With offshore structures constructed of steel tubular members, the greatest stresses occur in joints, making them the weakest parts of the entire structure. This paper presents the results of static and fatigue tests conducted on tubular T-joints. Test specimens included stiffened and unstiffened joints. The former were complex joints stiffened by means of an internal ring in the chord. The load was applied to the branch in the axial direction. Test results showed that a stiffening ring used in the chord significantly increases both the static and fatigue strength of the T-joint. These tests were also useful in establishing the static and fatigue design method for T-joints stiffened by an internal ring. INTRODUCTION Steel pipe is widely used for the structural members of offshore structures because it exhibits excellent resistance to fluid forces caused by currents and waves. Other advantages of steel pipe include buoyancy and the non-directionality of section properties. But steel pipe is susceptible to local loads applied vertically to its surface, a drawback that makes tubular joints the weakest points of an offshore structure; Moreover, since offshore structures are constantly subjected to fluctuating loads due to wave forces, joints in which significant stresses occur are prone to fatigue failure. Because tubular joints are of intricate geometrical configuration, it is difficult to determine their stress distributions and ultimate static strength by means of analytical methods. Therefore, laboratory tests have been used as the primary means of obtaining these data. API 1 and AWS 2 design criteria for tubular joints, which are now widely used in the design of offshore structures, were established through laboratory tests, as reported by Marshall. We conducted static and fatigue tests to evaluate stiffening methods for tubular joints. In these tests, we used unstiffened T-joints as well as complex joints stiffened by a ring placed inside the chord. This paper reports the test results for the static and fatigue strength of T-joints under axial loads on the branch. In designing offshore structures, ultimate static strength and local strains must be estimated, and the appropriate S-N curve selected. Accordingly, we made these studies a part of our tests. The method of stiffening with an internal ring can be applied to any steel tubular joint having a diameter large enough to permit a stiffening ring to be welded on the inside. STATIC TEST Testing Method and Test Results We used two unstiffened and seven stiffened joint specimens. In each of the seven stiffened joints, a ring of a different width wall thickness and material was placed inside the chord. The specimen configuration is shown in Fig. 1. Fig. 2 shows the test setup. The specifications of the specimens are shown in Table 1, together with tensile test results for the steels used. Internal rings for stiffened joints, with the exception of specimen R-1-3, consisted of four pieces built up by butt welding, as shown in Fig. 1. For comparison, a single-piece ring was used for specimen R-1-3.
A flexible string is used at various places. Typical examples are a cable for power transmission and communication and a wire for cranes and elevators. In addition, there are the uses such as a fly-fishing line, climbing rope and a mowing machine to cut turf by turning a nylon string. In the past study, a string pendulum was paid attention as the basic motion of the string. An analysis model of the string pendulum was made and appropriateness of the modeling and the behavior of the string pendulum were clarified. However, axial elongation is not considered. In fact, the axial elongation affects their behavior such as axial micro vibration or restoring force of the elongation. Therefore, it is necessary to consider the axial elongation to grasp the behavior of the string more exactly. In this study, an accelerated motion of the string such as the casting motion is focused on and analyzed in consideration the axial elongation then appropriateness of the modeling is verified by comparing experimental results and analysis results. Furthermore, relations of the axial elongation and an accelerated motion are investigated. In this paper, the behavior is investigated focusing on the velocity of the tip of the string. As a result, strain energy is stored and then converted into kinetic energy, thereby increasing the velocity.
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