References of "Maas, Stefan 50002241"
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See detailModel updating for structural health monitoring using static and dynamic measurements
Schommer, Sebastian UL; Nguyen, Viet Ha UL; Maas, Stefan UL et al

in Procedia Engineering (2017), 199

Structural health monitoring is tracking static or dynamic characteristics of a structure to identify and localize stiffness reductions for damage detection. Different damage indicators are used and any ... [more ▼]

Structural health monitoring is tracking static or dynamic characteristics of a structure to identify and localize stiffness reductions for damage detection. Different damage indicators are used and any indicator presents advantages and drawbacks. Hence the idea comes up to combine them in a model-updating procedure using a finite element model. In a first step, a model is fit to match the healthy reference state of the examined structure. Therefore it relies on minimizing a special objective function adding and weighting the differences between measured and calculated static and dynamic structural characteristics. For doing structural health monitoring the measurements are repeated in distinct time intervals and the finite element model is updated again, using the same objective function and minimization procedure. Damage can be identified and localized by highlighting reductions in the stiffness matrix of the model compared to the initial model. The efficiency of the method is illustrated by in-situ tests, where a single beam is examined that was part of a real prestressed concrete bridge. For static tests, 8 displacement transducers were disposed along the length of 40m, while the beam was mass-loaded and the deflection line is analyzed. Modal analysis was performed with swept sine excitation with constant force amplitude to identify eigenfrequencies and mode shapes. Stepwise artificial damage was provoked by cutting multiple prestressed tendons inside the concrete beam. A finite element model with a mapped mesh was created, allowing a variation of Young’s modulus in grouped sections. On real bridges temperature is neither homogenous nor constant over time, which often has a considerable influence on measured static and dynamic characteristics as the stiffness of asphalt and/or bearings can be affected. The proposed methods show their efficiency when temperature effects were excluded or compensated after measurement, which is a topic on its own and not discussed here. [less ▲]

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See detailBiomechanik neuer Implantate für die HTO
Pape, Dietrich; Diffo Kaze, Arnaud UL; Hoffmann, Alexander et al

in Orthopade (Der) (2017), 46(7), 583-595

Biomechanical characteristics of 5 tibial osteotomy plates for the treatment of medial knee joint osteoarthritis were examined. Fourth-generation tibial bone composites underwent a medial open-wedge high ... [more ▼]

Biomechanical characteristics of 5 tibial osteotomy plates for the treatment of medial knee joint osteoarthritis were examined. Fourth-generation tibial bone composites underwent a medial open-wedge high tibial osteotomy, using TomoFix™ standard, PEEKPower®, ContourLock®, TomoFix™ small stature plates, and iBalance® implants. Static compression load to failure and load-controlled cyclic fatigue failure tests were performed. All plates had sufficient stability up to 2400 N in the static compression load to failure tests. Screw breakage in the iBalance® group and opposite cortex fractures in all constructs occurred at lower loading conditions. The highest fatigue strength in terms of maximal load and number of cycles performed prior to failure was observed for the ContourLock® group followed by the iBalance® implants, the TomoFix™ standard and small stature plates. PEEKPower® had the lowest fatigue strength. All plates showed sufficient stability under static loading. Compared to the TomoFix™ and the PEEKPower® plates, the ContourLock® plate and iBalance® implant showed a higher mechanical fatigue strength during cyclic fatigue testing, suggesting that both mechanical static and fatigue strength increase with a wider proximal T‑shaped plate design together with diverging proximal screws. Mechanical strength of the bone–implant constructs decreases with a narrow T‑shaped proximal end design and converging proximal screws (TomoFix™) or a short vertical plate design (PEEKPower®). Published results indicate high fusion rates and good clinical results with the TomoFix™ plate, which is contrary to our findings. A certain amount of interfragmentary motion rather than high mechanical strength and stiffness seem to be important for bone healing which is outside the scope of this paper. [less ▲]

