STEEL HOT-ROLLED, COLD-FORMED, AND HOT-FINISHED STRUCTURAL HOLLOW SECTIONS AN EXPERIMENTAL STABILITY STUDY

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1 LIGHTWEIGHT STRUCTURES in CIVIL ENGINEERING CONTEMPORARY PROBLEMS Monograph from Scientific Seminar Organized by Polish Chapters of International Association for Shell and Spatial Structures University of Warmia and Mazury Faculty of Geodesy, Geospatial and Civil Engineering XXII LSCE 2016 Olsztyn, 2 December, 2016 STEEL HOT-ROLLED, COLD-FORMED, AND HOT-FINISHED STRUCTURAL HOLLOW SECTIONS AN EXPERIMENTAL STABILITY STUDY J. Szafran 1) K. Juszczyk 2) M. Kamiński 3) 1) Adjunct Professor, 2) M.Sc. Eng, Chair of Structural Reliability, 3) Professor, Head of Chair of Structural Reliability, Department of Structural Mechanics, Faculty of Civil Engineering, Architecture and Environmental Engineering, Łódź University of Technology ABSTRACT: The main aim of this study was to analyze the behaviour of steel circular hollow sections which are widely used in engineering structures such as masts or towers. The laboratory tests consisted of a series of axial compression trials which were executed until the moment of global stability loss has been reached. The tests were performed with nine samples made of S355 grade steel in a dedicated testing machine. They had been manufactured as cold-formed seamed profiles, seamless hot-rolled ones, and using seamed hot-finished technology. The obtained buckling capacity and manufacturing quality have been juxtaposed for different manufacturing methods. The secondary purpose consisted of the verification if a presence of a seam in a structural member significantly affects the failure mechanism of the tested element. The main conclusions are the method of production for circular hollow sections affects their buckling capacity which depends largely on material properties and manufacturing accuracy, and the seam has no influence on an element failure mechanism. Keywords: circular hollow sections, seamless tubes, welded tubes, hot finished profiles, buckling resistance, experimental study 1. INTRODUCTION Structural steel members of tubular cross sections for decades found applications in myriad of engineering fields such as towers or masts (Ref. 10). They provide supporting structures for various transmitter-receiver devices utilized in cellular networks and radio or television broadcasting. In view of the contemporary technological requirements, they often take form of slender objects of considerable heights. They usually utilize a type of spatial bracing, where the elements are primarily subjected to tensile and compression forces due to particular system of load: telecommunication devices are located at the top and the structure exhibits susceptibility to wind action, a key characteristic for such types of structures. Circular, hollow cross-sections are therefore a perfect solution willingly implemented due to both resilience and computational benefits. The former are connected to advantageous geometric parameters while under compression; the latter to identical cross-section characteristics in all directions considerably simplifying the calculations. Additionally, such shapes have favourable aerodynamic properties. The common utilisation of circular cross-sections is sparking interest in such elements not only in hands-on engineering, but also in scientific communities. A considerable part of the research in this scope concerns behaviour and bearing capacity of large diameter, fabricated steel tubes subjected to various loads which can be found mainly in sewage, water supply, and gas systems; and sometimes in construction. For instance, the influences of pipe forming angle, weld strength overmatch, and material strength anisotropy on the tensile strain capacity were investigated by means of parametric finite element analysis by Van Minnebruggen et al. in Ref 16. Additionally, numerical simulation of the buckling of steel pipes which are welded helicoidally, where the authors examine combined actions of external pressure: axial compression and bending moment, was the subject of Ref. 11. A similar topics were discussed in works of S.H.J. van Es et al. (Ref part I) and D. Vasilikis et al. (Ref part II). The authors presented experimental research aiming to investigate the structural behaviour of large-diameter, spiralwelded, steel tubes under bending. They tested thirteen 42-inch-diameter, spiral welded, steel tubes with the diameter to thickness ratios ranging between 65 and 120. They performed a series of large-scale, four-point bending tests to determine the structural behaviour of the tubes. They took into consideration the actual material characteristics investigated through uniaxial tensile and compression coupon tests. The initial geometrical imperfections were also examined. In the second part of their investigation, numerical simulations were conducted using nonlinear finite element simulations to define the bending strength and deformation capacity. Moreover, an analysis of practical utilisation of spirally welded tubes as a body of a wind turbine was performed in another work, Ref. 9. Apart from large-diameter tubes made from sheet