136 research outputs found
Linear Motion Systems. A Modular Approach for Improved Straightness Performance
No full textThis thesis deals with straight motion systems. A modular approach has been applied in order to find ways to improve the performance. The main performance parameters that are considered are position accuracy, repeatability and, to a lesser extent, cost. Because of the increasing requirements to positioning systems, concerning accuracy, repeatability and velocity, it becomes more and more difficult to meet these requirements when using merely mechanical means. The errors in position occurring as a result of e.g. external forces, wear or thermal expansion, are the reason that open-loop straight motion systems can only operate in the sub-micrometer range when very strict conditions are applied. When the required accuracy becomes smaller than the manufacturing tolerances achievable, active compensation of errors becomes indispensable. On the other hand, by the application of active error compensation, the requirements to the manufacturing accuracy can decrease. By means of this feedback of straightness errors, the problems and bottlenecks shift from the mechanical domain towards the electronics and control engineering domain, where they are better solvable. The eventual performance of a feedback controlled system is not limited any more by fabrication tolerances, bearing properties, thermal influences or external loads, but is mainly limited by the resolution of the sensor applied, and by the dynamical behaviour of the mechanical system. When aiming at a position accuracy in the sub-micrometer range, also vibration problems become important. Therefor, a completely different approach has been applied. Instead of aiming for the highest possible open-loop stiffness of the positioning system relative to the ground, an actuator stiffness equal to zero is proposed. The zero stiffness bearing results in a decoupling of the position, while preserving the force coupling to the world. This means that the position of such an object (e.g. a specimen holder), supported by a zero stiffness bearing, may be coupled to the position of another object (e.g. a lens on which no external forces are allowed) by means of a control loop, resulting in a virtual stiffness relative to each other. The forces, needed for positioning the moving object, cause a reaction force on the world, while the vibrations (position errors) of the world do not influence the positioning accuracy of that object. This concept offers possibilities in the design of precision positioning systems, where otherwise vibrations from the outer world would have a too large disturbing influence. Within the scope of this research on zero stiffness actuators, a permanent magnetic bearing element has been designed and built, which is able to generate a static bearing force that is without mechanical contact, without stiffness and without power consumption. The permanent magnetic zero stiffness bearing principle has been patented in the Netherlands; an international patent application is pending.Design, Engineering and Productio
Species independent strength grading of structural timber
No full textTimber as a construction material has been used for millennia, but the research field covering the prediction of the strength of structural timber is still in development. Currently, the common conception is that the determination of strength properties has to be determined for every wood species individually. By combining these strength properties to features that can be measured at the timber (either visually or by machine measurements), strength graded timber can be supplied to the market. Potentially, there are more than 1000 commercially available wood species, the timber of which can be used in structures. The largest amount of these wood species are tropical hardwoods. These wood species are often used when high strength and high durability are required. Nowadays, (tropical) timber is increasingly coming from sustainably managed forests. By application of this method of forest management, the (tropical) forests are preserved and have an economic value for the local population. A result of this approach is that more and more unknown wood species in small quantities are coming on the market, the strength properties of which have to be determined. The present methods for the determination of strength properties of a wood species require extensive testing. An extra problem is that the timber that is tested has to be representative for the timber coming on the market. All future variations in the quality of the timber coming on the market have to be covered. To be able to use the timber in structures, grading rules have to be formulated that are related to the strength properties, determined by tests. For visual grading, features like knots and slope of grain are used. For machine grading, for example, the density and modulus of elasticity are used. For softwoods it has been proven that machine grading is more accurate and gives higher yields in the higher strength classes in comparison with visual grading. For tropical hardwoods, a major problem for visual grading is that the most important feature for the mechanical properties, the slope of grain, is very difficult to measure in practice. For this reason, only one visual grade is defined for tropical hardwoods and optimisation is not possible. A solution for abovementioned problems can be species independent strength grading, where only the influence of the measured features is taken into account, irrespective of the species. To investigate whether this would be possible, the research question dealt with in this thesis was: what are the influencing parameters for the development of species independent strength models, and can they be quantified to ensure safe, economic and sustainable use of softwoods and (tropical) hardwoods in structures ? To answer this question, a database consisting of a large number