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Afsluttede PhD Projekter

Actual stress estimation

Offshore structures accumulate damage during operation because environmental and operational forces vary and fluctuate continuously. The irregular forces cause propagation of cracks, which leads to fatigue failure. The practical design of offshore structures allows for these uncertainties by simplifying reality.

Many offshore platforms in the North Sea are reaching the end of their designed lifetime, but the actual integrity of the structures is unknown. This project aims to obtain better knowledge of the actual stresses during operation by the use of tools like operational modal analysis. This knowledge might lead to a better understanding of the remaining lifetime of the existing structures in the North Sea.

ABOUT THE PROJECT


Project title:  Actual stress estimation

Main supervisor: Prof. Christos Georgakis

Co-supervisor: Rune Brincker (DTU)

Research section: Civil and Architectural Engineering


Soil-pile interaction in soft soils

Precast driven piles or cast-in-place bored piles are often used in connection to construction work on sites with soft soil conditions. If the upper layers of soft soil later experience settlement, then the piles will, in addition to the building load, also be affected by a downwards acting force (termed negative skin friction or down drag) resulting from the adhesion between the soil and the pile in those layers which settle relative to the pile.

Hence, a reliable assessment of negative skin friction is a vital part of pile design when piles are installed in soft ground conditions. However, current guidelines and design practice in Denmark are believed to be overly conservative due to limited understanding of the governing mechanism and limited knowledge regarding the effect of pile material and ground conditions on the development of negative skin friction. Especially the influence of bitumen coating on the development of negative skin friction is poorly understood and in Denmark and also generally in the Nordic countries, there exist only very limited documentation to prove the actual effect of bitumen coating.

ABOUT THE PROJECT


Project title:
Soil-pile interaction in soft soils

Main supervisor: Prof. (Docent) Kenny Kataoka Sørensen

Research section: Civil and Architectural Engineering


Propagation and effects of vibrations in densely populated urban environments

Trains, road traffic, factories and similar sources can generate a significant amount of environmental vibrations to the nearby areas. This is especially evident in urban environments where sources of vibrations are close to the living and working spaces. This projects investigates the ground borne vibrations that propagate via the soil, enter the building through the foundations and in turn cause structure born noise inside the structure.

The main aim of the project is to create a computational model that can estimate the propagation of vibrations through the soil with a reasonable accuracy while still being computationally efficient. The exact mechanism by which the vibrations travel through soil is a complex phenomenon that is hard predict, especially in an urban environment. Therefore, there are multiple aspects that can have an effect and need to be considered in a computational model, such as soil stratification, structure-soil interaction, the transmission in the foundation-soil and soil-air boundaries, etc.

The obtained model will be very useful in the early project design phases when a large number of configurations is being investigated and on-site conditions are often uncertain.

ABOUT THE PROJECT


Project title: Propagation and effects of vibrations in densely populated urban environments

Main supervisor: Professer Lars Vabbersgaard Andersen

Research section: Civil and Architectural Engineering


InDirect Estimation of Loads from Abnormal Waves

Large breaking waves has been observed in areas of the North Sea that are normally not encountered. The potential impact of these extreme breaking waves on offshore installations is yet to be fully understood. Despite numerous models on load estimates from breaking waves by recognised parties such as DNV GL, the full-scale impact from these extreme waves remains uncertain. Many small-scale tests have been conducted with the aim of describing the kinematics of the breaking waves. However, due to the nature of the breaking wave such as nonlinearities and significant scaling effects, further investigation is needed in order to evaluate the response on real structures.

The aim is to use system identification techniques such as Operational Modal Analysis, OMA, and Autoregressive Moving Average, ARMA, to evaluate indirectly the effect of extreme sea states on offshore installations through the structural response. Throughout the project, the objectives are to gather new knowledge about wave loads and statistics of abnormal waves and possibly improve the basis of design. 

The project is carried out in close co-operation between Department of Engineering at Aarhus University and the Danish Hydrocarbon Research and Technology Centre DHRTC, also referred to as Centre for Oil and Gas – DTU.

