Jónas Snæbjörnsson

Interrelations in multi-source geohazard monitoring for safety management of infrastructure systems

In this paper a methodology is put forward for enhancing safety by linking multi-source monitoring to multi-hazards threatening infrastructure systems. An application is demonstrated with a conceptual model comprising two systems. One system includes the infrastructure, exemplified by a reservoir and its dams, along with settings for geohazards. The other system contains the associated monitoring possibilities. Interrelations are quantitatively explored by applying an existing method of systems theory, extended to include multiple systems. This is used to investigate geohazard-triggering potential, interaction intensity and dominance. It is further used to explore the reaction of monitoring components to geohazard action as well as safety values of the monitoring system and its components. The interrelations established have a general relevance for the consideration of geohazards threatening reservoirs and dams, and the associated monitoring. This is explained with a case study. Moreover, the interrelations can be used for defining hazard chains in multi-hazard assessment, planning of monitoring programmes and detecting precursory pathways within a monitoring system. The methodology constitutes the basis for comprehensive safety and risk management embracing, multi-hazard assessment as well as structural health monitoring.

Measurements of Surface-Layer Turbulence in a Wide Norwegian Fjord Using Synchronized Long-Range Doppler Wind Lidars

Three synchronized pulsed Doppler wind lidars were deployed from May 2016 to June 2016 on the shores of a wide Norwegian fjord called Bjornafjord to study the wind characteristics at the proposed location of a planned bridge. The purpose was to investigate the potential of using lidars to gather information on turbulence characteristics in the middle of a wide fjord. The study includes the analysis of the single-point and two-point statistics of wind turbulence, which are of major interest to estimate dynamic wind loads on structures. The horizontal wind components were measured by the intersecting scanning beams, along a line located 25 m above the sea surface, at scanning distances up to 4.6 k m . For a mean wind velocity above 8 m · s - 1 , the recorded turbulence intensity was below 0.06 on average. Even though the along-beam spatial averaging leads to an underestimated turbulence intensity, such a value indicates a roughness length much lower than provided in the European standard EN 1991-1-4:2005. The normalized spectrum of the along-wind component was compared to the one provided by the Norwegian Petroleum Industry Standard and the Norwegian Handbook for bridge design N400. A good overall agreement was observed for wave-numbers below 0 . 02 / m . The along-beam spatial averaging in the adopted set-up prevented a more detailed comparison at larger wave-numbers, which challenges the study of wind turbulence at scanning distances of several kilometres. The results presented illustrate the need to complement lidar data with point-measurement to reduce the uncertainties linked to the atmospheric stability and the spatial averaging of the lidar probe volume. The measured lateral coherence was associated with a decay coefficient larger than expected for the along-wind component, with a value around 21 for a mean wind velocity bounded between 10 m · s - 1 and 14 m · s - 1 , which may be related to a stable atmospheric stratification.

Time-Domain Analysis of Wind-Induced Response of a Suspension Bridge in Comparison With the Full-Scale Measurements

The present study compares the buffeting response of a suspension bridge computed in the time-domain with full-scale measurement data. The in-service Lysefjord Bridge is used as a study case, which allows a unique comparison of the computational results with full-scale buffeting bridge response observed during a one year monitoring period. The time-domain analysis is performed using a finite element approach. Turbulent wind field is simulated according to the governing bridge design standard in Norway for three different terrain categories. The time-domain analysis indicates that the non-linear components of the wind loading are of limited importance in the present case, contributing by less than 5% to the standard deviation of the lateral displacement. The contribution of the buffeting loads on the main cables, hangers and towers to the lateral dynamic response of the bridge girder is about 6%. With the time-domain method, mode coupling as well as the influence of cables and towers are well captured in the simulation results. The buffeting response, estimated in terms of the standard deviation of acceleration, is found to be in good agreement with the field measurement data. Comparison suggests that the proposed numerical method, with the non-linear force model, is able to predict the bridge response reasonably well.