of test results from bending tests on European softwoods, temperate hardwoods and tropical hardwoods was investigated. This database was built-up in the last ten years in cooperation with the Dutch industry. Based on a literature survey, it was concluded that both the strength and stiffness of clear wood depend on the density of the timber, irrespective of the wood species. The natural variation in test values for both properties strength and stiffness are correlated. As a result, the stiffness is a good predictor of the strength for clear wood. Based on structural mechanics, mathematical models were formulated describing the reduction of strength and stiffness caused by the presence of knots and grain angle deviation. Because the density defines the maximum possible basic strength of the timber species, independent strength grading by visual grading is not possible. For some softwood species and temperate hardwoods, the grow ring width can be a measure for the density. For the majority of hardwood timber, there is no significant correlation. The examination of the visual measurement of the slope of grain has revealed that it is very difficult to accurately estimate the slope of grain for tropical hardwoods before a destructive bending test. As a consequence, the variation in strength properties between test samples from the same wood species can be very large. To determine the strength of timber brought on the market under the same trade name with sufficient safety, a reduction factor has to be applied to the test results. Because it is not known how large the variation can be in the slope of grain for tropical timber brought on the market under the various trade names, it is not possible to determine this reduction factor. By means of machine strength grading it is possible to detect the variation in slope of grain. The reduction of the stiffness (the modulus of elasticity) can be described with the same equation (the well-known Hankinson equation) as the reduction of the bending strength, only with other constant values. Because of this, the modulus of elasticity and the density are parameters that, together, can be used for machine strength grading for timber showing grain angle deviation. The reduction equation describing the strength due to the presence of knots has the same form as the reduction equation describing the stiffness due to the presence of knots, only with other constant values. Because of this, the modulus of elasticity and the density together are also the parameters suited for species independent machine strength grading of timber containing knots. Because the influence of knots and slope of grain on the modulus of elasticity cannot be distinguished from each other in the modulus of elasticity measurement, timber has to be divided into two groups for species independent machine strength grading: timber for which failure is induced by knots and timber for which failure is induced by slope of grain. Therefore, it is necessary to perform a visual assessment, to check for the group containing grain angle deviation that only knots of limited sizes are present in the timber. Furthermore, the visual check has to ensure removal of pieces with features that cannot be detected by machine readings, such as compression failures. A feature like a compression failure causes an unpredictable strength reduction and is therefore not allowed in structural timber. In practice, the modulus of elasticity can be determined in a simple manner by means of vibration measurements. On the basis of mathematical relationships between on the one hand the features knots and slope of grain and on the other hand the density and the modulus of elasticity, it is possible to formulate prediction models of the strength based on the measured density and modulus of elasticity of a piece. The values for the bending strength, the modulus of elasticity and the density for the standardized strength classes are related to timber with a moisture content of 12%. Correction factors have been derived to be able to adjust the test result of timber tested with a different moisture content to this reference moisture content, For structural sizes, no adjustments with regard to the reference sizes are necessary. The shape of the scatter around the prediction lines is theoretically derived on the basis of the distribution of the prediction values. The shape of the scatter turns out to be different for the prediction model for timber containing knots and for the prediction model for timber containing grain angle deviation. A method to derive the shape of the scatter on the basis of available data has been formulated and verified. To actually grade timber, "settings" have to be determined. These are limit values for the prediction values that determine which strength class the timber can be assigned to. The strength values of timber can only be verified on the basis of the properties of a sample that is tested destructively. For small numbers of pieces in a sample, the characteristic values of a strength grade can vary significantly between tested samples. To overcome this problem, a method was developed which takes into account the distribution of the prediction values and the scatter of the prediction model. The characteristic strength value of a strength grade for the required probability can be determined by it, irrespective of the number of pieces in a sample. With the developed prediction models it is possible to perform species independent strength grading. Especially for tropical hardwoods, the assigned strength classes can be determined in a reliable way and the yield in the higher strength classes can be increased. The research results contribute to an economic, safe and sustainable application of timber in structural applications.Structural EngineeringCivil Engineering and Geoscience
A seismic retrofitting design approach for activating dissipative behaviour of timber diaphragms in existing unreinforced masonry buildings