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See detailA Study of Temperature and Aging Effects on Eigenfrequencies of Concrete Bridges for Health Monitoring
Nguyen, Viet Ha UL; Mahowald, Jean UL; Schommer, Sebastian UL et al

in Engineering (2017), 9

This paper discusses the influence of environmental factors and of normal material aging on the eigenfrequencies of concrete bridges based on monitoring data registered during 4 years of a specific bridge ... [more ▼]

This paper discusses the influence of environmental factors and of normal material aging on the eigenfrequencies of concrete bridges based on monitoring data registered during 4 years of a specific bridge. It is a new composite steel-concrete bridge built in 2006 in Luxembourg. The measurements are analyzed and compared to literature data. The final objective is the use of real monitored eigenfrequencies for structural health monitoring and damage detection based on identification of stiffness losses in practical applications. Therefore, it is very important to identify and compensate for outdoor influences namely temperature, excitation force level and normal aging effects, like creep and shrinkage of concrete and their impact on material properties. The present paper aims at describing these effects in order to separate them from damage effects. It is shown that temperature change rates and temperature gradients within the bridge have an influence on the eigenfrequencies. Hence the key idea for assessment from the full database is to select only measurements with small temperature differences and slow temperature change rates. [less ▲]

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See detailPost-occupancy evaluation of residential buildings in Luxembourgwith centralized and decentralized ventilation systems, focusing onindoor air quality (IAQ). Assessment by questionnaires and physicalmeasurements
Ferreira Silva, Marielle; Maas, Stefan UL; Artur de Souza, Henor et al

in Energy and Buildings (2017), 148

tComplete knowledge about habits of the occupants, including their opinions regarding ventilation sys-tems is an important condition for reducing the consumption of natural resources and improving ... [more ▼]

tComplete knowledge about habits of the occupants, including their opinions regarding ventilation sys-tems is an important condition for reducing the consumption of natural resources and improving indoorcomfort. In addition, uncomforted occupants tend to take measures to improve their situation, whichmay increase energy consumption. Advanced thermal models for buildings can perhaps predict interac-tions between the IAQ determinants, e.g. energy consumption, ventilation and comfort, but do not takeinto account the behavior of residents. By questionnaires and physical measurements this study evalu-ated dwellings equipped partly with centralized and partly with decentralized ventilation systems withheat recovery. This field study involved two post-occupied residential buildings situated in the city ofEsch-sur-Alzette, Luxembourg, during spring season 2015. Thus, both the physical measurements andquestionnaires were considered. The results obtained demonstrated that more than 80% of the residentswere satisfied and the perceived IAQ was judged “normal”, “good” or even “very good”. Furthermore,the measurements performed detected in some cases malfunction of ventilation devices, wherefore theoccupants were unable. [less ▲]

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See detailHealth Monitoring based on Dynamic Flexibility matrix: Theoretical Models versus in-situ Tests
Schommer, Sebastian UL; Mahowald, Jean; Nguyen, Viet Ha UL et al

in Engineering (2017), 09(02), 37-67

The paper focuses on damage detection of civil engineering structures and especially on concrete bridges. A method for structural health monitoring based on vibrational measurements is presented and ... [more ▼]

The paper focuses on damage detection of civil engineering structures and especially on concrete bridges. A method for structural health monitoring based on vibrational measurements is presented and discussed. Experimentally identified modal parameters (eigenfrequencies, mode shapes and modal masses) of bridge structures are used to calculate the inverse stiffness matrix, the so-called flexibility matrix. By monitoring of the stiffness matrix, damage can easily be detected, quantified and localized by tracking changes of its individual elements. However, based on dynamic field measurements, the acquisition of the flexibility matrix instead of the stiffness matrix is often the only choice and hence more relevant for practice. But the flexibility-based quantification and localisation of damage are often possible but more difficult, as it depends on the type of support and the location of the damage. These issues are discussed and synthetized, that is an originality of this paper and is believed useful for engineers in the damage detection of different bridge structures. First the theoretical background is briefly repeated prior to the illustration of the differences between stiffness and flexibility matrix on analytical and numerical examples. Then the flexibility-based detection is demonstrated on two true bridges with real-time measurement data and the results are promising. [less ▲]