metal plates, structural hollow steel sections are also commonly utilised in construction engineering. Currently there are circular cold-formed and circular hotrolled profiles available on the construction market. Cold-formed sections have seam along the longitudinal axis of the element due to the use of manufacturing technology where it results from the thermal or electrical welding. Hot-finished profiles may be produced as the cold-formed ones: by bending metal sheet and then welding them along the length of the element, wherein the final processing is performed using high temperature thermal treatment: seamed tubes, or as hot-rolled, seamless tubes. One of the production methods (TSR) was broadly described in Ref. 18. When the technology of the production is taken into consideration, it is worth emphasizing that hot-rolled, seamless circular sections are up to approximately 30% more expensive than their cold-formed seamed counterparts. Besides the production costs, the differences are also visible in material properties. Hot-rolling of structural steel sections is generally carried out above the re-crystallisation temperature of the material (typically around 850 ºC) in accordance with EN (Refs 3, 4). The resulting sections have homogeneous material properties, consistent hardness, good ductility, and relatively low residual stresses. Conversely, cold-formed sections are produced at ambient temperatures in accordance with EN (Refs 5,

2 6) and undergo plastic deformation during forming. Plastic deformation causes cold-working of the material resulting in enhanced strength, but also in the corresponding loss of ductility. Non-homogeneity of material properties and variation in hardness around the section typically arise due to the uneven levels of plastic deformations created during forming. Plastic deformation in cold-formed, structural sections is also associated with nonuniform formation of the residual stresses; these generally appear as through-thickness bending residual stresses (Ref. 8). Experimental studies on column buckling of high strength, steel, welded, circular tubes are presented in Ref. 13. The authors carried out 24 axial compressions column tests on samples with a nominal diameter of 273 mm and thickness of 6 mm taking into consideration their geometrical imperfections and axial misalignments concerning load application. The obtained buckling models of the tubes and their experimental bearing capacities were discussed. Non-linear finite element (FE) models which captured the experimental observations were developed. Based on the validated FE models, further parametric analyses incorporating 60 HSS, numerically generated, welded, circular, tubular columns of various section sizes were carried out. The resulting FE and experimental results were compared with the existing column design curves from Chinese, European, and American codes. Similar topics are discussed by authors in Ref. 14, who also analysed buckling behaviour of cold-formed (welded) sections of increased strength. This time, however, they were of square cross-section. Their paper focuses on the effect of the different material properties, imperfections, and residual stresses on the global buckling behaviour of HSS members and on proposing designs for the applicable column buckling curves for steel grades between (S420) S500 and S960. Interesting results were obtained regarding experimental and numerical investigation into the structural performance of stainless steel, circular hollow sections (CHS) under combined compression and bending moments by the authors of Ref. 20. Their experimental programme examined four CHS sizes made of austenitic stainless steel and included material tensile coupon tests, four stub column tests, and twenty combined loading tests. Sample lengths under axial compression were chosen in such a way that their destruction was not due to buckling, but rather to the bearing capacity loss of the cross-sections. The obtained data was compared with FEM models and codes in force. The research referring to hot-rolled sections of elliptic cross-sections and hot-finished ones of square and rectangular cross-sections, which concerned their strength against compression and flexural behaviour, were discussed accordingly in Refs 1 and EXPERIMENTAL INVESIGATION 2.1. Stub column tests The tests were done for nine circular hollow section samples made with S355 grade steel of dimensions 88.9 x 5.0 mm and lengths of 1500 mm. Three samples (no 1-3) have been manufactured as seamed, cold-formed profiles. Next elements (no 4-6) have been produced as seamless, hot-rolled ones; and the others (no 7-9) were made using hot-finished, seamed technology. All the elements have been subjected to axial compression in the testing machine (Fig. 1). The tests were being carried on up to the expected global stability loss. The device utilized in performing the experiment was set in a particular way to ensure the stability of the elements being investigated. The endings of the tubes of length 105 mm were inserted into specially adjusted holes of diameters slightly larger than the shapes of elements. By that means it allowed for eventual minimal turning