ABOUT THE PROJECT


Project title:
InDirect estimation of loads from abnormal waves

Main supervisor: Prof. Christos Georgakis

Research section: Civil and Architectural Engineering


Analytical and Numerical Modelling of Reinforced Concrete including Tension-Stiffening Effects

Initially when designing reinforced concrete structures, focus is on the ultimate limit state (ULS), assuring the structure's strength by preventing failure from happening. Afterwards, the serviceability limit state (SLS) is investigated where stress limits as well as requirements concerning crack widths and deformations should be met. Due to the fact that stress levels and crack development are highly dependent on the reinforcement ratio and configuration, the choices made in the ULS concerning the design of the reinforcement have a high impact on the behaviour of structures in SLS.

The stiffness of a material with non-linear behaviour like reinforced concrete is closely associated with the level of crack development. Though, often, the material is assumed fully cracked or not cracked at all where the effect of tension-stiffening is disregarded. This can have consequences on the behaviour, for example in statically indeterminate structures where the static system in the elastic stage is dependent on the stiffness.

Tension-stiffening is an effect that causes the stiffness to be considerably larger than the one of a fully cracked member. The concrete between adjacent cracks is modelled to carry tensile stresses which are transferred from the reinforcement to the concrete by means of the bond.

The aim of this project is to establish the link between ULS and SLS through understanding of the actual physical behaviour. This will be described through analytical models based on concrete mechanics combined with principles of elastic energy. Advantage is taken of numerical modelling to support experimental results as well as to investigate relevance and reliable magnitudes of different material parameters concerning tension-stiffening.

ABOUT THE PROJECT


Project title:
Analytical and Numerical Modelling of Reinforced Concrete including Tension-Stiffening Effects

Main supervisor: Prof. (Docent) Lars German Hagsten

Research section: Civil and Architectural Engineering


Dynamic modelling of ventilation airflow and indoor air quality in naturally ventilated buildings

Natural ventilation is extensively used in intensive livestock production systems. Maintaining desired environmental conditions contributes to the productivity and welfare of the animals in the livestock buildings, which is dependent on the design and control of the ventilation system. On-line measurement or estimation of ventilation airflow rate is important for control of indoor thermal condition, air quality and airborne contaminant removal via building ventilation. However, such a measurement or estimation is a challenge for a naturally ventilated system.

Thus, the objectives of the project are to develop, investigate and validate a new modelling concept for describing the airflow characteristics and estimation of ventilation airflow rate in a naturally ventilated building based on building configuration geometry, wind conditions, etc. The hypothesis is that the characteristic link between indoor air and wind for a defined building structure and varied ventilation openings can be found based on spectral analysis of measured flow data.

In order to achieve this goal, the project will use both a numerical modelling method and analysis of the field data collected in field measurements to discover the approaches for a dynamics modelling of indoor airflow and estimation of the total ventilation rate of a naturally ventilated building.

ABOUT THE PROJECT


Project title: 
Dynamic modelling of ventilation airflow and indoor air quality in naturally ventilated buildings

Main supervisor: Senior Researcher Guoqiang Zhang

Research section: Civil and Architectural Engineering


Development of retrofit solutions for utilisation of the smart grid potential in existing one-family dwellings

The Danish Government has issued that by 2050 the total energy production in Denmark entirely based on renewable energy sources. Renewable energy production introduces a range of new challenges in relation to ensuring both grid stability and energy security.

To address these issues, research in smart energy technology has been on a steady incline throughout recent years. A smart energy system is by definition an energy distribution system that is able to take information about consumption and production rates into account, but also to distribute such information between energy system actors to increase energy efficiency and security.

This PhD project aims at investigating the potential of introducing smart energy technology in existing one-family dwellings, before and after a retrofit has been carried out. The work is intended to identify the economically viable balance between investments in energy savings and smart energy management during a building retrofit.

ABOUT THE PROJECT


Project title: 
Development of retrofit solutions for utilisation of the smart grid potential in existing one-family dwellings

Main supervisor: Assoc. Prof. Steffen Petersen

Research section: Civil and Architectural Engineering


Optimisation of building retrofit in an integrated energy system based on renewable energy

Old buildings account for much of the energy consumed in society and need to be energy renovated and retrofitted in order to cope with the ambitious political goals requiring a more sustainable and flexible energy supply, based on renewable energy technologies. But how do we identify the optimal retrofit solutions that we should rely on?