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See detailUntersuchung des thermischen Komforts und der Luftqualität an vier neuen energieeffizienten Luxemburger Schulen mit und ohne Lüftungsanlagen
Maas, Stefan UL; Brensing, Jessica UL; Flies, Max et al

in Bauphysik (2017), 39. Jahrgang

A study was conducted in four new energy efficient schools in Luxembourg with excellent primary energy consumption values, where the indoor thermal comfort and the air quality was assessed by technical ... [more ▼]

A study was conducted in four new energy efficient schools in Luxembourg with excellent primary energy consumption values, where the indoor thermal comfort and the air quality was assessed by technical measurements and by surveys. Two schools were equipped with mechanical ventilation with heat recovery and two were without, i.e. ventilation there was manually controlled by opening the windows. Further to the C02-concentration as indicator for the air quality all other important comfort parameters were measured, why the well-known indicators according to FANGER could be calculated, i.e. the „Predicted Mean Vote (PMV)“ and the „Predicted Percentage of Dissatisfied (PPD)“. The individual perceived indoor comfort was analyzed by standardized questionnaires for a period of four weeks. One aim of the study was to compare natural ventilation by opening and closing the windows with the mechanical ventilation in all named aspects. In total there was a good agreement between the measurements and the survey. The results show that in general mechanical ventilation in schools lead to better physical measured values and to better subjective perception of the users. However those buildings suffer from a very low relative humidity, though it is rarely sensed by the occupants. Furthermore, in one of the mechanically ventilated buildings the air velocity was very high and led to draft-perception. The detected air temperature was heterogeneous and independent of the ventilation concept, leading to an individual judgement of PMV for the different buildings. Users should always have the possibility of interaction and control to adjust the indoor climate according to their individual needs and to correct smaller technical shortcomings. [less ▲]

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See detailAn advanced approach to design experiments to investigate the biomechanics of the pelvis
Ricci, Pierre-Louis UL; Maas, Stefan UL; Gerich, Torsten et al

Scientific Conference (2017)

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See detailStatic load testing with temperature compensation for structural health monitoring of bridges
Nguyen, Viet Ha UL; Schommer, Sebastian UL; Maas, Stefan UL et al

in Engineering Structures (2016), 127(2016), 700-718

The paper presents a series of repeated static loading tests on a prestressed concrete beam, which was originally part of a real bridge and then subjected to stepwise artificial damage. The tests were ... [more ▼]

The paper presents a series of repeated static loading tests on a prestressed concrete beam, which was originally part of a real bridge and then subjected to stepwise artificial damage. The tests were done during a one-month period that four levels of damage were introduced by cutting tendons until visible cracking occurred. The deflection line was measured by means of several displacement sensors and the retrieved information is used in different ways for damage detection. At first, the sensor spacing requirement is analyzed with respect to measurement accuracy as well as necessary resolution for the numerical derivations of the deflection line to obtain the rotational angle and the curvature of the beam. These derived quantities may be used as damage indicators in addition to the deflection. Damage of concrete goes very often along with non-linear phenomena like cracking of concrete and plastic strain of reinforcement steel. These effects are discussed and their influence on the repeated loading tests as well the test procedure for condition monitoring is deployed. Progressive damage goes along with progressive sagging of the bridge due to gravity, which can also be used as damage indicator. Finally, the effect of varying outdoor temperatures are discussed and assessed. Though these effects can be reduced by choosing cloudy days without high temperature changes and without high solar irradiation, the outdoor temperature is never constant. Hence, a compensation algorithm is proposed which reflects the measured data according to a reference temperature. This compensation visibly improved the regularity of data. [less ▲]

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See detailWie bilanzieren die europäischen Staaten in Luxemburg, Deutschland, Belgien, der Schweiz und in Frankreich? Ein Vergleich verschiedener nationaler Berechnungsvorschriften der Gesamtenergieeffizienz anhand von 9 klimatisierten Bürogebäuden und deren Realverbrauch
Render, Wolfgang; Maas, Stefan UL; Scholzen, Frank UL

in cci Zeitung - Führende Fachzeitung für die LüKK (Lüftung- Klima- Kältebranche) (2016)

