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Three seismic refraction profiles obtained between 42°N and 43°N along the median valley of the Gorda Ridge, an active spreading center, allow the computation of the velocity structure underlying the valley. Wide angle reflections which appear on the seismic records suggest the existence of a velocity inversion underlying layer 3 and were analyzed in combination with refraction arrivals. The resulting velocity model has a low velocity zone with a directly-determined velocity of 5.72 km/sec, between crust of velocity 6.48 km/sec and Moho of velocity 7.54 km/sec. The velocity inversion is 0.7 km thick and lies 3 km below acoustic basement. Consideration of the velocity structure of the Gorda Ridge, together with other information on processes involved in oceanic crustal formation, suggests a model which is consistent with current knowledge on oceanic spreading centers. In the proposed model, the rise of asthenospheric material on the ascending limb of a convection cell causes the generation of a small percentage of partial melt. The molten fraction tends to coalesce near the top of the ascending limb, forming a region of significant partial melt under the ridge crest. This molten material is the immediate source reservoir for mid-ocean ridge magmas. The geophysical expression of the reservoir is a region of low seismic velocity and low density. As the magma cools from the upper surface, heavy minerals tend to work their way downward, forming a layer of cumulate ultramafic rocks at the base of the crust, while the lighter constituents work upward to form the cumulate gabbros of oceanic layer 3. The injection and extrusion of magmatic material upward leads to the formation of layer 2. The crust under the median valley is in isostatic equilibrium with the partial melt during formation, but as it is displaced laterally from the magmatic center, the entire lithosphere becomes competent and the isostatic depth of compensation moves downward into the mantle. This is thought to cause the familiar ridge crest topography of a median valley and adjacent axial mountains observed at slowly spreading ridges. The features of this general model in the specific case of the northern Gorda Ridge between 42°N and 43°N have been tested by the comparison of theoretical and observed gravity and magnetic anomalies. The computation of the theoretical gravity anomaly for this model gives values which match the observed anomaly. The magnetic data show only the pattern of anomalies expected from sea floor spreading and magnetic field reversals. / Graduation date: 1978

Key point to development of environmentally friendly timber structures, appropriate to urban ways of living, is the development of high-rise timber buildings. Comfort properties are nowadays one of the main limitations to tall timber buildings, and an enhanced knowledge on damping phenomena is therefore required, as well as improved prediction models for damping. The aim of this work has consequently been to estimate various damping quantities in timber structures. In particular, models have been derived for predicting material damping in timber members, beams or panels, or in more complex timber structures, such as floors. Material damping is defined as damping due to intrinsic material properties, and used to be referred to as internal friction. In addition, structural damping, defined as damping due to connections and friction in-between members, has been estimated for timber floors. The thesis consists of six main parts. The first part is entitled “Contexts”, and is composed of four chapters. A general overview of the wood material and its structural use in buildings is presented in Chapter 1. Chapter 2 gives a thorough literature review on comfort properties of (timber) floors. Chapter 1 and Chapter 2 serve as justifications for the motivation of this work, expressed in Chapter 3, and the aim of the work, expressed in Chapter 4. The next part “Backgrounds” briefly describes the basic theories used along the thesis, for the analytical studies (Chapter 5), the experimental studies (Chapter 6), and the numerical studies (Chapter 7). The part “State of the art” is a general literature review on damping (Chapter 8). A particular accent is set on the derivation of various damping prediction models. The “Research” part summarizes the original research work. Chapter 9 briefly presents the background and main findings for each study, and Chapter 10 concludes and proposes suggestions for further research. The studies are detailed in four journal papers, which are integrally reported in the “Publications” part. Paper I focuses on the evaluation of material damping in timber beam specimens with dimensions typical of common timber floor structures. Using the impact test method, 11 solid wood beams and 11 glulam beams made out of Norway Spruce (Picea Abies) were subjected to flexural vibrations. The tests involved different spans and orientations. A total of 420 material damping evaluations were performed, and the results are presented as mean values for each configuration along with important statistical indicators to quantify their reliability. The consistency of the experimental method was validated with respect to repeatability and reproducibility. General trends found an increasing damping ratio for higher modes, shorter spans, and edgewise orientations. It is concluded from the results that material damping is governed by shear deformation, which can be expressed more conveniently with respect to the specific mode shape and its derivatives. Paper II deals with the prediction of material damping in Timoshenko beams. Complex elastic moduli and complex stiffness are defined to derive an analytical model that predicts the hysteretic system damping for the whole member. The prediction model comprises two parts, the first related to bending, and the second related to shear. Selected experimental damping evaluations from Paper I are used to validate the model and obtain fitted values of loss factors for two types of wood. The good agreement of the derived model with experimental data reveals an efficient approach in the prediction of material damping. In Paper III, a semi-analytical prediction model of material damping in timber panels is described. The approach is derived from the strain energy method and input is based on loss factors, which are intrinsic properties of the considered materials, together with material properties and mode shape integrals, whose calculation can easily be implemented in most finite element codes. Experimental damping evaluations of three types of timber panels are performed. These are particleboards, oriented strand board panels and structural laminated veneer panels. Fair goodness-of-fit between the experimental results and the prediction models reveals an efficient approach for the prediction of material damping in timber panels with any boundary conditions, knowing only the loss factors and the mode shapes. In Paper IV, dynamic properties of two timber floors are experimentally evaluated by impact method. Each floor uses one specified type of connectors, either screws or nails. A numerical model is developed using constrained degrees-of-freedom for the modeling of connectors. Numerical analyses have been performed, and show good agreement with experimental results. A procedure is written using the commercial finite element software Abaqus to predict material damping from a strain energy approach. Estimation of structural damping is performed as the difference between the experimentally evaluated total damping and the predicted material damping. The contribution from floor members to material damping is extensively investigated, and the needs for better prediction of damping are discussed. Specific details of some aspects of the work are included in the “Appendix” part.

