Static And Fatigue Tests On T-Joints Stiffened By An Internal Ring

Abstract

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.

Chord failure resistance of 3D cut welded connections with CHS columns and through I‐beams

“…According to the same approach currently adopted by the EC3 part 1.8 in case of CHS to externally welded beam connections, the flexural resistance of through-all beam connections can be predicted by multiplying the axial load defined in (9) and the beam height: = (4 + 20 2 ) 0 2 5 ℎ 1 (11) As it has already been shown in paragraph 4 about Voth's proposal, it is possible to prove the inaccuracy of (11) in predicting the flexural resistance in the case of through-all beam connection. For this reason, basing on the FE outcomes, a new analytical model based on the original approach of Togo [18], has been proposed (Figure 16).…”

Mechanical Modelling of the Strength and Stiffness of Circular Hollow Section Tube under Localised Transverse Compression and Tension

“…Even though this solution requires a relevant number of joints, it has the beneficial effect of optimising the design of the beams limiting their depth and, consequently, the oversizing of brittle nodal components. Nevertheless, the main drawback of this practice is the need to manufacture complex beam-to-column connections [ 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 ], usually characterised by the use of collar plates and stiffeners.…”

Component Modelling of Connections Between Circular-Hollow-Sections and Through-All Members

“…Nevertheless, the 3D-LCT is a good solution when very complex geometrical components have to be manufactured; an example of such a case is represented by steel beam-to-column connections between circular hollow section (CHS) columns and double-tee beams. In fact, until now, except for the study of the behaviour of tubular profiles ( [1]- [4]), some different solutions for coupling the aforementioned profiles have been conceived: i) the beam is simply welded to the external surface of the column [5]; ii) plates, welded to the flanges of the double-tee profile, intersect the column [6]; iii) the beam is welded to collar plates externally welded or bolted to the column [7]; iv) the connection is filled with concrete, while the beam is welded to the external surface of the column [8]. The first solution is very easy to manufacture, but the obtained connection provides low stiffness and resistance such that it can be adopted only in the case of pinned or semi-continuous frames [9].…”