The PhD project aims at setting up a platform/tool to quantify the operational energy demand and demand-side flexibility of the different types of buildings in a city district depending on their application, location, construction year etc. The purpose of the platform is to enable the identification of cost-optimal solutions that minimise the life-cycle energy need (i.e. include embodied energy), maximise the demand flexibility and guarantee high-quality indoor environments for the end-user. These solutions should not only consider the performance of individual buildings but the whole city district including operation of the energy supply. 

The platform/tool is initially set up and calibrated using Aarhus energy district as case study, but should be generally applicable when finalised.

ABOUT THE PROJECT


Project title: 
Optimisation of building retrofit in an integrated energy system based on renewable energy

Main supervisor: Assoc. Prof. Steffen Petersen

Research section: Civil and Architectural Engineering


Development of retrofit solutions for utilisation of the smart grid potential in existing residential buildings

Project description:
The Danish Government has passed an agreement that by 2050 the total energy production in Denmark should constitute of renewable energy sources. This implies a new challenge as renewable energy production is fluctuating with weather conditions. Therefore, a flexible relation between the consumers and producers, referred to as smart energy, is necessary.

Taking offset in the retrofit demonstration project READY, this PhD project aims at investigating the smart energy potential in existing residential buildings. Retrofit solutions that increases the buildings' smart grid potential is then identified and tested in an actual retrofit case. The objective is to set up a general methodology for targeting and quantifying the effect of different energy flexibility technologies related to energy retrofit of buildings.

ABOUT THE PROJECT


Project title: 
Development of retrofit solutions for utilisation of the smart grid potential in existing residential buildings

Main supervisor: Assoc. Prof. Steffen Petersen

Research section: Civil and Architectural Engineering


Pore water pressure response and heave of Palaeogene clays in connection to deep excavation and pile driving

Constructions in the city centers have changed over the last 10-20 years. The buildings are increasingly getting higher, moving into coastal areas and more often include excavations for multi-story basements. When removing a large amount of soil due to deep excavations the foundation substrate is relieved from a massive load – often even when the weight of the building is included. When building in areas with clay of high plasticity (Palaeogene Clay) near the surface, the relieving pressure on the underlying substrate pose a challenge. Deep excavations generate negative pore water pressures within the clay which cause the soil to heave. However, it is believed that positive pore pressures generated by pile driving can reduce the subsequent heave.

Through literature/archive study, field monitoring at construction sites and testing in the laboratory we aim at extending our current knowledge of the pore pressure development and ground deformations in connection to pile driving in combination with deep excavations in clay. The results of the work should provide the basis for the development of an analytical method which can be used to predict ground deformations and pore pressure development in Danish Palaeogene Clays. The project will furthermore aim at establishing guidelines of how to include the partial equalization of the pore pressures in the design of future foundations.

ABOUT THE PROJECT


Project title:
Pore water pressure response and heave of Palaeogene clays in connection to deep excavation and pile driving.

Main supervisor: Prof. (Docent) Kenny Kataoka Sørensen

Research section: Civil and Architectural Engineering


Adaptive Smart (Natural) Ventilation Control for Cattle Housing and Integrated Climate Sensing

Natural ventilation is an increasingly popular approach to offer a good indoor climate without any mechanical technology aid. Comparing with the mechanical ventilation, natural ventilation has a very significant advantage in terms of energy savings. However, obvious defects such as the absence of precise control of the air movement, vulnerability to the persistent severe situation and lack of adaptability restrict the natural ventilation in becoming more widespread. Therefore, innovative design and control are needed to improve its performance to ensure optimal indoor climate.

To achieve this goal, knowledge on animal heat loss and thermal well-being influenced by air temperature, speed, radiation and evaporation effects in a space is crucial.

The aim of this project is to develop an adaptive smart (natural) ventilated barn for cattle. An investigation of integrated climate sensing methods and precision zone ventilation techniques will be conducted. Both experiment and numerical simulation methods will be applied in the project.

ABOUT THE PROJECT


Project title:  
Adaptive Smart (Natural) Ventilation Control for Cattle Housing and Integrated Climate Sensing

Main supervisor: Senior Researcher Guoqiang Zhang

Research section: Civil and Architectural Engineering


Automated Operational Modal Analysis

In the future, the application of Structural Health Monitoring (SHM) will be a key element when considering structures such as large span bridges, high-rise buildings or wind turbines.