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This investigation was part of a greater research initiative regarding the seismic vulnerability of precast hollowcore floor systems. The primary focus throughout the research programme has been to investigate the susceptibility to loss of vertical support of the floor system, from the seating beam. Previous research firstly focussed on identifying and understanding preconceived deficiencies with existing seating connection details. This was followed by the validation of amended, superior performing, 'new' seating connection details. However, little consideration has been given to retrofit techniques for already existing buildings, with potentially poor performing existing seating connections. A two-dimensional, single hollowcore unit, seating connection sub-assembly is used to experimentally investigate the seismic behaviour of previously un-tested existing seating connections pre- and post-retrofit. Three existing seating connection configurations, with the hollowcore unit seated directly on the bare concrete seating ledge and with varying seating lengths were tested. These tests were followed by a fourth retrofitted specimen. Both relative rotation between the hollowcore unit and seating beam, and beam elongation 'pull-off' deformations (resulting from the supporting frame deformations) were imposed on the test specimens. In conjunction with this experimental investigation and with prior knowledge from previous investigations, three primary failure mechanisms for existing hollowcore seating connections are summarised. A suite of conceptual retrofit techniques which target the critical structural weaknesses attributed to causing the primary failure mechanisms are outlined. In general, unfavourable performance was exhibited by the existing seating connections in the experimental investigation, resulting in loss of vertical support of the hollowcore unit under imposed 'pull-off' effects. In contrast, when the retrofit strategy was implemented, a higher level of seismic performance, leading to collapse prevention was achieved. A review is carried out into existing beam elongation numerical models, which are simple and involve only hand-type calculation procedures. The aim of this was to investigate potential methods for predicting the 'pull-off' effects on suspended floor systems. From this, a modification is made to an existing, loading dependent method developed by Matthews (2004). The modified method aimed to more accurately represent the loading dependant nature of beam elongation (and the resulting 'pull-off' effects) as described by Lee and Watanabe (2003). A number of beam elongation predictions for a suite of experimental beam elongation data sets were carried out with the modified method. Good agreement was generally seen, both in terms of prediction of the magnitude of elongation and the shape of the elongation profile.

Diss. (sammanfattning) Stockholm : Karol. inst. / Härtill 6 uppsatser.

Thesis (Ph. D.)--University of Wyoming, 2006. / Title from PDF title page (viewed on June 16, 2008). Includes bibliographical references.

Thesis (doctoral)--Technische Universiteit Delft, 1997. / Includes bibliographical references (p. [169]-176).

Thesis (doctoral)--Technische Universiteit Delft, 1997. / Includes bibliographical references (p. [169]-176).