Die EU-Mitgliedsstaaten sind gemäß den Richtlinien 2002/91/EG und 2010/31/EU verpflichtet, die Gesamtenergieeffizienz von Gebäuden abhängig von ihrer Art der Nutzung zu ermitteln und in einem Energiepass ... [more ▼]

Die EU-Mitgliedsstaaten sind gemäß den Richtlinien 2002/91/EG und 2010/31/EU verpflichtet, die Gesamtenergieeffizienz von Gebäuden abhängig von ihrer Art der Nutzung zu ermitteln und in einem Energiepass zu belegen. Die Ausgestaltung der Details des Berechnungsverfahrens und der Grenzwerte obliegt den Mitgliedsstaaten im Rahmen ihres nationalen Entscheidungsspielraums. Für 9 bestehende mittlere bis große Bürogebäude mit Teil- und Vollklimatisierung wurden die Berechnungen der Gesamtenergieeffizienz nach den jeweiligen Berechnungsmethoden durchgeführt und mit dem gemessenen realen Verbrauch verglichen. Die Abweichungen zwischen der berechneten Endenergie und dem tatsächlichen Verbrauch betragen zwar etliche 100% beim einzelnen Objekt, aber ca. 20 % im Mittel bei der Heizenergie. Bei der berechneten Endenergie des Stroms bestehen auch riesige Differenzen beim Einzelobjekt, aber im nationalen Vergleich im Mittel nur geringe Abweichungen zum Verbrauch, wobei der Stromverbrauch für die Büroausstattung geschätzt wurde. Die Abweichungen bei der Heizenergie und beim Strom im Mittel können als gut bezeichnet werden, wobei vor allem Belgien durch Über- und Unterschätzen beim Strom auffällt. Rund ¾ der verbrauchten Primärenergie ist auf den elektrische Energie zurück zu führen, wobei der Mittelwert von rund 400 kWh/m2 darauf hindeutet, dass die realen Gebäude noch weit vom europäischen Ziel der „Nearly Zero Energy Buildings (NZEB)“ entfernt sind. [less ▲]

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See detailEin Vergleich der nationalen Berechnungsvorschriften zur Gesamtenergieeffizienz und deren Realverbrauch anhand von 14 Bürogebäuden in Luxemburg, Deutschland, Belgien, der Schweiz und in Frankreich
Render, Wolfgang; Maas, Stefan UL; Scholzen, Frank UL

in Bauphysik (2016), 38

Die EU-Mitgliedsstaaten sind gemäß den Richtlinien 2002/91/EG und 2010/31/EU verpflichtet, die Gesamtenergieeffizienz von Gebäuden abhängig von ihrer Art der Nutzung zu ermitteln und in einem Energiepass ... [more ▼]

Die EU-Mitgliedsstaaten sind gemäß den Richtlinien 2002/91/EG und 2010/31/EU verpflichtet, die Gesamtenergieeffizienz von Gebäuden abhängig von ihrer Art der Nutzung zu ermitteln und in einem Energiepass zu belegen. Die Ausgestaltung der Details des Berechnungsverfahrens und der Grenzwerte obliegt den Mitgliedsstaaten, denen man damit einen nationalen Entscheidungsspielraum zugesteht. Für 14 bestehende einfache kleinere Bürogebäude ohne Lüftung und Klimatisierung wurden die Berechnungen der Gesamtenergieeffizienz nach den jeweiligen Berechnungsmethoden durchgeführt und mit dem gemessenen realen Verbrauch verglichen. Die Abweichungen zwischen der berechneten Endenergie und dem tatsächlichen Verbrauch betragen zwar bis über 100% beim einzelnen Objekt, aber nur ca. + 28 % im Mittel bei der Heizenergie und nur wenige Prozent beim Strom, allerdings unter Annahme eines geschätzten Stromverbrauchs für die Büroausstattung. Die Abweichungen bei der Heizenergie und beim Strom können als gut bezeichnet werden. [less ▲]

