J. J. R. Cheng

Buckling of stiffened steel plates—a parametric study

The stability of plates stiffened with tee-shape stiffeners was investigated using a finite element model. Four series of stiffened plate panels were modeled using a finite strain fournode shell element. The model was validated using the results of tests on full-size stiffened plate specimens and was subsequently used to perform the study of various parameters presented in this paper. The parameters investigated are: the shape and magnitude of initial imperfections in the plate; residual stress magnitude and direction of applied uniform bending; plate slenderness ratio; plate aspect ratio; and plate to stiffener cross-sectional area ratio. The effect of the investigated parameters on the axial load carrying capacity and the mode of failure of stiffened plates is investigated both in the elastic and inelastic ranges. A comparison of these results with design guidelines formulated by Det norske Veritas and the American Petroleum Institute indicates that the guidelines are generally conservative for cases where initial imperfection magnitudes do not exceed the guidelines’ prescribed maximum.

Punching of Two-Way Concrete Slabs with Fiber-Reinforced Polymer Reinforcing Bars or Grids

This paper presents 4 tests that compare the behavior of steel and fiber-reinforced polymer (FRP) reinforced slab-column connections. Existing test results for punching of FRP-reinforced slabs are also summarized. Two approaches for predicting punching strength of a slab-column are studied. The 1st is an empirically based fit of the available data following the same format of design equation as offered by Matthys and Taerwe (2000). The 2nd is an adaptation of an equilibrium-based model for slab-column connection behavior proposed by Alexander and Simmonds. Both methods give acceptable results when compared with available test data.

Behavior and design of gusset plate connections in compression

Abstract A total of thirteen full-scale tests were conducted to investigate the compressive behavior and strength of gusset plate connections. The test parameters included gusset plate thickness, size, and brace angle. In addition, the effects of frame action on the compressive behavior of gusset plate connections were also investigated. In general, the gusset plate specimens were failed by sway buckling of the connection since no out-of-plane restraint was provided from the bracing member. The test results indicated that, in general, significant yielding of the gusset plate specimens occurred prior to reaching the ultimate load. However, only limited yielding was observed for the thin specimens with a plate thickness of 6.5 mm. The ultimate load of the specimens increased almost linearly proportional to the gusset plate thickness and decreased with increasing plate size. A slight decrease in the ultimate load of the specimens was observed when a 30° brace angle was used instead of a 45° one. The beam and column moment had only negligible effects on the ultimate load of the specimens; however, yielding of the specimens was detected at a load level significantly lower than that had no framing moment. The numerical simulation provided good agreement with the test results.

Design for lateral torsional buckling of coped I-beams

Abstract The lateral torsional buckling of coped steel I-beams was investigated extensively by a number of researchers. Design recommendations have been proposed by Cheng and Yura. They proposed a set of interaction equations to evaluate the elastic lateral torsional buckling capacity of coped steel I-beams based on the interaction of the lateral torsional buckling capacity of the coped (tee-section) and the uncoped regions. However, these interaction equations may not be appropriate for sections with a small ratio of cope length to web depth of the tee-section because local web buckling of the tee-section may occur. In this paper, modified interaction equations based on an analytical study of tee-sections are proposed with improvement over Cheng and Yuras interaction equation. For symmetrical loading, the use of the equivalent moment coefficient C b in Cheng and Yuras design interaction formula for calculating the critical moment of coped I-beams may produce non-conservative results. Subsequently, a modified interaction formula is proposed.

Initial imperfection models for segments of line pipe

Initial imperfections have long been acknowledged as having an effect on the behavior of shell structures, affecting both the global and local behavior. Yet, despite their significance, initial imperfections are rarely included in analytical models for pipelines. This is usually because of the complicated nature of initial imperfections, the difficulty in measuring them, and the small amount of available literature that describes their geometry. Some recent investigations at the University of Alberta in Edmonton have focused on the effect of initial imperfections on the behavior of segments of line pipe. Imperfections measured across the inside surface of pipe test specimens were incorporated into a finite element model that was developed to predict the experimental behavior of the specimens tested under combined loads of internal pressure, axial load and bending moment. Test-to-predicted ratios for the load carrying capacity of the test specimens had a mean value of 1.035 with a coefficient of variation of 0.047. The improvements in the accuracy of the finite element analysis models that include the initial imperfection pattern indicate their importance in modeling the experimental behavior. Once the importance of initial imperfections was established, idealized patterns were developed to simplify numerical modeling. This paper presents the results of different patterns investigated for both plain and girth-welded segments of line pipe and provides recommended simplified assumed initial imperfection patterns.

