The objective of this paper is to analyze and characterize high-resolution measurements of geometric imperfections taken from a set of seven slender tapered steel tubes in order to provide insights that can improve methods for predicting buckling behavior. The seven tubes are each similar to 3400 min long with diameters between 800 mm and 1100 mm, diameter to thickness ratios between 300 and 350, and taper angles between 0.67 degrees and 0.86 degrees. The tubes are manufactured from steel plates using an innovative spiral welding process. The geometric imperfections of these tubes are characterized with harmonic analysis of the overall imperfection measurements and with regression analysis of the measured shapes of weld depressions. The results show a consistent imperfection signature caused by the manufacturing process including distinct features attributed to both the rolling and welding processes, i.e. anticlastic deformations and weld depressions. Variability in the imperfection measurements is also analyzed and used to generate a probabilistic scheme capable of generating random fields of geometric imperfections that are consistent with the measurements considered here.

The objective of this study is to develop and validate a practical finite-element modeling protocol for predicting the flexural strength and collapse behavior of thin-walled spirally welded tapered tubes that can be used as steel wind turbine towers. The overall modeling protocol consists of two parts: (1)a meshing protocol is developed considering the effects shell element type, aspect ratio, inclination angle, and density on the buckling moment relative to theoretical predictions; and (2)two patterns of geometric imperfections (eigenmode-affine and so-called weld depression) scaled to the thresholds of fabrication tolerance quality classes in Eurocode 3 are considered in nonlinear collapse shell finite-element models and the results are compared to a series of eight large-scale flexural tests of spirally welded tubes. The computational results are compared with test results in terms of moment-rotation response, stiffness, and buckling modes and show sufficient agreement to justify the further development of nonlinear analysis methods for the design of steel wind turbine towers made from thin-walled spirally welded tapered tubes.

The objective of this paper is to advance the design of slender steel tubes by developing a practical approach for utilizing high-resolution measurements of geometric imperfections to estimate the buckling location and strength of such tubes in bending. This approach includes a novel measure of imperfection severity that is designed to be insensitive to noise. The ability of this measure to predict buckling behavior of slender tubes in bending is assessed through comparison with eight large-scale tests, and, for this set of data, the predictions are promising. This study is intended to be a starting point in the development of a simple, but accurate, design method to quantify the impact of imperfections on the buckling behavior of slender tubes.

SummaryThis paper revisits the phenomenon of dynamic soil‐structure interaction (SSI) with a probabilistic approach. For this purpose, a twofold objective is pursued. First, the effect of SSI on inelastic response of the structure is studied considering the prevailing uncertainties. Second, the consequence of practicing SSI provisions of the current seismic design codes on the structural performance is investigated in a probabilistic framework. The soil‐structure system is modeled by the sub‐structure method. The uncertainty in the properties of the soil and the structure is described by random variables that are input to this model. Monte Carlo sampling analysis is employed to compute the probability distribution of the ductility demand of the structure, which is selected as the metrics for the structural performance. In each sample, a randomly generated soil‐structure system is subjected to a randomly selected and scaled ground motion. To comprehensively model the uncertainty in the ground motion, a suite of 3269 records is employed. An extensive parametric study is conducted to cover a wide range of soil‐structure systems. The results reveal the probability that SSI increases the ductility demand of structures designed based on the conventional fixed‐based assumption but built on flexible soil in reality. The results also show it is highly probable that practicing SSI provisions of modern seismic codes increase the ductility demand of the structure. Copyright © 2016 John Wiley & Sons, Ltd.