along their longitudinal axes O 89 O O Fig. 1 Scheme of the testing machine. The load was being increased gradually while the values of the acting force were constantly being registered. When the boundary value (causing the loss of the element stability) has been reached, the load was kept being applied to observe the deformations. The deformed samples after the experiments have been carried out are shown in Fig. 2. Their deformed endings as the result of the attachment method are presented in Fig. 3. The subject of latticed, telecommunication supporting structures made with solid round steel members was also discussed by the authors of Ref. 12. They drew attention to the fact that not only the strength and stiffness, but also all stability issues should be considered in designing members of this kind of structures. They also set out recommendations for economical design. As mentioned before, cross-sections of nominally similar geometries, but from the two different production routes may vary significantly in terms of their general material properties, geometric imperfections, residual stresses, material response, general structural behaviour, and load-bearing capacity of the produced sections (Ref. 8). Such a fact undeniably deserves further study which would simplify the dilemma of engineers of which section type to use. Motivation to conduct this tests and to write this article was high frequency of use circular hollow sections as structural elements in many engineering structures in which slender elements subjects to the loss of stability are dominating. This article shows results of laboratory research concerning axial compression of cold-formed, hot-finished, and hot-rolled tube elements of circular cross-sections. Their most essential experimental property: buckling load capacity has been analysed as far as the method of production is concerned, the elements quality was also taken into account. The results have been compared with theoretical data based on standardized procedures. The material parameters were determined based on tensile coupon tests. The influence of the element production method on its failure mechanism has been also closely monitored (the seam behaviour under load in particular). Fig. 2 Plastic deformations of the samples: cold-formed (left), hot-rolled (middle), and hot finished seamed technology (right). Fig. 3 Plastic deformations at the endings of the examined elements.

3 Tab. 1. Experimental and theoretical buckling capacities of the examined elements. Technology Cold-formed Hot-finished Circular section type Seamed Seamless Seamed Sample no Experimental buckling resistance [kn] Theoretical buckling resistance [kn] Tab. 2. Thickness measurements for the examined sections [mm]. Technology Cold-formed Hot-finished Circular section type Seamed Seamless Seamed Measurement point Sample no Expected values of the thickness E[t] (mm) Standard deviation σ[t] , , , , , , , , , , , , , , , , (2%) (4%) 0.28 (6%) Tab. 3. Measurements of section diameters [mm] (2%) Technology Cold-formed Hot-finished Circular section type Seamed Seamless Seamed Measurement point Sample no Expected values of the diameter E[t] (mm) Standard deviation σ[t] , , , , , , , , (3%) 0.13 (3%) 0.11 (2%) 89, , (<1%)

4 The results showing the breaking forces were included in Table 1; the relations of the applied force to longitudinal deformation of an element (both elastic and plastic parts) are presented in Fig Geometric measurements Taking the geometric measurements of the samples had constituted an important stage in the process. It allowed for checking the precision and quality of a section production method. 16 Measurements of material thickness (8 per ending) and 8 of the diameter were taken for every sample (Tables 2, 3). Fig. 5 presents a draft of the layout for the measurement points. experiment have been presented in Fig. 6. The obtained results were presented in Table 4. The graphs with relation of sample elongation to the applied force were presented in Figs 7a and 7b. Circular section type Cold-formed (seamed) Tab. 4. Material parameters of the examined samples. Specimen no Yield stress f y [MPa] Tensile strength R m [MPa] Hot-finished (seamless) Hot-finished (seamed) Fig. 4 Relationships between the acting force and the shortening of a sample: cold-formed (top), hot-rolled (middle) and hot finished seamed technology (bottom) Fig. 5 Measurement points layout for the material thickness and diameter Tensile coupon tests The basic stress strain properties of the investigated hot-rolled and coldformed sections were obtained through tensile coupon tests. These tests were conducted in accordance with EN (Ref. 2). One flat coupon had been cut out in parallel for each of the nine CHS samples (Fig. 6). The experiment was carried out using a hydraulic testing machine, in which each sample was being subjected to tension until the tensile failure. The yield point (Re) and tensile strength (Rm) have been measured each time. Samples before (top) and after (bottom) the 8 7 Fig. 6. Tensile coupon samples before (top) and after tests (bottom). Fig. 7a Relationship between the acting force and the elongation of a cold-formed sample.