Basalt Fibers as New Material for Reinforcement and Confinement of Concrete

Basalt Fibre Reinforced Polymer (BFRP) is a new material in civil engineering and has shown to be a promising material for infrastructure strengthening. In comparison to carbon fiber, glass fiber and other composites, it has some advantages such as high-temperature resistance and low cost. At the Structural and Composite Laboratory at Reykjavik University (SEL) several research projects involving strengthening concrete beams and columns by using FRP materials have been on-going in recent years. These tests have shown improvements in strength and durability compared to unstrengthened concrete members. The benefit of using basalt fiber or other FRP material is that they are non-corrosive, which is a good choice for reinforcing concrete structure exposed to de-icing salts, for examples in bridge decks and parking garage elements. Also for concrete exposed to marine environment, such as seawalls, water breaks and buildings or other structures located near a waterfront. Two research projects are presented in this paper; a test of prestressed concrete with internal basalt rods instead of steel and a test of columns strengthened by wrapping fibre-reinforced composite sheets around the columns to increase their strength and ductility. These experimental tests show increasing strength and ductile for both the beams and the columns.

Temperature Effects on the Modal Properties of a Suspension Bridge

The paper studies temperature effects on the modal parameters of a suspension bridge across a Norwegian fjord. The approach used is a full-scale ambient vibration testing, where an automated Covariance-Driven Stochastic Subspace Identification (SSI-COV) method is used to identify the modal parameters. The bridge site, the bridge structure and the monitoring system are presented, followed by a summary of the data analysis procedure and the parameters used for the automated SSI-COV method applied. The operational modal analysis is based on 6 months of continuous acceleration records providing seasonal and diurnal variations of the natural frequencies of the bridge and the modal damping ratios. Temperature effects were observed with details that are scarcely available in the literature. In particular, the pronounced daily fluctuations of natural frequencies and seasonal effects are documented.

Surface Strain Rate Tensor Field for Iceland Based on a GPS Network

The theme of this paper is the transformation of velocities from a GPS network into a surface strain rate field that illustrates recent and underlying movements within the tectonic system of Iceland. The surface strain rates presented are derived using GPS measurements at the base stations of the National Land Survey of Iceland reference network. The GPS observations are discussed, and the derived average velocity field for an 11-year period, from 1993 to 2004, is presented. The measurements at the westernmost and the easternmost points, which are located on the oldest parts of the country, i.e., the North America plate and the Eurasia plate, respectively, show the rigid movement of the two crustal plates. The average northwards velocity of the two plates is 25 mm/year, and the differential movement is 22 mm/year in the direction 281° from north. The definitions of strain rate tensor and vorticity tensor are outlined and applied in a numerical analysis to derive tensor strain rates from the GPS data. The strain rate tensor field is displayed on maps showing normal and shear strain rate fields, along with principal and dilatation rate fields, as well as the vorticity field. The derived tensor fields are discussed and interpreted in relation to the present-day view on the tectonism of Iceland and recent tectonic activity. The results indicate that most of the significant strain is concentrated on a tongue zone that goes from the North American plate into a slit in the Eurasian plate. The results clearly show the strained volcanic zone of Iceland and the rigid zones with zero strain on either side of it.

On the Manifestation of Ground Motion Model Differences on Seismic Hazard Sensitivity in North Iceland

In this study, probabilistic seismic hazard assessment (PSHA) for North Iceland is explored in terms of its sensitivity to one of its key elements, the selected ground-motion models (GMMs). The GMMs in previous PSHA studies for Iceland are reviewed and in some cases recalibrated to the Icelandic dataset using a Markov Chain Monte Carlo (MCMC) algorithm which is useful in regions where the earthquake records are scarce. To show the ground motion model variability as it is manifested in PSHA uncertainties, the hazard maps of standard deviation and coefficient of variation (CV) of PGA at two hazard levels for GMMs before and after recalibrating are shown. The results indicate that the recalibrated models are promising candidates to be applied for future hazard studies in Iceland, but more importantly they show how to what extent and how the epistemic uncertainty of the GMMs contribute to patches of heightened hazard uncertainties, especially at near- and far-fault distances where there is a particular lack of data.

Dynamic Characteristics of Multi-Story Reinforced Concrete Buildings

Having a realistic estimate of structural parameters, such as natural frequency and damping is important for design purposes. In this study, available wind and earthquake induced acceleration data from four multi-story reinforced concrete buildings are utilized to examine structural behaviour and system parameters. The buildings measurement systems are described and the recorded structural response data presented. The data stems from two different sources of excitation, i.e. wind and earthquake, and are recorded for various excitation levels and environmental conditions. System identification analyses of the buildings are carried out applying previously verified parametric methods to the recorded data. The natural frequencies and critical damping ratios established from the recordings are compared to values estimated using design guidelines and international data compilations for reinforced concrete structures of similar type. Considerable variability is discovered between the different estimation formulas and the observed natural frequencies of the buildings are found to lie at the upper limit of the prediction formula.