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See detailDamage detection for bridge structures based on dynamic and static measurements
Nguyen, Viet Ha UL; Schommer, Sebastian UL; Zurbes, Arno et al

Scientific Conference (2016, March)

Some results of damage detection for real bridge structures are reported in the present paper based on both dynamic and static measurements. Dynamic analysis relates to the identification of modal ... [more ▼]

Some results of damage detection for real bridge structures are reported in the present paper based on both dynamic and static measurements. Dynamic analysis relates to the identification of modal parameters and deduced variables… The processing of static data is based on the analyses of deflection line and its derivatives, i.e. slope and curvature. Detection methods were applied in several real concrete bridges in Luxembourg. The results are encouraging and useful for Structural Health Monitoring in civil engineering structures. [less ▲]

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See detailStructural health monitoring based on static measurements with temperature compensation
Nguyen, Viet Ha UL; Schommer, Sebastian UL; Zürbes, Arno et al

in QUALITY SPECIFICATIONS FOR ROADWAY BRIDGES, STANDARDIZATION AT A EUROPEAN LEVEL (2016)

The paper presents the main results from static tests in a prestressed concrete beam taken out from a real bridge. The tests were achieved during about one month with several scenarios of damage that ... [more ▼]

The paper presents the main results from static tests in a prestressed concrete beam taken out from a real bridge. The tests were achieved during about one month with several scenarios of damage that loaded and unloaded states were monitored for each scenario. Damages in 4 levels were simulated by cutting prestressed tendons. There were 8 transducers distributed along the length’s beam to measure displacements. Deflection lines resulted from the static measurements from every state allow discovering the location of damages. Moreover, the calculation of slope and curvature lines leads also to very interesting issues for damage localization. [less ▲]

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See detailEnergy consumption of non-retrofitted institutional building stock inLuxembourg and the potential for a cost-efficient retrofit
Hoos, Thorsten UL; Merzkirch, Alexander UL; Maas, Stefan UL et al

in Energy and Buildings (2016), 123(2016), 162-168

The public building stock of a country, consisting of schools, offices, accommodation facilities, single-and multi-family homes, accounts for a high consumption of electrical and heat energy. Therefore ... [more ▼]

The public building stock of a country, consisting of schools, offices, accommodation facilities, single-and multi-family homes, accounts for a high consumption of electrical and heat energy. Therefore, thisstock is often subject to actions with the goal of lowering this energy usage by increasing the efficiencyof those buildings. This is usually done by applying measures to the building envelope like insulationand/or new windows and by using a more efficient HVAC technology. But often, in the initial state, thecurrent energy consumption of such a stock is unknown or only known for single buildings. In this case,the calculation of energy and cost savings is either impossible or not exact. This paper shows a way toquantify and categorize the end-energy for heat use of the public building stock in Luxembourg, whichconsists of a gross area of 1.744 million m2. This analysis was carried out in cooperation with the nationaladministration of public buildings.A certain amount of sample buildings was analyzed and then separated into three groups of low,normal and high end-energy use. The boundaries of these groups were chosen according to literaturevalues, derived from European retrofit projects, which also served as the source for possible renovationcosts. This data was extrapolated to the whole stock. This information serves as a basis for future decisionsconcerning the retrofit of those buildings and makes a calculation of costs possible.As a result, the type of buildings with the highest potential for retrofit measures was identified. Schools,offices and accommodation facilities with a “high” consumption of more than 190 kWh/(m2a) show thehighest economic potential with retrofit costs of 0.04–0.08 D /kWh if their energy consumption is loweredto values of around 90–100 kWh. Other groups of buildings show higher costs of around 0.07–0.19 D /kWh. [less ▲]