5 4. ANALYSIS OF EXPERIMENTAL RESULTS The dimensions of the samples were selected in such a way that their stability conditions determined their bearing capacities. As can be seen in Fig. 2, each of the examined tubes gave in to buckling, exhibiting plastic deformations under persistent load. Due to the characteristics of the crosssections of the elements (being hollow) they became damaged (dented). Consecutively, the largest deformations could be found in case of the coldformed tubes, while the least deformations could be found for the hot-rolled ones. A similar relationship could be seen in Fig.3 exposing the plastic deformations resulting from the method of attachment in the testing machine: the cold-formed elements have exhibited the largest deformations, while the hot-rolled ones have shown almost no such effects. It should be also noted that the seam along the element has not shown any connection to the failure mechanism in any test; the mechanism has been practically unaffected by it. Analysing the characteristics of the relationships between the applied load and the shortening of the samples, attention should be paid to the force at which the loss of the stability occurred - it has been slightly lower for the cold-formed elements. The characteristics of the graphs are also different: in the case of cold-finished tubes the characteristic was milder, while in case of sections made by using hot temperatures the strain grew with the force and fell faster after the maximum value has been reached, which has been noticeable particularly for hot-rolled elements. Fig. 7b Relationships between the acting force and the elongation of a sample: hot-rolled (top), and hot-finished seamed technology (bottom). 3. STANDARD FORMULAS The standard in force concerning the design of steel constructions (Ref. 7) proposes the following formula used for determination of buckling capacity of elements under tension: A f y Nb, Rd, (1) M1 where A is cross-section area of an element under tension, f y is the yield point of the steel, χ is the buckling coefficient corresponding to the definitive criterion for buckling form, and γ M1 is the partial coefficient used for checking the element stability, which is 1.0 according to Polish criteria. In case of elements compressed axially the value of coefficient χ is determined according to Eqns (2, 3) depending on relative slenderness λ w, and an appropriate buckling curve: 1, but 1, 0, 2 2 w 2 0,5 1 w 0,2 w, (3) where α is a parameter of imperfections which depends on the section production method and the type of steel. Its value for steel of type S355 is equal to α = 0.21 for hot-finished elements and α = 0.49 for cold-finished ones. The relative slenderness is determined by Eqn. (4): L cr w, (4) i 1 where L cr is the length of element buckling (L cr = 1.29 m for the analysed samples), i is radius of gyration, and λ 1 is comparative slenderness described by Eqn. (5): E 1, (5) f y where E is the modulus of longitudinal steel elasticity (Young modulus). Theoretical values of buckling capacity for the elements are shown in Table 1. (2) Comparing the results obtained from stub column tests, it can be unequivocally stated that the highest buckling capacity was exhibited by hot-rolled elements, while the lowest by cold-finished sections. The differences between the following production methods (cold-formed elements versus hot-finished with seam versus hot-rolled) are about 5%. Far greater differences: from about 15% for hot-formed elements with a seam to about 20% in case of cold-finished tubes, are visible in juxtaposition of experimental and theoretical bearing capacities. The reasons for such imbalances are certainly material properties of the steel, especially yield point, where the theoretical and the experimental values differ significantly. This parameter is assumed, according to standards, as f y = 355 MPa for structural steel S355, while its values in static tensile tests range from f y = 398 MPa (hot-finished, seamed) to f y = 449 MPa (hotrolled) so they are higher in every case, and by about 20% on average. The highest yield point was obtained for cold-rolled samples, while the lowest yield point was exhibited by the hot-finished tubes with a seam. It clearly supports the assumption that the yield point for cold-rolled elements may be higher due to the technology of production such as work of sheet metal plates. The situation is a little bit different for tensile strength f u. The standardized value for this parameter was f u = 510 MPa, but lower values were obtained for cold-formed samples (498 MPa, 500 MPa, 504 MPa), and the values for hot-finished elements were higher by about 10%. Analysing the graphs for the load versus elongation of the samples (Figs 7a, 7b), some regularities could be noticed: there was no plateau of plasticity for cold-formed elements, the