New Lattice-Tubular Tower for Onshore WEC - Part 1: Structural Optimization

Currently, onshore wind turbines are economical for 2.5 mega-watt inland installation [1]. One of the alternatives to achieve the higher power capacity in onshore wind turbines is to go to higher altitude, which increases the wind speed and has more uniform wind pressure, leading to higher possible annual energy production and relatively lower maintenance costs due to wind shear induced vibrations. This paper deals with a predesign of a new type of tower that should overcome known disadvantages of steel tubular towers for heights over 150 meters. This first part of the paper addresses the viability of a hybrid support structure in respect of ultimate limit states and structural dynamics as part of the requirements in exploring the feasibility of the hybrid tower concept. The hybrid wind turbine tower concept consists of a lattice (lower) structure and a tubular tower for the higher tower segment. The study cases found in literature were designed with maximum height of 160m and the 2.5MW wind turbine. Moreover, they used commercial "L" sections which leads to high number of connections and bolts. The project that supports the present paper aims at using optimally design hollow cross sections, to reduce the number of connections, and low maintenance type bolts. The developed study case is made for 220m tower height and using 5MW wind turbine.Steel & Composite Structure

Fatigue experimental characterization of preloaded injection bolts in a metallic bridge strengthening scenario

All over the world, the number of civil engineering structures, particularly bridges with long service periods, has been increasing. The most common evidences of damages are the presence of corroded metallic elements and cracks in structural details due to the fatigue phenomenon. A large number of cases were found in which fatigue cracks were detected in structural details, such as riveted connections. Different strategies can be implemented for repairing and strengthening operations of old metallic riveted bridges. However, the use of injection bolts has been considered as an alternative with important advantages. In this regard, it is essential to study their performance under fatigue loading. In this investigation, an experimental campaign has been performed to assess the fatigue strength of injection bolts by means of comparison with standard bolts. These fatigue tests are defined to be representative of a structural strengthening scenario of an old metallic bridge. Experimental results show that injection bolts contribute to significantly reduce the scatter in the data related to fatigue resistance. For double shear specimens, the characteristic curve proposed for connections with injection bolts presents a detail category with 15% higher value when compared to non-injected specimens. The beneficial effect is also verified in terms of slip deformation. For single shear specimens, the overall effect of the adhesive is not clear.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.Steel & Composite Structure

Reliability of Fatigue Strength Curves for Riveted Connections Using Normal and Weibull Distribution Functions

In North America and Europe, there is a significant number of centenary metallic bridges that require maintenance and strengthening operations. These structures need to be adapted to increasing traffic intensities, and their structural integrity should be constantly evaluated. Fatigue damages were not considered in the original design of old metallic bridges, and riveted connections are one of the most frequent sources of fatigue damages on these structures. This paper intends to be a contribution for the reliable assessment of the fatigue behavior of riveted connections by proposing S-N curves for these structural details. Experimental fatigue data is analyzed from multiple bridges across Europe, and different statistical methods were implemented. A normal distribution function was implemented following the ISO 12107 standard, and the results were compared with the implementation of the two-parameter Weibull distribution function. Different estimation methodologies were implemented to determine the parameters of the Weibull distribution. S-N curves obtained by the statistical analysis were then compared with design recommendations from North American and European standards. 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. Erratum DOI: 10.1061/AJRUA6.0001081 The following correction should be made to the original paper: in all places where it is written “probability of failure,” the text should read “probability of survival,” including in Tables 2 and 3.Steel & Composite Structure

Fatigue crack growth modelling for S355 structural steel considering plasticity-induced crack-closure by means of UniGrow model

The UniGrow model is an analytical procedure to assess the fatigue crack growth based on elastic–plastic crack tip stresses and strains. The assumption is that fatigue crack growth (FCG) can be considered as a process of successive crack re-initiations resulting from material damage in the crack tip zone. The main parameters of this model are the crack tip radius and the elementary block size. Experimental FCG data obtained for S355 carbon steel showed that assuming the elementary block size with the same value of the crack tip radius to collapse FCG data using UniGrow model is non-coherent with experimental evidence. In this sense, a new approach is proposed by establish a clear distinction between crack tip radius and elementary block size. The value of the crack tip radius, ρ, was defined by correlation with experimental and numerical values of residual compressive stress field ahead of the crack tip while for the elementary block size, ρ∗, a new expression was proposed which relies on effective stress intensity factor range and cyclic yield strength. This research intends to be a valuable contribution for the implementation of UniGrow model to assess the fatigue crack growth of a material.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.Electronic InstrumentationSteel & Composite Structure