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See detailField tests of centralized and decentralized ventilation units inresidential buildings – Specific fan power, heat recovery efficiency,shortcuts and volume flow unbalances
Merzkirch, Alexander UL; Maas, Stefan UL; Scholzen, Frank UL et al

in Energy and Buildings (2016), 116(2016), 376-383

tThe energy efficient operation of mechanical ventilation systems depends on various parameters. InLuxembourg, field tests with 20 centralized and 60 decentralized mechanical ventilation systems insingle ... [more ▼]

tThe energy efficient operation of mechanical ventilation systems depends on various parameters. InLuxembourg, field tests with 20 centralized and 60 decentralized mechanical ventilation systems insingle- and multi-family homes were conducted in order to measure the actual performance of thosesystems in occupied buildings. The considered parameters were: Main air flows, internal and externalrecirculation, sensitivity to differential pressure, specific fan power and heat recovery efficiency. In manycases, the performance of the ventilation units was lower than expected. The systems showed deviationsbetween supply and exhaust flows of up to 60%. In particular, the air flow in decentralized units wasstrongly influenced by pressure differences between the inside and outside due to wind or stack effectswhich leads to a decreased heat recovery efficiency. The total mean recirculation ratio was 6.5%, with astandard deviation of 12.5% for centralized and 13 ± 6.2% for decentralized devices. As a consequence,the delivered flow of fresh air is smaller by the amount of recirculation. The specific fan power, theratio between air flow and power consumption, was measured with 0.475 ± 0.37 Wh/m3for centralizedand 0.22 ± 0.023 Wh/m3for decentralized systems. The lower value for the decentralized systems canbe explained by lower pressure losses due to the lack of ductwork. The heat recovery efficiency was0.65 ± 0.24 for centralized systems and 0.7 ± 0.17 for decentralized systems which is significantly lowerthan nominal values provided by the manufacturers. The results of this study show that the overall energyefficiency of ventilation devices installed in residential buildings under real working conditions are oftenlower than expected. These findings could possibly serve as indicators for future research & developmentat manufacturer and commissioning level. [less ▲]

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See detailSome remarks on the influence of temperature-variations, non-linearities, repeatability and ageing on modal-analysis for structural health monitoring of real bridges
Maas, Stefan UL; Schommer, Sebastian UL; Nguyen, Viet Ha UL et al

in MATEC Web of Conferences (2015, October 19), 24(Article No. 05006),

Structural Health Monitoring (SHM) intends to identify damage by changes of characteristics as for instance the modal parameters. The eigenfrequencies, mode-shapes and damping-values are either directly ... [more ▼]

Structural Health Monitoring (SHM) intends to identify damage by changes of characteristics as for instance the modal parameters. The eigenfrequencies, mode-shapes and damping-values are either directly used as damage indicators or the changes of derived parameters are analysed, such as e.g. flexibilities or updated finite element models. One common way is a ontinuous monitoring under environmental excitation forces, such as wind or traffic, i.e. the so-called output-only modal analysis. Alternatively, a forced measured external excitation in distinct time-intervals may be used for input-output modal analysis. Both methods are limited by the precision or the repeatability under real-life conditions at site. The paper will summarize everal field tests of artificially step by step damaged bridges prior to their final demolishment and it will show the changes of eigenfrequencies due to induced artificial damage. Additionally, some results of a monitoring campaign of a healthy bridge in Luxembourg are presented. Reinforced concrete shows non-linear behaviour in the sense that modal parameters depend on the excitation force amplitude, i.e. higher forces lead often to lower eigenfrequencies than smaller forces. Furthermore, the temperature of real bridges is neither constant in space nor in time, while for instance the stiffness of asphalt is strongly dependant on it. Finally, ageing as uch can also change a bridge’s stiffness and its modal parameters, e.g. because creep and hrinkage of concrete or ageing of elastomeric bearing pads influence their modulus of elasticity. These effects cannot be considered as damage, though they influence the measurement of modal parameters and hinder damage detection. [less ▲]

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See detailMesstechnische Ermittlung der U-Werte von Altbaufassaden im Vergleich mit Typologiewerten für den Gebäudebestand
Hoos, Thorsten UL; Merzkirch, Alexander UL; Maas, Stefan UL et al

in Bauphysik (2015)

