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This paper presents a study on the behaviour of welded "T" joints between RHS sections under brace axial loading. A finite element model was developed to investigate the influence of some geometrical variables on the joint's response. The brace load (always in tension) was incremented up to joint failure, while the chord was kept unloaded. In the companion paper (part II) a complementary study including chord axial loading is presented. The force-displacement curves corresponding to the different geometries are analyzed and compared, focusing on the failure loads and elastic stiffness. Different failure criteria are discussed and applied to the present curves and a comparison of the numerical results with the Eurocode 3 provisions is presented and discussed. KeywordsFinite element method; hollow section joints; deformation limit; plastic analysis; elastic stiffness.Resistance and elastic stiffness of RHS "T" joints: part I -axial brace loading INTRODUCTIONThe use of hollow sections is quite common in steel structures (Figure 1), partly due to their mechanical and aesthetical characteristics. The most common structural hollow sections are rectangular (RHS), square (SHS) or circular (CHS). The precision of design methods for these sections has a major importance considering the economical and safety points of view, and available analytical formulations to predict their behaviour have been included in modern design guides codes such as the

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.

Nowadays the wind turbines with the high power capacity are installed for the onshore wind parks. In order to achieve higher wind speeds and more stable wind, wind turbine towers are designed for higher altitude. Moreover, higher altitude reduces turbulence and wind shear induced vibrations. The convectional tubular towers are assembled using welded ring flanged joints. Moreover, preloaded bolts are used to connect the segments' flanges together. Therefore, the fatigue problem is predominant in the tall towers and fatigue failure due to welded connection and/or due to the bolts in tension may be the governing design situation. To tackle the fatigue problem in the bolted joints and discard the welded connections, a new friction connection is designed for tubular segment assembly. The tower segments are designed with long open slotted holes. The preloaded bolts are used to provide contact between the slotted holes and the intermediate auxiliary plate. Therefore, the connection will resist only by the friction in between the plates. In order to provide necessary friction between the plates, sufficient pre-load should be applied on the bolts. Moreover, it should be guaranteed that the load loss in the bolts is trivial. Therefore, Bobtail® bolt as free maintenance preloaded bolts are used in the friction connections. This paper deals with the identification of a function for bolts load loss behavior in the friction connections and provide an estimation for lifetime loss. A monitoring setup has been settle to measure the load in the bolts and the environmental temperature simultaneously for almost a year. Consequently, the Hammerstein-Weiner method, was applied to create the nonlinear dynamic model of the friction connection preloading behavior regarding the environmental temperature variation. Furthermore, the identified function output for the measured temperature was compared with the measured force and then the error was calculated. Based on the model, the load loss for constant temperature was calculated for the period of the measurement and was estimated for 20 years of life time span of the structure. The findings demonstrated acceptable amount of load loss in a year and estimation showed that bolts loads retain constant pre-load after a two years period. Moreover, it is obtained the initial fastening conditions were important to achieve free-maintenance bolts.

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The purpose of this paper is to present an evaluation of the robustness of fixed offshore structures, through the understanding of, i) major historical accidents, and its repercussions in the codes and regulations, ii) review the methods used to assure robustness, with special focus in offshore standards, but also with civil Eurocodes; iii) Robustness assessment and measurement and, iv) which factors and mechanical properties of structures influences robustness. To be able to measure the Robustness of a structure under accidental or unforeseen events it is necessary to quantify its ultimate capacity and establish its post collapse behaviour. Therefore, a case study consisting of Non-Linear analysis (pushover) of a fourlegged jacket is developed using USFOS software, where several structural properties are made variable and the effect in the robustness is measured.