Axial monotonic and cyclic testing of micropiles in loose sand

Micropiles, which are small-diameter deep foundation solutions with diameters that can measure up to 300 mm, are often used to reinforce new and existing foundations. Their use in the foundations of structures with high eccentricity, such as wind towers when subjected to wind loads, may lead to more efficient and economical solutions. As the new generation of wind towers will reach more than 150 m tall, very large and uneconomical gravity foundations are required. In regions of high seismicity this problem is aggravated. To evaluate the behavior of micropiles under variable loading and predict the improvement of the reinforced solution, load tests were performed on steel micropiles under controlled laboratory conditions. A total of 36 tests were conducted on 3-m-long pipe micropiles, both while isolated and in 2 by 2 groups, with three different spacings. The micropiles were installed in a cylindrical container filled with calibrated sand and tested under monotonic and cyclic loading, first without grout, then when low-pressure grouted and retested, with the aim to evaluate the improvement caused by the grout injection, the micropile spacing, and application of cyclic loading both in terms of resistance and stiffness. An improvement both in stiffness and resistance due to the grouting was obtained and, for the applied cyclic loading, there was no clear reduction in micropile cyclic stiffness. The presented results provide a tool for the calibration of numerical models to estimate the behavior of real-scale micropiles installed in higher density sand.Steel & Composite Structure

Hotéis atractivos a hóspedes seniores: A experiência da Regiao do Algarve

Housing Quality and Process InnovationOTB Research Institute for the Built Environmen

Influence of execution tolerances for friction connections in circular and polygonal towers for wind converters

Friction connections with long open slotted holes have been proven to be a competitive alternative to the conventional flange connections in steel tubular towers for wind energy converters. As full-scale tests are not available, results of Finite Element Analysis (FEA) of the real-scale tower geometry are used in this paper to investigate the influence of tower cross section shape, execution tolerance (gap between the shells) and length of the connection on the bending resistance. Buckling behaviour of the shell in the vicinity of the friction connection in circular and polygonal towers is compared. The friction connection is thoroughly examined and recommendations for execution tolerances are given. The influence of two types of the execution tolerances on the connection strength is considered: inward bended “fingers”, leading to inclined gaps, and a parallel gap created by different tower diameters.Steel & Composite Structure

Influence of cold-formed angle on high strength steel material properties

This paper describes a study of the S650 high strength steel material properties including the effect of cold-formed angle. Coupon specimens with different cold-formed angles (90°, 100°, 120°, 140°, 160°and 180°) and different thicknesses (4 mm and 6 mm) were examined. Relationships between cold-formed angle and yield stress as well as tensile stress of the material were determined, based on the tensile coupon test results. Yield and tensile stresses assessed by consid ering the influence of the cold-formed angles were compared with those without considering this influence. Analyses revealed that both yield and tensile stresses decreased with increasing cold-formed angle. Ductile-damage material models available in the finite element analysis software ABAQUS were used to simulate tensile coupon tests. The experimental and numerical results showed good agreements.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.Steel & Composite Structure

Alternative steel lattice structures for wind energy converters

Purpose: In the last decades, the demand and use of renewable energies have been increasing. The increase in renewable energies, particularly wind energy, leads to the development and innovation of powerful wind energy converters as well as increased production requirements. Hence, a higher supporting structure is required to achieve higher wind speed with less turbulence. To date, the onshore wind towers with tubular connections are the most used. The maximum diameter of this type of tower is limited by transportation logistics. The purpose of this paper is to propose an alternative wind turbine lattice structure based on half-pipe steel connections. Design/methodology/approach: In this study, a new concept of steel hybrid tower has been proposed. The focus of this work is the development of a lattice structure. Therefore, the geometry of the lattice part of the tower is assessed to decrease the number of joints and bolts. The sections used in the lattice structure are constructed in a polygonal shape. The elements are obtained by cold forming and bolted along the length. The members are connected by gusset plates and preloaded bolts. A numerical investigation of joints is carried out using the finite element (FE) software ABAQUS. Findings: Based on the proposed study, the six “legs” solution with K braces under 45° angle and height/spread ratio of 4/1 and 5/1 provides the most suitable balance between the weight of the supporting structure, number of bolts in joints and reaction forces in the foundations, when compared with four “legs” solution. Originality/value: In this investigation, the failure modes of elements and joints of an alternative wind turbine lattice structures, as well as the rotation stiffness of the joints, are determined. The FE results show good agreement with the analytical calculation proposed by EC3-1-8 standard.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.Steel & Composite Structure

Resistance of cold-formed high strength steel angles

This paper describes a study of the behaviour of cold-formed high strength steel angles. Thirty-six specimens with different cold-formed angles (90°, 100°, 120°, 140°, 160°, and 170°) and different thicknesses (4 mm and 6 mm) were considered. The initial geometric imperfections of the specimens were determined using the 3D laser scanning method. The magnitudes of these geometric imperfections for torsional and torsional-flexural buckling and flexural buckling analyses were proposed. The commercial finite element analysis (FEA) programme ABAQUS with shell elements S4R was used for finite element analyses. Different material strengths in corner and flat parts along with different proof stresses (0.2%, 0.01%, and 0.006%) were considered in the numerical models. The experimental and FEA results showed good agreement. Influence of cold-formed angle on non-dimensional slenderness and reduction factor curves of the 4 mm thick columns with 90° and 120° cold-formed angles was analysed.Steel & Composite Structure