Die Wirtschaftlichkeit von Renovierungsvorhaben an Altbauten hängt im Wesentlichen von der erwarteten Wertsteigerung, der Miethöhe und den zu erreichenden Energieeinsparungen ab. Um diese Einsparungen zu ... [more ▼]

Die Wirtschaftlichkeit von Renovierungsvorhaben an Altbauten hängt im Wesentlichen von der erwarteten Wertsteigerung, der Miethöhe und den zu erreichenden Energieeinsparungen ab. Um diese Einsparungen zu berechnen, wird ein rechnerischer Vergleich zwischen dem aktuellen Verbrauch und dem Verbrauch nach der Sanierung angestrebt. Zahlreiche Studien zeigen auf, dass der rechnerische Energiebedarf von Gebäuden häufig von dem tatsächlichen Verbrauch abweicht. Dies gilt vor allem für vor 1970 errichtete Altbauten. Ein wichtiger Eingangsparameter der Berechnungsmethodik sind die U-Werte der Aussenwände, welche typischerweise vom Energieexperten vor Ort aus Typologien des Gesetzgebers für die jeweilige Wandkonstruktion ausgewählt werden. In dieser Veröffentlichung werden Mess- und Simulationsergebnisse von verschiedenen Wandtypen präsentiert. Die ermittelten Werte lagen zwischen 0.9 und 1,2 W/(m2K) und somit deutlich unter den in Luxemburg angenommenen Typologiewerten von 1,4 bis 2,1 W/(m2K). Die beschriebenen Abweichungen zwischen Energiebedarfsberechnung und tatsächlichem Energieverbrauch können somit zum Teil auf fehlerhafte Annahmen des ausstellenden Experten vor Ort zurückgeführt werden, welche sich an Typologiewerten orientieren. [less ▲]

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See detailInvestigation of temperature-dependent stiffness variation of a layer of asphalt and their possible effect on the deformation behaviour of concrete structures
Waldmann, Danièle UL; Erdenebat, Dolgion UL; Scherbaum, Frank et al

in 6th International Conference on Experimental Vibration Analysis for Civil Engineering Structures, Duebendorf 2015 (2015, October)

In the time of increasing maintenance costs, the continuous inspection and the earliest possible damage detection become more and more important. In order to minimize future maintenance costs, the exact ... [more ▼]

In the time of increasing maintenance costs, the continuous inspection and the earliest possible damage detection become more and more important. In order to minimize future maintenance costs, the exact evaluation of the condition of the structure and the exact assessment of potential damages are of essential importance. Therefore the University of Luxembourg carries out projects to investigate an efficient application of different assessment methods taking into account praxis relevant test conditions. As a part of this project especially the changing temperatures which influence the stiffness of the materials are analysed. As a consequence, for the condition assessment of structures, the asphalt layer cannot only be taken into consideration as a mass applied as load on the structure. Due to bond effects of the asphalt layer to the load carrying element its changing stiffness induced by changing temperatures influences the stiffness of the whole structure. Within this paper this effect will be illustrated. First the load carrying behaviour and the stiffness of pre-stressed concrete slabs realized with and without an additional asphalt layer will be investigated in a climate chamber and the results will be compared for different temperatures. [less ▲]

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See detailPrimary energy used in centralized and decentralized ventilation systems measured in field tests in residential buildings
Merzkirch, Alexander UL; Maas, Stefan UL; Scholzen, Frank UL et al

in Proceedings of the 26th AIVC Conference, Effective Ventilation in high performance buildings (2015, September)

Ventilation systems can save heat energy by using heat recovery, but consume electrical energy to power the fans. In practice, the energy efficiency of those systems can be lower than expected, when ... [more ▼]

