Fatigue design consideration in columns under wave cyclic loading - Research Paper Example

Fatigue Design Consideration in Columns under Wave Cyclic Loading Designing of structures, particularly, columns and beams are measures that require several factors to be kept in focus. One of the most important factors of these is the fatigue design considered for columns under cyclic wave loading. Studies reveal that there are significant calculations and estimations that need to be considered to determine the effects of wave conditions on these structures. The present study focuses on these factors, and the types of fatigues that are possible. This would bring into light the necessity for experts to consider an extensive study on the structures, the materials employed and the conditions of the waves such that effective measures may be applied in the process of construction. Introduction: Considering the issue of cyclic wave loading, it is necessary to understand that both cyclic strains and residual strains or strains that are permanent in nature are developed when a sequence of cyclic loads affect the underground soil. The permanent or residual strains stay behind at the end of each cyclic load reflecting a growing effect that gets accumulated with the effects left by earlier storms. Greater attention has been provided to evaluations of peak cyclic displacements that occur along with a storm. However there are greater needs for studies on issues of permanent displacement. While considering these issues, it is also necessary to take into consideration the relationships among the properties of the soil, cyclic loading as well as well as the residual strains and the shear stress (Marr et al, 1981, pp.1129-1130). Concrete components used in offshore structures encounter forces of strong wave owing to frequent storms and are open to tremendously rigorous conditions of the environment leading to decay of steel that is fixed in the structure and worsening of the concrete material. Fatigue loading tests are conducted in this regard in order to obtain fatigue properties of column joints that are already stressed, the results of which can then be compared to those obtained from static loadings. Considering joints and columns in the offshore structures, it has been found that many types of offshore structures constituting concrete that is already stressed have been built up with intentions to make use of the space of the offshore and to take advantage of the natural resources in offshore regions (Kiyomia et al, 1988, p.139). The present study focuses on the concept of fatigue design implemented in columns under cyclic wave loading. Literature Review: Kiyomia et al (1988, pp.139-140) in their studies had discussed about the determination of the conditions of waves for fatigue designs. According to them the waves of sea are an assortment of numerous sinusoidal waves reflecting different periods of time and amplitudes. Moreover, the conditions of the wave vary based on the site of the construction. The conditions of the waves at every location need to be estimated and assessed by statistical procedures by means of wave records. “The relationships between the numbers of waves and the wave heights are needed to determine the fatigue limit state according to the service life of the facility when using Miner's rule for calculating the fatigue strength of the joints. To estimate the serviceability of the joints, the wave conditions, expressed as the number of waves and wave heights over a period of 1 month to 1 year, are needed”. A study considered by International Maritime Organization discussed that the likelihood of damages to fatigue designs owing to cyclic wave loading need to be taken concern of in the “design of self-elevating and column-stabilized units” (International Maritime Organization, 1990, p.72). Such analyses of the fatigue designs are required to be conducted with the studies based on the intended mode and the operations involved in the designs of the units. Also, the analysis needs to consider the life of the intended design and the convenience of individual structural constituents for assessment. While deciding on the arrangement of the structures, particular considerations are given to all the details in regard to the exposures to high local loadings resulting from external damages, impacts of waves, as well as from tanks that are not filled completely or from bottom bearing operations (International Maritime Organization, 1990, pp.71-72). Research Methodology: The interpretivism method of research philosophy has been applied in this study that takes into account the human experiences and their beliefs and opinions to form an authenticated outcome that presents a study that lay hidden in such human experiences (Collins, 2010, pp.38-39). The research has been based on a secondary study involving qualitative and research method involving a natural approach to the topic of study and tries to result in outcomes that can be derived from the facts and figures provided by the studies (Thomas, 2003, pp.1-2). Secondary resources have been collected and used for analysis in this particular research study thus involving the information collected from studies that have already been conducted by previous researchers (Malhotra, 2008, pp.106-107). The sources that have been used in this study mainly include materials obtained from the internet websites, the books, articles and journals, and the previous studies conducted on the current topic. Ethical factors have been duly kept in focus while conducting the research work. The collection and analysis of data have been done with due care to ensure that professional ethics is not harmed by any wrongdoings. Also, utmost care has been taken to ensure that the websites used from the internet are not unauthentic and they contain proper articles and information provided by different theorists or authors. Books and journals have been considered as they are authentic in nature and provide with useful information on the concerned topic. Since the research study is only secondary sources based, the main limitation of the research has been obtained in the availability of data. Also, the credibility and authenticity of the data had to be kept in note. Not all websites provide with authentic