrast to other methods - as e.g. measuring the rise of mean temperature during fatigue loading - the proposed methodology is based on measurements during the stabilized temperature regimen and offers very high spatial resolution of localized phenomena. Investigations have been made on mildly notched cylindrical and also on welded specimens. The results confirm the close relation between the local temperature signal and typical fatigue phenomena. The new methodology allows for a much better localization and quantification of effects as cyclic plasticity, crack initiation, crack growth etc. The following paper presents considerations and experimental results of an application of thermography to the local assessment of fatigue damage.

The social dimension of bridge sustainability assessment - Impacts on users and the public

In the past, bridge design usually concentrated on structural aspects. The cultural significance of a bridge as well as the interaction with the surrounding environment were frequently neglected. Within the scope of a growing demand for a holistic assessment of building activities, many evaluation criteria going beyond the pure manufacturing cost come to the fore. They are, however, not completely developed for infra-structure buildings. While methods like life-cycle costing and life-cycle assessment are already applied for in-frastructures, especially in the field of social assessment a lot of inaccuracies, assessment problems and me-thodological difficulties exist. This article summarizes different existing assessment approaches and tries to classify them. Furthermore, a method to quantify the impacts on users and the public by calculating the ma-croeconomic damage is proposed. Current developments of international standardization are incorporated in the considerations.

Effects of High-Frequency Peening Methods on the Surface Layers and the Fatigue Strength of Welded Details

Investigations have been carried out regarding the influence of high-frequency peening methods on the fatigue strength. These methods cause plastifications at the weld toe, which change the local weld toe geometry and produce compressive residual stresses at the surface as well as surface hardening. Two different techniques, High-Frequency Impact Treatment (HiFIT) and Ultrasonic Impact Treatment (UIT), are compared. Laser measurements of the weld seam prove that both methods change the overall weld toe radii to a more uniform shape. Residual stress measurements verify the introduction of compressive residual stresses at least up to a depth of 1–1.5 mm which can reach values up to the yield strength. Further analyses show that the local effects increase the fatigue strength. Crack detection methods prove that, due to the material mechanical effects, the crack initiation and surface layer propagation phases are extended. These effects lead to a significant increase of the fatigue strength.

Ultrasound excited thermography of thickwalled steel load bearing members

Ultrasonic lockin and burst-phase thermography establish more and more as failsave NDT methods in the scope of inspection of light-weight composite structures. The methods were not fully transferred to heavy load bearing members used in constructional steelwork so far. The investigation of three massive components, a steel plate, a 3 m long hot rolled girder and an extremely thick-walled welded joint revealed the applicability of ultrasonic lockin, burst and sweep procedures. The influence of the coupling location and the thermal response behaviour of single and multiple defects are discussed. The use of differential laser vibrometry is introduced in order to examine the extent of relative motion of crack faces and to analyse the connection between mode III oscillation velocity and crack detectability.

Use of Ultrasound Excited Thermography Applied to Massive Steel Components: Emerging Crack Detection Methodology

In the field of nondestructive testing of structural components, active thermography methods are increasingly in demand. The most experience is available in testing of carbon-fiber-reinforced plastic (CFRP) or similar types of composite material. One of the emerging techniques is ultrasound excited thermography, which has not been fully transferred to massive steel members used in constructional steelwork thus far. The idea of ultrasound excitation is to generate elastic waves that propagate inside the investigated structure. In case of internal flaws, such as cracks, the boundary faces move relative to each other. The resulting rubbing and clapping of crack faces generate frictional heat, which is detected by means of an infrared camera. This paper demonstrates the usage of high-frequency mechanical excitation to detect cracks in hot-rolled girders and reduced plate specimens. The ultrasonic lock-in and the ultrasonic sweep thermography approaches are presented. Localized heating of the crack regions and distributed heating patterns caused by material damping can be observed. The influences of the tuned frequency and the crack depth as well as effects of prestressing and repeated excitation are discussed. In addition to experimental results, a finite-element simulation of the thermostructural problem is conducted. The model can be easily adjusted to match the experimental results of a performed ultrasonic sweep thermography.

FE Simulation of Residual Welding Stresses: Aluminium and Steel Structural Components

One of the decisive criteria in the selection of material between steel and aluminium could be the welding RS (residual stresses), which play an important role for the fatigue behavior of the structures under cycling loading. In the current paper simulations in commercial FE software ANSYS were carried out, in order to calculate the welding RS field for three different materials: structural steel S355 and the aluminum grades EN AW-6060 and EN AW-5754. In the case of EN AW-6060 influence of recrystallization on the yield strength of the HAZ (heat affected zone) was taken into consideration.