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.@en

This paper is the second part of the study on the cold-formed high strength steel circular and polygonal sections intended to be used in tubular wind towers. Results from 32 numerical finite element analysis (FEA) models were compared with and calibrated against results of the tests on 32 corresponding specimens. The FEA results agreed well with the experimental results in terms of resistances and load-displacement curves. Further investigations on the numerical models were performed. Yield stress used in the FEA significantly affected the resistances of the numerical models. Using 0.2% proof stress leaded to higher resistance than the experimental results. Corners significantly influenced buckling behaviour in the polygonal section models. Analyses of an oval opening in the tubular specimens showed that peak stresses around the opening were considerably higher in the polygonal section models than in the circular section models. Finally, investigation of sensitivity to geometrical imperfections indicated that failure modes of numerical models with geometrical imperfections according to EC3 significantly differed from those of tested specimens and numerical models with geometrical imperfections obtained from the 3D scans.@en

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.@en

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.@en

There is a significant number of old metallic bridges with high levels of structural degradation due to their long service period. Fatigue problems are especially important in these structures since the majority of them were not designed taking into account this phenomenon. Several investigations showed that riveted joints are critical details since several fatigue cracks were found in these joints. In this sense, strengthening methodologies need to be studied. The strategy that has been considered a good solution is the implementation of injection bolts to replace faulty rivets. The structural performance of injection bolts has been demonstrated essentially under quasi-static conditions presenting good results. This paper intends to contribute to the scientific knowledge regarding the fatigue behavior of connections with preloaded injection bolts in the context of a bridge strengthening scenario. An experimental investigation was conducted to compare the fatigue performance of connections with preloaded injection bolts and preloaded standard bolts. Single and double shear connections were tested. New S–N design curves were proposed based on a statistical analysis of the results and compared with the S–N curves proposed in EC3-1–9. The obtained results showed that the use of injection bolts lead to lower scatter and improvement of fatigue life. It was verified that the Eurocode 3 is not able to represent the fatigue strength of connections whose performance is influenced by old metallic materials. Additionally, the fatigue behavior of these connections was assessed by numerical analysis. The relevance of the fatigue crack initiation was evident.@en

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.@en

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.@en

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.@en

A steel hybrid structural solution for onshore wind turbine towers was proposed in the European project SHOWTIME. This solution is used in the lattice structure for the lower portion of the tower. Recently, a procedure for fatigue life estimation of steel half-pipes bolted connections applied in global structural models using multiaxial Smith-Watson-Topper (SWT) criteria was proposed by Öztürk et al. In this paper a procedure for design S-N curve modelling of steel half-pipes bolted connections is proposed. This procedure is based on a local approach using multiaxial fatigue criteria together with an elastoplastic analysis using the finite element method. The materials to be used in this analysis are the S355 and S690 steels. This evaluation to be performed is calibrated with experimental results of fatigue tests of the connection under consideration.@en

The maintenance and safety of ancient bridges is a major concern of governmental authorities. In particular, the safety of old riveted bridges fabricated and placed into service at the end of the 19th century deserves particular attention. These structures are susceptible to exhibit high fatigue damage levels due to their long operational period with increasing traffic intensity associated to an original design not covering the fatigue phenomenon. This paper reviews recent fatigue behaviour investigations on single and double shear riveted joints performed by Universities of Porto (Portugal), Trás-os-Montes e Alto Douro (Portugal), and Wrocław (Poland), in particular concerning the fatigue characterization of riveted joints extracted from representative Portuguese riveted bridges, namely the Eiffel, Luiz I, Fão, Pinhão and Trezói bridges. In order to overcome the influence of scatter and establish a reliable assessment for the obtained experimental data, two statistical approaches were used: implement linearized boundaries following the recommendation in ASTM E739 standard and defining probabilistic S–N fields using the Castillo & Fernández-Canteli model. This statistical analysis allows to propose design S–N curves for single and double riveted joints and evaluate the applicability (safety) of using the design curves suggested in Eurocode 3 as well as design curves proposed by Taras and Greiner.@en