information. Hence limited sources were available. Findings: Over the recent years, there have been several considerations in the interest of developing fatigue strength in the components of concretes. The reasons for such interests include apprehensions about the effects of frequent loads on structures like crane beams and bridge slabs. Moreover there are new uses of these concrete components demanding for a product whose performance is high with guaranteed fatigue strength. Also, there is latest identification of the effects of repeated loading on a component, even though repeated loading has not been found to cause a failure in the fatigue (ACI Committee 215, 1997, p.2). Fatigue Properties of Component Materials: When plain concrete is subjected to frequent loads it is found to show evidence of unwarranted cracking and may in due course stop working after an adequate number of load repetitions. It may even not work then if the maximum load is less than the static strength of a related variety. “The fatigue strength of concrete is defined as a fraction of the static strength that it can support repeatedly for a given number of cycles” (ACI Committee 215, 1997, p.2). Fatigue strength in plain concrete is controlled by a range of loading, loading rates, unconventional behavior of loading, history of load, the properties of material, and the conditions of the environment (ACI Committee 215, 1997, p.2). Low Cycle Fatigue: The state of limit for low-cycle fatigue that is observed during pre-qualification assessments of moment connections when “ductile tearing of the metal in the creases of the beam plastic hinges after a few large-amplitude cycles, is indirectly accounted for in the pre-qualification test requirements” (Lee & Stojadinovic, 2004). Alternatively, the resistance of low-cycle fatigue has been widely considered in bridge structures. Studies have obtained that the plastic behavior of steel under cyclic loading is nonlinear and much dependent on history. In addition the response of the stress-strain in changes within the steel that occurs considerably with straining of cyclic into the range of the plastic. Therefore, “fatigue life in the plastic range may be more accurately described as a function of the cyclic strain amplitude than the cyclic stress amplitude” (Lee & Stojadinovic, 2004). The estimation of the fatigue design and the effects has also been determined by different studies. The test data of low-cycle fatigue for a group of diverse invariable strain amplitude tests are generally exhibited by a “logarithmic plot of strain amplitude versus the number of cycles to failure at that amplitude” (Lee & Stojadinovic, 2004). In case of plastic strain amplitude, the total strain amplitude may also be used for the test. These plots characteristically represent around the linear relation “between the number of cycles to failure and strain amplitude in the log-log space. Using a log-log linear approximation, expected fatigue life may be computed from an S-N curve suggested by Manson and Coffin as follows: NSm = K where N is the number of cycles to failure, S is a constant total or plastic strain amplitude, and K and m are material properties obtained from tests. Experiments show that m has a value of approximately 2 for plastic strain amplitudes and approximately 3 for total strain amplitudes” (Lee & Stojadinovic, 2004). Fatigue Loading of Structural Members: Patterns of cyclic loading may be represented through the figure provided in Appendix A. There are three different types of fatigue loading that are considered for structural components including columns under the cyclic wave loading. These include the zero-to-max-to-zero, varying loads superimposed on a constant load, and fully reversing load. Zero-to-max-to-zero is the type where a part of the structure which is carrying no load is subjected to a load later, and again the load is removed, hence the first part is returned to no-load condition. Example of this type includes chain used to haul lugs behind a tractor. An example of the varying loads superimposed on a constant load is the suspension wires in a railroad bridge. The wires constitute an unvarying static tensile load that is obtained from the weight of the bridge, accompanied by a supplementary tensile load when the train is on the bridge. Fully-reversing load is the type where once this type of loading occurs “when a tensile stress of some value is applied to an unloaded part and then released, then a compressive stress of the same value is applied and released. A rotating shaft with a bending load applied to it is a good example of fully reversing load” (Cyclic/Fatigue Loading of Structural Members, n.d., pp17-18). Discussion: From the literature review and the findings it can be clearly said that the use of fatigue designs in column structures prove to be a significant factor as far as concerns regarding cyclic wave loading is concerned. Hence the estimations and calculations of the fatigue designs also prove to be highly essential. Fatigue failures have generally been obtained to occur at the surface of the materials. The reasons for these include that fibers that are the mainly highly-stressed are positioned at the exterior or surface, particularly in case of bending fatigue and, the inter-granular defects which reflect impulsive tension failures are more regularly obtained at the surface. In this regard it is necessary to identify and recognize the level of endurance of a material (Cyclic/Fatigue Loading of Structural Members, n.d., p.21). Considering structural designs, there are two kinds of failures that are taken into concern- fatigue and fracture. The fatigue of a component is hard and critical to design. “If a material is about to fail due to cyclic loading, this means the maximum stress values are less than the ultimate tensile stress limit, and thus the material will eventually fail to a smaller subjected load” (SS- Fatigue, 2010). From the significance of the study, it can be observed that owing to the fatigue failures in the columnar structures, cracks are gradually formed, that may accumulate at a microscopic level of the material and hence not visible at the initial stage. Eventually these cracks would reach the exterior of the material and the gradual cracks would lead