SHM is a network of sensors placed wisely on the structure, monitoring the physical parameters of the structure. The sensors inform the control centre or the maintenance crew if any change in the structure is detected revealing crack growth or failure of secondary structural elements. This increases the safety of the structure and makes it possible to replace parts or fix the structure before failure.

The SHM system consists of sensors and an analysis part to process the measured signals. In this process, the identification of the structures’ physical parameters is essential. As it will be an enormous amount of data which have to be analysed, this procedure will have to be automated.

The focus of this project is on Automated OMA. OMA is short for Operational Modal Analysis and is used in modal testing to find the modal parameters such as eigenfrequency, modeshapes and damping ratio of a machine or a structure. The process of automating the estimation process when working with OMA demands development of an algorithm that is stable and need no interaction from the user.

The main objective of the project is to investigate the use of well-known identification techniques in both time domain and frequency domain in combination with filtering techniques to automate the identification process. The identification of several criteria for choosing the physical parameters is another main focus of the project.

ABOUT THE PROJECT


Project title:
Automated Operational Modal Analysis

Research section: Civil and Architectural Engineering


Change Management i Building Performance: An Engineering Design Methodology for Sustainable Retrofitting

A sustainable energy sector has a balance in energy production and consumption and has no, or minimal, negative impact on the environment (within the environmental tolerance limits). It allows for the opportunity for a country to employ its social and economic activities. It can be seen as the final goal: a balance of social activities, economic activities and the environment. In this framework, compounding the typical challenges of retrofitting implementation, due to the potential of different types of stakeholders, is the lack of a comprehensive methodology to enhance both communication in a learning process among different stakeholders and multi-optimisation among different criteria in a sustainable perspective.

Therefore, in a real sustainable retrofitting context, an understanding of the level of connectivity and level of criticality of the dependencies (building features, limitations, constrains, etc.) is required initially towards application of an appropriate decision support system which needs to be embedded in the early design stages (conceptual design phase).

The motivation for this research is related to identifying and developing a suitable methodology to carry out and improve learning using a mix of methods. This involves the application of systematic approaches to identify and catch the complexity in retrofitting of existing buildings when dealing with complex problems and multiple decision makers as well as multiple criteria based on a holistic vision. It also includes considering and addressing the processes involved in an optimised retrofitting process among the existing alternatives using MCDM methods.

ABOUT THE PROJECT


Project title:
Change Management in Building Performance: An Engineering Design Methodology for Sustainable Retrofitting

Main supervisor: Prof. Poul Henning Kirkegaard and Prof. Rossella Corrao, Department of Architecture, University of Palermo

Research section: Civil and Architectural Engineering


Ductility of Confined Concrete

Confinement is known to significantly improve the compressive strength and ductility of concrete; two properties which are highly desirable to exploit in practical designs. The main issue in applying the existing constitutive confinement models in practical design is the determination of the confining stress. In the most common models, the confining effect is defined as an uniform external stress which actively confines the member.  In actual reinforced concrete members, the confining effect is, however, provided as a passive effect by the reinforcement.

Thus, the aim of the project is to relate the passive and active confinement to fully understand the confining effects that the reinforcing arrangement has on the concrete member.

The first step is to study the physical behaviour of the confining effect through a theoretical understanding on a micromechanical level. The knowledge of the behaviour on a micromechanical level will be compared to the already existing models which describe the effect of external confining pressure.

The principal approach to the project will be based on analytical modelling. However, experimental studies will be essential in verifying any theoretical models and to conduct parametric studies on the topic.

ABOUT THE PROJECT


Project title:  
Ductility of Confined Concrete

Main supervisor: Prof. (Docent) Lars German Hagsten

Research section: Civil and Architectural Engineering


Influence of Smectite Content on the Deformation Behaviour of Clays

Understanding the high plasticity Palaeogene clay has been a challenge for the geotechnical community in Denmark for decades. The settlement and heave potential of this type of clay exceeds what is commonly expected for Danish clay deposits. Typically, the Palaeogene clays have high strengths but a rather large settlement and heave potential, which means that they do not follow what is normal for most clays where strength and stiffness follow one another. Many relationships exist linking the plasticity index (a state parameter easy to assess) to the engineering properties for clays based on rules of thumb. However, they rarely fit with the behaviour of the Palaeogene clays.

