Structural behavior of pressure locked gratings

Abstract

Pressure locked gratings have many uses, both on land and at sea. This is mainly due to the high degree of flexibility in dimensions and load bearing capacity. Despite the many uses in our surroundings, the term pressure locked grating is unknown to many. This term refers to the way in which the most important connection in pressure locked gratings is manufactured. Pressure locked gratings are galvanized and/or coated to prevent corrosion, galvanization is almost always used. This thesis is primarily focused on the behavior of galvanized pressure locked gratings. A pressure locked grating is made up of three type of bars: load bearing bars, cross bars and support bars. The connections between the load bearing bars and the cross bars are realized by pressing the cross bar into premade notch in the load bearing bar. The bars will deform plastically, which results in a connection based only on friction. This connection is called a pressure locked connection. The load bearing bars serve primarily for load transfer in one direction. The pressure locked connection weakens the load bearing bars. The cross bars redistribute a local load over the load bearing bars outside of the loading area and reduce the buckling length of the individual load bearing bar. The support bars are spot-welded to both the load bearing and cross bars. Pressure locked gratings are calculated according to RAL-GZ-638, a standard published by the Deutsches Institut für Gütesicherung und Kennzeichnung e.V. In this standard, pressure locked gratings are calculated on the basis of linear elasticity theory. A calculation according to the EN 1993-1-1 is not possible because it is not suited for pressure locked gratings. The main objective of this thesis is to determine if a calculation based on the plasticity theory is possible for pressure locked gratings. Through the application of this theory the design strength can be greatly increased, which has a large economic potential. This research is divided up into four parts that are stated below. • Analytical study • Experimental study • FEM validation • Parametric study The vast diversity in size and properties and the complex functioning of the pressure locked connection make pressure locked gratings a difficult product to research. The pressure locked connection also has a big influence on the strength and stiffness. In this galvanizing has a big influence. The analytical study shows that pressure locked gratings fail through: local lateral torsional buckling of one of the load bearing bars, global lateral torsional buckling for the entire pressure locked grating or by a mechanism due to a plastic hinge in de load bearing bars. To research the behavior, numerical models were developed in the FEM-package ABAQUS. To determine the accuracy of these models, experiments were carried out on a number of pressure locked grating geometries. The results of the tests were compared with FEM simulations which made it possible to validate the FEM-models. The validation has taken place on the basis of four test series. Three test series focused on failure mechanisms, namely: collapse due to a mechanism as a result of plastic hinges in the load bearing bars, test series 1A, collapse due to local lateral torsional buckling, test series 2A and collapse due to global lateral torsional buckling of the entire pressure locked grating, test series 2B. The last test series, test series 1B, is aimed at examining the structural behavior of a common used pressure locked grating. After this validation it is possible to make FEM-models for a large number of different geometries. The behavior of these geometries can then be analyzed. The validation shows that failure mode and failure load are correctly determined by the FEM models. There is however a notable difference between the stiffness of the FEM-model and the stiffness of the test specimens. This difference is likely the result of the influence of the pressure locked connection between the load bearing and cross bars. The research shows that all aforementioned failure modes play a role depending on the dimensions of load bearing bars and cross bars. For relatively thick-set load bearing bars pressure locked gratings fail due to a mechanism as the result of plastic hinges in the load bearing bars. For slender load bearing bars pressure locked gratings fail as a result of instabilities. With the aid of FEM models, it is possible to estimate the extent in which the plastic capacity is present in actually produced pressure locked gratings. This analysis shows that in 72% of the produced pressure locked gratings, the full plastic capacity of the load bearing bars can be reached to 96% or more. It is however important to keep in mind that, the dimensions of the load bearing bars are determined by deflection or by strength. This is determined by the load bearing bar height and span. When deflection is determining the dimensions of the load bearing bars, a plastic calculation methodology is not useful, because it only increases the design strength. It can thus be stated that the full plastic capacity cannot be fully used. On the basis of this study it is not yet possible to apply a plastic calculation methodology on pressure locked gratings. This is because this research shows that the pressure locked connection can have a big influence on plastic strength. It is not clear if the pressure locked connection always has the same influence on the plastic capacity. It is possible that the plastic capacity is influenced by fatigue, corrosion and manufacturing tolerances.