to fractures in the materials (SS- Fatigue, 2010). It is also studied that the fatigues in the materials do not occur as a sudden process but occurs in gradual manner. Hence a significant period of time is involved in the process. The figure below represents the location where the fatigue actually occurs: Figure 1: Fatigue Occurrence (SS-Fatigue, 2010). It can be observed and learnt from the figure that the fatigue takes place between the eventual tensile strength and fracture stress. Cracks are generally formed before the tensile strength. However, the fatigue design may be built after this stage as well. Thus it can be said from the study that the fatigue designs in columns under cyclic wave loading need to consider the conditions of the waves significantly before designing. This requires extensive calculations and estimations of the wave conditions as well as of the fatigue failures. The modes and operations of the designs building are largely dependent on these factors. Moreover the type of the fatigue failure also needs to be analyzed. The frequent loads on the structures of columns are of severe concern that has led to several studies in regard to fatigue designs. Newer identifications are thought of to bring solutions to these issues as well. Figure 2: S-N relationship (Vibration Fatigue Theory, n.d.). From the figure it is reflected that under constant amplitude cyclic loading, a linear relationship exists between cycles to failure N and applied stress range S when plotted on log-log paper. Conclusion and Research Limitation: From the above study it can be concluded that fatigue design in columns under cyclic wave loading are a matter of significance considering the effects of frequent loads on the structures. The design needs to depend on the level of failure that can occur on the material and its surfaces gradually over a period of time causing damage and fatigue in the structures. Hence careful and sincere estimations and calculations are essential to be determined in order to successfully adapt the design that might prevent the fatigue of the columnar structures if affected through cyclic wave loading. The types of fatigue might also depend according to the structure and the loads that the structures are encountered with. It is highly essential to prevent the fatigues that would otherwise lead to fractures of the structures, the two primary failures known to affect the structures and their materials. The primary limitation of the study has been the use of secondary resources. Since the study has been based on secondary studies, no primary study could be conducted and the entire study has been dependent on earlier researches. Moreover, since the secondary resources might not always be trusted, care was needed to select the correct and authentic sources such that the study could be determined as authentic and reliable as was possible. Moreover there were not much sources available to discuss on the concerned topic that too created limitations for the research study. Recommendations: From the above study it can be recommended that while designing the structures of joints, and beams, the wave cyclic loading of the regions need to be studied from before to have a clear idea and concept of the effects that the structures might encounter in the later periods of time. When measurements of calculations and estimations are available, experts need to be discussed and these methods need to be carefully made use of to determine the status of the structures as well as the conditions of the waves. Prior determinations of these factors, and extensive research on the types of fatigues and their consequences would help the construction experts to determine materials and measures accordingly. Moreover, there needs to be monitoring measures to obtain the conditions of the structures on a regular basis such that fatigues when initiated might be detected and corrective measures taken before they gradually turn into fractures for the structures. References 1) ACI Committee 215 (1997), Considerations for Design of Concrete Structures Subjected to Fatigue Loading, civilwares, available at: http://civilwares.free.fr/ACI/MCP04/215r_74.pdf (accessed on October 17, 2012) 2) Collins, H. (2010), Creative Research: The Theory and Practice of Research for the Creative Industries, West Sussex: AVA Publishing 3) Cyclic/Fatigue Loading of Structural Members (n.d.), WITS, available at: http://wiredspace.wits.ac.za/bitstream/handle/10539/4714/CHAPTER%202.pdf?sequence=2 (accessed on October 19, 2012) 4) International Maritime Organization (1990), Resolutions, IMO Publishing 5) Kiyomia, O. (1988), Fatigue Properties of Prestressed Beam Column Joints, With Emphasis on Offshore Structures Subject to Wave Loading, PCI Journal, pp.139-163, available at: http://www.pci.org/view_file.cfm?file=JL-88-NOVEMBER-DECEMBER-9.pdf (accessed on October 17, 2012) 6) Lee, K. & Sojadinovic, B. (2004), Low-cycle Fatigue Limit on Seismic Rotation Capacity for US Steel Moment Calculations, IITK, available at: http://www.iitk.ac.in/nicee/wcee/article/13_90.pdf (accessed on October 18, 2012) 7) Malhotra, N.K. (2008), Marketing Research: An Applied Orientation, 5/E, India: Pearson Education India 8) Marr, W.A. et al (1981), Permanent Displacements Due to Cyclic Wave Loading, geocomp, available at: http://www.geocomp.com/files/technical_papers/Permanent%20Displacements%20Due%20to%20a%20Cyclic%20Wave%20Loading.pdf (accessed on October 16, 2012) 9) Shi, G. et al (2012), Experimental and modeling study of high strength structural steel under cyclic loading, Engineering Structures, Vol.37, pp.1-13, available at: http://www.sciencedirect.com/science/article/pii/S0141029611005013 (accessed on October 19, 2012) 10) SS- Fatigue (2010), miesandpeas, available at: http://miesandpeas.blogspot.in/2010/06/ss-fatigue.html (accessed on October 19, 2012) 11) Thomas, R.M. (2003), Blending qualitative & quantitative research methods in theses and dissertations, London: Corwin Press 12) Vibration Fatigue Theory (n.d.), mscsoftware, available at: http://www.mscsoftware.com/training_videos/patran/reverb3/index.html#page/Fatigue%20Users%20Guide/fat_vibration.09.7.html (accessed on October 19, 2012) Appendices Appendix A: Cyclic Loading Patterns (Shi et al, 2012pp.1-13). Read More