Ventilation systems can save heat energy by using heat recovery, but consume electrical energy to power the fans. In practice, the energy efficiency of those systems can be lower than expected, when compared to the nominal values provided by the manufacturer. In this paper, results of a comprehensive field tests with 20 centralized and 60 decentralized ventilation systems for residential buildings and the calculation of the primary energy savings of those devices are presented. Factors like volume flow unbalances, shortcuts, temperature change rates and specific fan power have been addressed by tracer gas technology and other means and been used as input factors to calculate the primary energy balance of those devices. Every system showed positive primary energy savings. The mean value for centralized systems was 2.92 Wh/m3 with a high standard deviation of 2.23 Wh/m3, while the decentralized systems showed higher savings of around 4.75 Wh/m3 with a standard deviation of 0.01 to 0.15 Wh/m3. In general, the calculated savings in field tests were significantly lower compared to the case of using nominal values as input parameters. [less ▲]

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See detailDesign rules for autofrettage of an aluminium valve body
Sellen, Stephan UL; Maas, Stefan UL; Andreas, Thomas et al

in Fatigue & Fracture of Engineering Materials & Structures (2015)

The following paper is intended to improve the fatigue behaviour of a complex aluminium valve geometry under high internal cyclic pres sure loading. The autofrettage process helps to increase the f atigue ... [more ▼]

The following paper is intended to improve the fatigue behaviour of a complex aluminium valve geometry under high internal cyclic pres sure loading. The autofrettage process helps to increase the f atigue durability and a simple, but efficient design method for this proces s is deployed. Based o n non-linear material’s behaviour, fin ite eleme nt simulations of t he crack-free geometry help to determine the minimum and maximum autofrettage pressure to be used, without iterative crack simulations , which would require higher computational effort. Material tests under inverse plastifications were performed in order to determine the correct material model. The derived design method was validated with simplified specimens subjected to different autofrettage pressure levels and su bsequent cyclic fatigue tests. [less ▲]

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See detailBiomechanical properties of five different currently used implants for open-wedge high tibial osteotomy
Diffo Kaze, Arnaud UL; Maas, Stefan UL; Waldmann, Danièle UL et al

in Journal of Experimental Orthopaedics (2015), 2(14),

Background: As several new tibial osteotomy plates recently appeared on the market, the aim of the present study was to compare mechanical static and fatigue strength of three newly designed plates with ... [more ▼]

Background: As several new tibial osteotomy plates recently appeared on the market, the aim of the present study was to compare mechanical static and fatigue strength of three newly designed plates with gold standard plates for the treatment of medial knee joint osteoarthritis. Methods: Sixteen fourth-generation tibial bone composites underwent a medial open-wedge high tibial osteotomyn(HTO) according to standard techniques, using five TomoFix standard plates, five PEEKPower plates and six iBalance implants. Static compression load to failure and load-controlled cyclic fatigue failure tests were performed. Forces, horizontal and vertical displacements were measured; rotational permanent plastic deformations, maximal displacement ranges in the hysteresis loops of the cyclic loading responses and dynamic stiffness were determined. Results: Static compression load to failure tests revealed that all plates showed sufficient stability up to 2400 N without any signs of opposite cortex fracture, which occurred above this load in all constructs at different load levels. During the fatigue failure tests, screw breakage in the iBalance group and opposite cortex fractures in all constructs occurred only under physiological loading conditions (<2400 N). The highest fatigue strength in terms of maximal load and number of cycles performed prior to failure was observed for the ContourLock group followed by the iBalance implants, the TomoFix standard (std) and small stature (sm) plates. The PEEKPower group showed the lowest fatigue strength. Conclusions: All plates showed sufficient stability under static loading. Compared to the TomoFix and the PEEKPower plates, the ContourLock plate and iBalance implant showed a higher mechanical fatigue strength during cyclic fatigue testing. These data suggest that both mechanical static and fatigue strength increase with a wider proximal T-shaped plate design together with diverging proximal screws as used in the ContourLock plate or a closed-wedge construction as in the iBalance design. Mechanical strength of the bone-implant constructs decreases with a narrow T-shaped proximal end design and converging proximal screws (TomoFix) or a short vertical plate design (PEEKPower Plate). Whenever high mechanical strength is required, a ContourLock or iBalance plate should be selected. [less ▲]

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