No full textThe region of Groningen (NL) has experienced increasing human-induced seismicity caused by gas extraction in the last decades. The local building stock, not designed for seismic loads, consists for more than 50% of unreinforced masonry buildings with timber diaphragms. In this context, a detailed seismic characterization of timber and masonry structural components has taken place, and a retrofitting technique for timber floors activating their energy dissipation has been developed. Besides, specific analytical and numerical modeling strategies for as-built and retrofitted timber floors have been formulated. This work presents a design approach for creating strengthened dissipative timber diaphragms, and maximizing the seismic capacity of existing masonry buildings through this retrofitting method. The results from the performed numerical analyses prove that the proposed design approach for timber floors can increase the energy dissipation capacity of masonry buildings, while improving the box behavior at both damage and near-collapse limit state.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Bio-based Structures & Material
De Effectieve Kiplengte van Houten Liggers (Lateral Torsional Buckling)
No full textIn dit Bachelor eindwerk is onderzoek gedaan naar de effectieve kiplengte van liggers met rechthoekige doorsneden. In een voorgaand eindwerk is hier al onderzoek naar gedaan door Roeland van Straten. Daarom zijn er bepaalde delen uit het desbetreffende onderzoek overgenomen ter ondersteuning van dit eindwerk. Het gaat hierbij om de paragrafen 1.4 en 2.1. Tevens zijn er in paragraaf 2.2 delen gedeeltelijk overgenomen. Dat wil zeggen dat ze gebruikt zijn ter ondersteuning, maar zodanig aangepast dat ze beter in dit eindwerk pasten. Allereerst is er op analytische wijze een oplossing gevonden voor het kritieke kipmoment van een ligger, belast met een constant moment over de liggerlengte. Bij een moment ter grootte van zal de ligger gaan kippen. Aangezien er voor andere belastingsgevallen geen uitdrukking gevonden kan worden op deze wijze is gebruik gemaakt van energievergelijkingen om het kritieke kipmoment te berekenen. Deze energievergelijkingen zijn gebaseerd op het feit dat de totale energie in de ligger tijdens het kippen gelijk moet blijven. Zo bezit de ligger potentiële energie ten opzichte van een referentielijn gelijk aan , waarbij de massa in kg is, de valversnelling in en de hoogte boven deze referentielijn. Door het kippen buigt de ligger dichter naar de referentielijn toe zodat de potentiële energie afneemt, maar de vervormingsenergie is dan wel toegenomen. Aangezien men in de praktijk niet alleen te maken heeft met één enkele ligger maar met meerdere steunpunten is gekeken wat hier de invloed van is. Dit was van groot belang bij de berekening van de kritieke last bij het fysieke model. Daar is de proef op de som genomen en voor een aantal belastingsgevallen getest of het theoretische kipmoment overeen komt met het werkelijke kipmoment.Structural MechanicsStructural EngineeringCivil Engineering and Geoscience
Quasi-static cyclic in-plane tests on masonry components 2016/2017
No full textApplied MechanicsBio-based Structures & Material
Magnetic support system
No full textThere is described a support system enabling supporting an object such as a platform (1) free from vibration, in that bearing elements (50) have a stiffness (k) which at a working point (z0) equals zero. A bearing element (50) comprises two magnetic couplings (51, 52) provided by permanent magnets (61, 63). One coupling (51) has a positive stiffness (k51), and the other coupling has a negative stiffness (k52); in the working point, the absolute values of those stiffnesses are equally great. Alternatively, electrostatic couplings are used.Mechanical, Maritime and Materials Engineerin
Influence of the seismic calculation method on a CLT constructed building: Subjected to human induced earthquakes
No full textCivil Engineering and GeosciencesStructural Engineerin
Test report on cyclic behaviour of replicated timber diaphragms representing a detached house
No full textThe in-plane strength and stiffness as well as the hysteretic behaviour of timber diaphragms is investigated by performing quasi-static cyclic tests. In this report the behaviour of traditional timber floors and roof configurations was studied representing a typical detached masonry house. The configurations are based on a case study from a detached Groningen house that was demolished. Characteristics of the demolished house were:- A detached house, built in 1920- At ground storey double wythe clay masonry, at first storey single leaf gables of clay masonry- Timber floors built-up with joists and planks nailed on top of the joists.- Timber roof with vertical rafters spanning from wall plate to top beam, small horizontal purlins on rafters with vertical planks nailed to the purlins.Based on the extracted samples the configurations and material properties could be determined. Based on these findings timber diaphragms (floors and roof) were replicated, built from new timber material and fasteners, with the same material properties as found in the extracted samples.Replicated specimens of approximately 4.0 m by 2.75 m were prepared and tested. After the testing of the replicated specimens representing the situation in practice (non-strengthened), the specimens were strengthened with plywood panels screwed on the timber planks.Bio-based Structures & Material
Test report on cyclic behaviour of replicated timber joist-masonry wall connections
No full textIn this report an experimental pilot study on the seismic response of timber-masonry connections is presentedBio-based Structures & Material
Test report on material properties of timber and fasteners extracted from existing buildings
No full textThis report presents the material properties of timber and fasteners specimen extracted from existing unreinforced masonry houses.Bio-based Structures & Material
- …