The latest research has indicated that the clay mineral smectite may govern the behaviour of a clay, which means that the settlement and heave potential may be dependent on this content. This project investigates this theory by execution of multiple tests of clays containing smectite.

ABOUT THE PROJECT


Project title: 
Influence of Smectite Content on the Deformation Behaviour of Clays

Main supervisor: Prof. (Docent) Kenny Kataoka Sørensen

Research section: Civil and Architectural Engineering


Vibration-Based Structural Health Monitoring

The main focus of this project is on vibration-based damage detection on civil as well as mechanical structures.

This subject can, in a global context, be classified as a “Structural Health Monitoring” (SHM) discipline. The SHM term covers a broad range of different methods and techniques with the coincident goal or purpose of detecting damage, position and magnitude in a structure.

For researchers as well as commercial companies, SHM is an area of great interest due to the fact that SHM has the potential to replace traditional damage detecting techniques which, in most cases, are mere visual inspections supplemented by material samples extracted from the structure. 

It is commonly known that damage, e.g. holes or cracks, in a structure will have an impact on the global dynamic properties, in other words the modal parameters. These parameters can be extracted by means of “Operational Modal Analysis”, sophisticated sensors and data acquisition equipment, even on large civil structures. Hence, damage in a structure can be detected if modal property shifts are observed. The aim of this project is to develop a method which is able to detect and pin-point damage on an arbitrary structure.

ABOUT THE PROJECT


Project title:
Vibration-Based Structural Health Monitoring

Research section: Civil and Architectural Engineering


Fullblown ODS

When measuring the dynamic properties of a civil structure, the information is always limited to the amount of sensors placed on the structure. This often results in sparse modal models and rough estimations.

The main focus of this project is to develop a model that provides information about the dynamics of a structure in points where no sensors have been placed.

This is done by making a transformation between a set of experimentally obtained mode shapes and a set of mode shapes from a Finite Element Model (FE). The overall principle is that a set of mode shapes can be described as a linear combination of another set of mode shapes as long as the changes between the two models are small. This is known as the Local Correspondence Principle.

The set of experimentally found mode shapes can be found by making an Operational Modal Analysis on the measured response and has the advantage of providing “true” information in a limited number of points.

On the other hand, the FE model provides a set of “fictive” mode shapes in a large number of points. As long as the FE model doesn’t differ too much from the real structure, the estimated mode shapes can be found successfully, making a linear transformation of the FE mode shapes.

The method will provide new ways of analysing structures. Displacements can be transformed to stains and stresses, which will be a helpful tool when analysing fatigue where the stress history is an important factor. Furthermore, the method will be suitable for monitoring of structures.

ABOUT THE PROJECT


Project title:
Fullblown ODS

Research section: Civil and Architectural Engineering


Complex Ventilation and Micro-Environmental Control in Livestock Housing

The aim of this project is to improve animal welfare and reduce environmental impact. A new concept for monitoring the thermal and airflow conditions in animal zones will be introduced. Furthermore, we will set up a dynamic predictive model to create a precision environment control strategy at individual animal or defined zone level.

Micro-complex ventilation involves integrating precision local ventilations in animal zones and near manure fouled floors or manure surfaces within the building ventilation. In order to gain knowledge about air motion and distribution in animal occupied zones and about system effects on emission reduction, an integrated micro ventilation concept in livestock housing will be investigated.

Data will be gathered by using both Computational Fluid Dynamics (CFD) simulations and full scale experiments. After the establishment of the system, optimisations are also needed. Then, to validate the optimal system, varied techniques including local cross ventilation, heat exchange and passive earth-air heat/cooling will be investigated. The proposed system combines the advantages of natural, mechanical and displacement ventilation, making it a technology with great efficiency and potential. 

ABOUT THE PROJECT


Project title:
Complex Ventilation and Micro-Environmental Control in Livestock Housing

Main supervisor: Senior Researcher Guoqiang Zhang

Research section: Civil and Architectural Engineering


Smart Grid Flexibility Potential in Model-Based Building Control

Much can be achieved by making the energy consumption of buildings more flexible so that the use of energy takes place at times when it is most convenient for the overall energy system.

