Civil Engineering Design最新文献

The strong hygroscopic behavior of wood can lead to severe moisture-induced swelling deformations, especially perpendicular to the grain direction. Fully threaded screws arranged in this direction can be used as reinforcement and to constrain those deformations. Depending on the wood moisture content increase, tensile stresses in the screws and compression stresses in the surrounding solid wood evolve. A sufficiently accurate prediction of the maximum stresses occurring is important for reliable structural designs. For this reason, moisture-induced swelling experiments for two different configurations of screw diameter and specimen dimensions were performed within this work. Due to its homogeneity and its high swelling rate perpendicular to the grain, beech laminated veneer lumber (LVL) was used. A moisture content increase from oven-dry to about 7% led to screw tensile stresses in the magnitude of the yield strength and to highly nonlinear effects in the surrounding wood material. By means of a two-dimensional finite-element-based numerical simulation model, taking into account orthotropic plasticity and moisture-dependent material properties, the experiments could be simulated with high accuracy. The numerical results agree well with experimental observations. Based on a small parameter study, new insights into the complex interaction behavior between beech LVL and fully threaded screws could be gained.

Timber-concrete composite (TCC) ceilings build on the idea of making use of the advantageous properties of both materials symbiotically. While concrete, as the upper layer, is used to absorb the compression forces, wood is used in the lower layer to absorb the tensile forces. Many systems have been developed with special attention paid to solutions with both a continuous concrete and wood layer. This article introduces a new system developed with the primary focus set on the most efficient material use by introducing a free space between the concrete and the wood layer using special vault shaped moldings. The first part of the paper contains an introduction including a short overview of different embodiments of TCC floor systems. The second part focuses on the design of the new system and gives an overview of the estimated structural performance. In the third part the environmental performance of the new system is discussed in comparison to chosen existing systems focusing at the the whole life-cycle including a re-use (A-D).

The residual structural lifetime of concrete bridges is often limited by fatigue of the prestressing steel. In practice, numerical calculations of the structure are combined with Miner's rule—based on load frequencies and stress amplitudes—to estimate the damage. Thereby, various parameters must be accounted for whereof a few are actually relevant. Structural monitoring is valuable to increase the accuracy of lifetime predictions, but usually experts choose the right parameters on experience only. A more objective assessment can be reached by sensitivity analysis. A powerful method is provided by Sobol's variance-based indices. They quantify the influence of a single parameter's variance on the model's total variance and account for interactions in nonlinear models, too. Exemplified on a reference structure, sensitivity indices are determined. Meanwhile specific characteristics like the nonlinear behavior of concrete after cracking are considered. In this case, the key elements of lifetime prediction turn out to be the time-dependent losses of prestress and the traffic loads.

New materials allow new building designs and construction types. However, initial construction projects using the new materials show that traditional construction principles - in the case of carbon reinforced concrete those derived from steel reinforced concrete - are still being used. In other words, conventional materials are merely substituted. Only combined with intelligent construction strategies it is possible to exploit the full potential of innovative materials. Detached from established patterns of thought, the fundamentals for a new way of building with concrete are to be created in the frame of the Collaborative Research Centre Transregio (CRC/TRR) 280 project “Design Strategies for Material-Minimised Carbon Reinforced Concrete Structures–Principles of a New Approach to Construction” at Technische Universität Dresden (TUD) and RWTH Aachen University. These are to be based on profound insights into the mechanical behavior of novel mineral structures. Innovative lightweight construction strategies and material composites reduce resource and energy consumption while maintaining high levels of usability, structural safety, and durability, while the aspirational esthetics make a valuable contribution to the culture of building. The long-term research alliance of TUD and RWTH pools the excellent competencies, inspiring research into material-minimized construction with mineral composites. The article highlights the research activities planned for the first period from July 2020 to June 2024.

Building in heavy rain is seldom beneficial, but common practice on site. It promotes inaccuracies and impairs the use of modern but sensible high-performance materials and costs time, since disruption in construction frequently causes complicated returns to the planning process. Nevertheless, a handcrafted production process is still considered the one and only alternative since all buildings are unique and thus must be manually constructed on site. Indeed? The priority program entitled “Adaptive modularized constructions made in a flux” funded by the German Research Foundation follows a completely new approach. Buildings are divided into similar modular precast concrete elements, prefabricated in flow production, quality-assured, and just-in-time assembled on site. Comparable to puzzles with many pieces, the uniqueness of the structure is maintained. The motto is: “Individuality on a large scale-similarity on a small scale”. The contribution presents approaches of modularization, production concepts, and linking digital models. Serial, stationary prefabrication enables short production times and resource-efficient modules that are assembled to load-bearing structures with low geometrical deviations. Stringent digitalization ensures high quality of all intermediate steps. These comprise fabrication, assembly, and the whole service life of the structure. The result is a lean production process.

Wind flow transfers forces to the wind turbine's rotor blades. These then set the rotor in motion. The hub and the gearbox, where present, transfer this rotational energy to the generator for conversion into electrical power. All the rotating components have significant mass and are located at the head of a slender, elastic load-bearing tower in which they induce dynamic effects. The resulting vibrations, generated at the upper end of the tower, are modified by the dynamic properties of the tower structure and pass through the foundations into the ground. Broadband seismometers record these ground vibrations not only directly adjacent to the wind turbine but also at greater distances of (up to) several kilometers from the turbine. We are aware that local residents and opponents of wind power consider that these vibration phenomena bear potential negative health effects. In the context of this paper, seismic vibrations were measured at the foundation of a 2 MW reference turbine. These seismic signals were compared to numerical simulations. Based on this, we explain the physical background. In the past, any ground vibrations measured have usually been attributed exclusively to the excitation frequencies from the rotor. However, the investigations presented here show that the structural properties of the tower structure significantly influence the type and intensity of the vibrations induced in the ground and dominate the ground motion amplitudes. Finally, we show that the targeted use of absorbers can significantly reduce the vibrations induced in the ground.

Cracks are an essential characteristic of reinforced concrete (RC) construction. Serviceability and durability, however, require a reasonable limitation of the maximum crack width. The prediction of the maximum crack width of RC, however, is not trivial and has been much debated over the past decades. This article starts with a fundamental comparison of basic procedures for determining the crack width, namely using a mechanical or calibrated model. Following, the behavior of reinforced concrete during crack formation is thoroughly discussed with a focus on the bond stress-slip relation at the reinforcement-concrete interface and with regard to the particular crack stage. Based on these fundamentals, a mechanical calculation model is then proposed for practical calculation of maximum crack width in RC members. The suitability of the proposed model is demonstrated by the comparison of predicted and experimentally obtained maximum crack widths for a database including 460 crack width measurements. It is shown that the current state of knowledge enables a mechanically plausible calculation of the maximum crack width. Although the formulation based on the bond law is not trivial, the calculation model can be prepared with appropriate simplifications in a practical way.