This paper is a part of an ongoing numerical and experimental study on steel spirally welded tapered tubes applicable to wind turbine towers. Such tubes are usually thin-walled and are known to be sensitive to different types of geometric imperfections and boundary conditions. The European Standard on the Strength and Stability of Metal Shells EN 1993-1-6:2007, has organized design of hollow shelled structures into three classes: Excellent, High, and Normal (designated A, B, and C, respectively) according to the quality of manufacturing and construction. EN 1993-1-6 allows shell finite element models to be used to determine capacity, a general method with great potential but unique challenges for the analyst/engineer. Adopting numerically generated imperfection patterns and magnitudes according to EN 1993-1-6, a set of material and geometric nonlinear shell finite element analyses has been performed on spirally welded tapered tubes recently tested under flexure at the STReSS Lab at Northeastern University. A comparison between the numerical models with different levels of imperfections and the tested specimens in terms of moment-rotation response, stiffness, and failure modes has been utilized to propose a modeling protocol for thin-walled tapered tubes using generated imperfections based on the design specification. The potential to use the modeling protocol to extend the test results to different sizes of the tubes and different load combinations resembling the loading conditions in the wind turbine tubes are discussed. 1. Introduction Imperfections in circular cylindrical shells have been recognized as a dominant factor in reducing buckling strength (Hutchinson 1971). According to specific tolerance limits of imperfections systematically measured on site or at workshop, Eurocode EC3 (European Standard on the Strength and Stability of Metal Shells EN 1993-1-6:2007) classifies shelled structures to be either Excellent (Class A), High (Class B), or Normal (Class C) (EN 1993-1-6:2007). The code specifies 1 Graduate Research Assistant, Dept. of Civil Engineering, Johns Hopkins University, asayed3@jhu.edu 2 Assistant Research Professor, Dept. of Civil Engineering, Johns Hopkins University, torabian@jhu.edu 3 Graduate Research Assistant, Dept. of Civil and Env. Eng., Northeastern University, jay.a@husky.neu.edu 4 Graduate Research Assistant, Dept. of Civil and Env. Eng., Northeastern University, mirzaie.f@husky.neu.edu 5 Assistant Professor, Dept. of Civil and Environmental Engineering, Northeastern University, atm@neu.edu 6 President, Keystone Tower Systems, eric@keystonetowersystems.com
 7 Professor, Dept. of Civil Engineering, Johns Hopkins University, schafer@jhu.edu

Taking more advantage of wind and other sources of renewable clean energy is essential for a sustainable future. To this end, the cost of wind generated electricity needs to be significantly reduced. One solution is to build taller turbine towers taking advantage of less turbulent and higher velocity wind at higher elevations, which in turn results in producing more power per turbine and reducing the wind energy production costs. However, taller towers require larger base diameters to be able to carry the flexural loads from the turbine. Currently, transportation limitations restrict tower base diameters to 4.3 m in the US, therefore any tower taller than 80 m cannot be optimally designed (in a lowest material weight sense). To overcome this challenge, spiral welding procedure used in the pipeline industry is being adapted to manufacture tapered tower sections on site. However, spiral welding causes unique pattern and magnitude of geometric imperfections, and since the buckling behavior of slender tubes is highly imperfection sensitive, the impact of imperfections associated with the spiral welding procedure on the ultimate flexural strength of the manufactured tubes needs to be thoroughly investigated. Geometric imperfections in slender tubes can arise for a variety of reasons, and for practical purposes can essentially considered to be random. Thus, statistical characterization of the manufacturing-induced imperfections is necessary to better understand the uncertainties in the buckling capacity of slender tubes. The research completed in this dissertation enhances the design of slender spirally welded tubes for use as wind turbine towers. The work includes proposing a simplified approach and a novel measure of imperfection severity that is designed to be insensitive to noise for predicting buckling strength of slender tubes using high-resolution geometric imperfection measurements; characterizing randomness in the weld-induced geometric imperfections of spirally welded tubes and proposing a probabilistic scheme capable of simulating random realistic geometric imperfections for slender tubes; providing a foundation for reliability-based design of spirally welded tubes by presenting a probabilistic view on their buckling capacity considering the randomness in the geometric imperfections; and examining the relationship between spiral welding, induced geometric imperfections, and induced residual stresses.

A new manufacturing process allows for the production of tapered spirally welded steel tubes. This paper describes a series of eight large-scale tests examining the behavior of such tubes in flexure and investigates the impact of imperfections on the flexural strength of the tube. The tests are performed on tapered circular steel tubes with diameters between 0.7 and 1.1m and maximum diameter-to-thickness ratios between 200 and 350. Specimen geometries are selected to provide flexural test data at slenderness ratios not commonly tested in the literature and to be representative of tapered tubes applied as wind turbine towers. The geometries of the specimens are measured with laser scanners before and during testing to characterize initial imperfections and the evolution of local buckling. Results are compared to design strengths per Eurocode EN 1993 1-6. All specimens meeting Eurocode manufacturing quality requirements exceed predicted strengths. The location and orientation of the local buckling region are correlated with the spiral seam welds on the specimens.