This project examines the flexibility potential in relation to controlling the energy system of a building. This is done by using a model of a building that includes weather forecasts, energy prices and occupant behaviour.

The approach is essential to finding the combination of future actuator set-points that result in the best outcome with respect to indoor climate and economy.

ABOUT THE PROJECT


Project title:
Smart Grid Flexibility Potential in Model-Based Building Control

Main supervisor: Assistant Prof. Steffen Petersen

Research section: Civil and Architectural Engineering


Incorporating Structural Health Monitoring in the Design of Slip Formed Concrete Wind Turbine Towers

For decades, the prevailing material for very tall chimneys for power plants has been concrete. Combined, Rambøll and MT Højgaard have been involved in all phases of the construction of the very highest in Denmark.

Most modern wind turbine towers are tubular steel towers, and until now the high labour costs have tipped the scale in favour of steel towers. But as technology advances rapidly towards larger turbines, it is only natural to assume that concrete will be the new first choice for multi MW designs as it did for chimneys. For this scale of structures, the mechanical properties of concrete can be superior to those of steel if the experiences and know-how from concrete chimney construction are applied throughout the project.

Structural health monitoring (SHM) is the perpetual process of monitoring the structures' integrity. By equipping SHM to a civil structure, the owner is provided with decision support. So far, for the structures that have been equipped with SHM, the value in terms of total life-cycle benefits has rarely been estimated.

By associating a risk optimised decision policy, the SHM effort can be optimised and the initial target safety of the structure may be recalibrated upfront. In some cases, this translates into reductions on the Partial Safety Factors, leading to reductions on initial cost.

This project implements the risk based SHM system design, which is a research heavy topic, on the development of cast concrete wind turbine towers for multi MW turbines.

ABOUT THE PROJECT


Project title:
Incorporating Structural Health Monitoring in the Design of Slip Formed Concrete Wind Turbine Towers

Research section: Civil and Architectural Engineering


Precision Zone Ventilation Design and Control in Pig Housing

The ventilation system of animal houses is important in livestock production due to its significant influence on local thermal conditions, indoor air quality and emission to the neighbouring atmosphere.

Precision zone ventilation consisting of direct air supply into the Animal Occupied Zone (AOZ) and precision exhaust ventilation from the source zone can provide more efficient climate control and improved air quality. The objective of this project is to develop the knowledge of precision zone ventilation in pig production buildings, aiming at achieving more effective ventilation and improving indoor air quality as well as reducing the required capacity of air cleaning devises.

Experimental investigations are carried out both in a pig production facility and in a 2-D chamber in the laboratory. The two-dimensional Laser Doppler Anemometry (LDA) is used for measuring velocity speed and characterising flow type in the boundary layer. N2O is applied as tracer gas. The artificial pigs developed at APL are used to simulate the heat production of pigs at different locations. Computational Fluid Dynamics (CFD) is used for computer modelling.

ABOUT THE PROJECT


Project title:
Precision Zone Ventilation Design and Control in Pig Housing

Main supervisor: Senior Researcher Guoqiang Zhang

Research section: Civil and Architectural Engineering


Continuous Members in Reinforced Concrete

The vast majority of concrete slabs are only supplied with longitudinal reinforcement while expensive shear-reinforcement is only placed in case of shear forces of extraordinary magnitude. Thus, in most situations, the capacity of the slabs with respect to shear-forces is provided by a combination of the concrete itself along with the longitudinal reinforcement. Unfortunately, the fundamentals of this vital mechanism that allow for such structures to be designed are still not fully understood, and most models used for design are empirical and lack a physical and rational basis.

In addition, such slabs are often designed as statically indeterminate structures. In order to achieve a certain level of robustness, the final structure must possess a certain level of ability to withstand larger deformations without failing.

It is well known that the ability of reinforced concrete to deform is due to development of cracks. Hence, in this respect, the progressive development of cracks is beneficial. However, when considering members without shear-reinforcement, the same cracks also introduce certain “weakened” regions which effectively reduce the capacity with respect to shear.

The aim of the project is, among others, to increase the understanding of this rather delicate relation between the inevitable development of cracks and the shear-capacity of slabs.

ABOUT THE PROJECT


Project title:
Continuous Members in Reinforced Concrete

Main supervisor: Prof. (Docent) Lars German Hagsten

Research section: Civil and Architectural Engineering