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.@en

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.@en

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. @en

This paper presents the safety assessment of Eurocode 3—part 1-1 (Eurocode in EN 1993-1-1, Eurocode 3: design of steel structures—part 1-1: general rules and rules for buildings, CEN, Brussels, 2005) rules for hot-rolled members with I and H cross-sections loaded in bending and compression. The safety assessment follows the recommendations of Annex D of EN 1990 (Eurocode in EN 1990, Eurocode—basis of structural design, CEN, Brussels, 2002). It is based on a large number (11,345) of numerical simulations (GMNIA) covering various relevant parameters. The assessment aimed at the justification of the partial factors to be used in the design of members under bending and compression as part of the ongoing process of revision of Eurocode 3—part 1-1. For that, it covered the application of the interaction factors given in Annex B/Method 2 of the current version of Eurocode 3—part 1-1 which will be the only method in the future version of Eurocode 3. In addition, the interaction factors were applied with the new buckling curves for lateral-torsional buckling, as they will appear in the new version. The assessment showed consistent results across the subsets considered with low variability and partial factors generally lower than 1.0. Finally, the γM* values are compared for flexural buckling of columns, lateral-torsional buckling of beams with the results obtained for beam-columns considering fy, cross-section geometry and E as random variables, using the same statistical characterization of the random variables. It is observed that the partial factors obtained beam-columns are lower than those obtained for columns and beams. In conclusion, based on combination of the results obtained it was possible to recommend a global value of γM1 = 1.0.@en

Wire Arc Additive Manufacturing (WAAM) has gained popularity due to its speed and cost-effectiveness. However, the current knowledge on the cyclic deformation behaviour of WAAM materials is limited. To address this issue, this study investigates the cyclic deformation behaviour of WAAM ER70S-6 carbon steel. Coupons were extracted from printed walls with horizontal and vertical orientations and from surface and interior locations. Low-cycle fatigue tests were performed under fully reversed strain-controlled conditions, spanning strain amplitudes from 0.20 % to 1.50 %. Regardless of the group, the material exhibited cyclic hardening for strain amplitudes above 0.60 % and cyclic softening for smaller strain amplitudes. Significant non-Masing hysteretic behaviour was also observed. Cyclic stress–strain curves and fatigue-life relationships written in terms of stress, strain and energy-based parameters were derived for all groups. Fatigue life was not significantly influenced by printing orientation or location across thickness. However, horizontal orientation resulted in a slightly superior fatigue response. A statistical analysis revealed no significant difference in the accuracy of fatigue-life relationships between the groups. Finally, a comparative study between WAAM materials and conventional steels showed promising results. Yet, conventional steels outperform WAAM carbon steel, at least doubling the fatigue life for the same value of the damage parameter.@en

Wire Arc Additive Manufacturing (WAAM) has gained popularity due to its speed and cost-effectiveness. However, the current knowledge on the cyclic deformation behaviour of WAAM materials is limited. To address this issue, this study investigates the cyclic deformation behaviour of WAAM ER70S-6 carbon steel. Coupons were extracted from printed walls with horizontal and vertical orientations and from surface and interior locations. Low-cycle fatigue tests were performed under fully reversed strain-controlled conditions, spanning strain amplitudes from 0.20 % to 1.50 %. Regardless of the group, the material exhibited cyclic hardening for strain amplitudes above 0.60 % and cyclic softening for smaller strain amplitudes. Significant non-Masing hysteretic behaviour was also observed. Cyclic stress–strain curves and fatigue-life relationships written in terms of stress, strain and energy-based parameters were derived for all groups. Fatigue life was not significantly influenced by printing orientation or location across thickness. However, horizontal orientation resulted in a slightly superior fatigue response. A statistical analysis revealed no significant difference in the accuracy of fatigue-life relationships between the groups. Finally, a comparative study between WAAM materials and conventional steels showed promising results. Yet, conventional steels outperform WAAM carbon steel, at least doubling the fatigue life for the same value of the damage parameter.@en

A short state-of-the-art on design verification of supporting structures for wind energy converters is presented. On-going trend in building of land based wind farms is to install larger capacity wind energy convertors at the higher towers. Preliminary studies have indicated that use of higher strength steel (HSS) would be advantages in such applications. However, because of a rather low fatigue detail class of a ring flange connection which is traditionally used for in-situ assembling of segments to a tubular tower, the use of HSS is not justified. That was the main reason to search for an alternative in-situ connections. A possible solution is based on a single overlapping friction connection obtained by preloaded bolts and using open slotted holes. This connection is studied in RFCS research projects, HISTWIN and HISTWIN2, High-strenght tower in steel for wind turbines and Higher steel tubular towers for wind turbines (2012 and 2014), respectively. Parts of these results are presented here. Based on this research a new knowledge is gained to in order to optimize tower design and achieve economic benefits. A case study example of a real tubular tower is used to provide insight in real design values. Other challenges that are associated with the tendency to build towers with higher hub heights are briefly discussed focusing on limits of tubular towers imposed by the traffic constraints. Finally, an outlook for innovation is shown providing an insight into the on-going research projects.@en