yield point was assumed as the value of stress at 0.02% of strain of the element. Moreover, the graph regarding plasticity was constant throughout a significant part of the experiment. The plateau of plasticity finally appeared in characteristics obtained for sections produced in technology using high temperature, and was more noticeable for hot-rolled tubes. Here, at the beginning of the plasticity range, the elongation of the sample was increasing along the increase in the acting force. An essential element which should be noticed, having direct influence on the values of buckling capacities of the examined elements, are geometric dimensions of their cross-sections: the diameter and material thickness. Based on the taken measurements, it could be stated that, while the actual values of diameters did not differ considerably from the theoretical dimensions for each of the production methods (the standard deviation is 1% at maximum), with the material thickness the differences were noticeable: in case of only one sample the thickness of the material is satisfactorily similar to the nominal value (5.01 mm). In other cases the measured values were not-standard (the differences between the theoretical values reach 7%). The thickest material was noted for the hot-rolled elements, the thinnest for cold-formed ones. The standard deviation for the thickness of the material in case of the rolled tubes reaches 6% which indicated a considerable randomness and small precision of element

6 production. The value was up to three times smaller for the cold-formed sections (σ = 2%). The hot-finished tubes with a seam found themselves at the middle of this range with the standard deviation twice smaller than for the rolled elements (σ = 3%) The thickness of the material corresponded directly to the area of the cross-section of the element which, in turn, affected its buckling capacity (Eqn. 1). It can be therefore assumed that the higher bearing capacity values in the case of hot-rolled elements depended, to a certain extent, on their cross-sections. The cross-sections themselves were, according to the performed experiments, adequately higher in comparison to the cross-sections of the elements produced using alternative methods. 5. CONCLUSIONS The goal of this article was to deliver a comparison of tube sections: coldformed, hot-rolled and made by hot-finished, seamed technology, which are commonly utilised in construction engineering. Based on the gathered data and the performed analyses, it can be draw following conclusions: the method of production for tube sections affects with certainty their buckling capacity. The highest buckling capacity is exhibited by hot-rolled elements, while the least one by coldrolled ones. The hot-finished tubes with a seam yield similar parameters to the rolled ones, the cause of this phenomenon lies in the properties of the material where the structure changed during heating up to high temperatures, welding, and cold work, and also in production quality, the method of production determines manufacturing accuracy, which is the highest (closest to the nominal values) for hotrolled tubes, which has a direct impact on its buckling capacity (it's the biggest one), real (experimental) buckling capacities are higher about 15-20% than buckling capacities estimated based on Standards, and the differences are due to different values of theoretical and experimental yield strength mainly, the elements made using particular methods differ from each other by the magnitude and character of plastic deformations: the largest could be detected for cold-formed sections and the smallest for hot-rolled ones, Eurocode procedure about buckling applies elastic range only, in confrontation with experiment it should be extended to additional part taking into account Johnson-Ostenfeld, Tetmajer-Jasiński or Engesser-von Karman theory, the influence of a seam on an element failure mechanism was not observed for any case. The conclusions of this research will assist the practicing engineers in the evaluation of load-bearing capacity for the existing antenna towers since even a small increase in strength against live load can make a difference between replacing a tower or leaving it in service for the long years ahead without a need of reinforcing certain members (Ref. 12). Moreover, the outcomes are valuable from the economical aspect as well when the differences in section prices, between elements with seam and seamless ones, are taken into consideration. ACKNOWLEDGMENTS The authors would like to thank Lena Wilków Sp. z o.o. for their financial and organizational support that made this project possible. Special thanks we are sending to: Julian Ziarno, Zbigniew Górzański, Marek Warzecha, Patrycjusz Pisarski and Grzegorz Suchan. REFERENCES 1. T.M. Chan, L. 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