Mechanical Properties of Cold Bend Pipes

Cold bends are frequently required in energy pipelines in order to change the vertical and horizontal orientations of the pipeline route. They are produced by plastically bending a pipe joint in a cold bending machine by creating a series of uniformly spaced incremental bends. This procedure acts to reduce the moment capacity and buckling strain of the pipe, and studying the changes in pipe properties caused by cold bending is valuable in assessing the level of this strength reduction. Accordingly, the initial imperfections and material transformations of five full-scale cold bend pipes were assessed in this research program. The imperfections were measured at several locations around the circumference of the specimens, along the entire bend length. It was determined that the distribution of imperfections was similar in shape to a sine function, and their amplitude ranged from 0.3mmto1.0mm. Tension coupon tests were conducted on the intrados, extrados, and virgin materials of the specimens. It was revealed that the extrados material exhibited an increase in yield strength due to work hardening and that the intrados material demonstrated a reduction in yield strength due to the Bauschinger effect. It was established that the imperfections, and material transformations in the specimens were predominantly unaffected by the incremental-bend magnitude or spacing that was employed during the cold bending procedure.

Full-Scale Tests of Cold Bend Pipes

An experimental program sponsored jointly by SNAM Rete Gas, Tokyo Gas Co., Ltd. and TransCanada Pipelines Ltd. was conducted on cold bend pipes under combined loading. These tests were designed to study the local buckling behavior and to develop the critical compressive strain criteria for cold bend pipes under combined loading. The test program includes eight full-scale specimens of NPS24 and NPS30 pipes with pipe thickness up to 14.3 mm. The test parameters include different D/t ratios (44, 69, and 93), material grades (X60, X65, and X80), bend angles (1.0 to 1.5 degree/diameter), and operation pressures (0%, 40%, 60%, and 80% of SMYS). In addition to full-scale tests, initial imperfections and residual strains due to cold bend processes were also measured. This paper describes the test specimens, test setup, instrumentation, and test procedures used in the program. A brief discussion of the test results is also covered in the paper.Copyright

Monitoring and Prediction of Pipe Wrinkling Using Distributed Strain Sensors

As exploration of energy resources develops further into the remote Canadian North, pipeline construction is being pushed further into the unknown. The new pipeline construction in the North has generated a great need for understanding and predicting the behaviour of pipelines under harsh northern environmental conditions. Continuous real-time monitoring technology using distributed strain sensors has become a possible method for monitoring the performance of these pipelines in the field. The aim of this research is to find the correlation between the distributed strains along the line pipes and the local buckling (wrinkling) of these pipes, and to study the contribution of these distributed strains to the detection of initial wrinkle of buried pipelines. Both experiments and finite element analyses (FEA) concluded significant findings in the current research. Conventional strain gauges and advanced Brillouin Scattering fibre optic sensors (BSFOSs) were employed in the experimental programs. BSFOSs can measure the distributed strains spacing as close as 50 mm along the line pipes, so that these sensors can detect the wrinkle location along the monitored pipeline. The distributed strains measured from the BSFOSs and from conventional strain gauges in longitudinal direction are presented in this paper. The methodology for detecting the initiation of pipe wrinkling and finding the optimal positions of installation of the distributed sensors are proposed.Copyright

Comparison of damage development in random fiber-reinforced polymers (FRPs) under cyclic loading

It has been well known that suppression of debonding and matrix cracking could improve fatigue resistance of fiber-reinforced polymers (FRPs). In this study, the roles of these two mechanisms on the damage development in FRPs with in-plane random glass fiber reinforcement have been investigated. Two polymers are used as the matrix - isophthalic polyester and polyurethane. Polyurethane-based FRP shows higher ultimate tensile strength (UTS) and strain to failure, but lower elastic modulus. Under zero-tension fatigue loading (with the maximum stress level equivalent to 50% of their respective UTS), the change in modulus, energy dissipation rate, and the corresponding damage development process are investigated. The damage development is analyzed at the macroscopic and microscopic levels, and found to be closely related to the modulus degradation and change in energy dissipation rate. The study concludes that the two FRPs show significantly different behavior under fatigue loading. The polyurethane-based FRP had better fatigue resistance, in view of the mild modulus change and the capability of absorbing energy through plastic deformation. Results from the study suggest that the excellent fatigue resistance of the polyurethane-based FRP is due to good toughness of the matrix.

Behavior of Cold Bend Pipes under Bending Loads

Ground movement along a buried pipeline often places the line in net compression. The local buckling resistance at cold bends is less than at straight sections, and hence, local buckling tends to occur at bends. Correspondingly, this full-scale testing program was conducted to investigate the local and global load-deformation relationships for seven cold bend pipes under applied loads. The specimens were loaded under bending, axial load, and internal pressure until local buckling occurred, and their curvatures were subsequently increased to establish their postbuckling behaviors. It was demonstrated though these tests that the buckling strain of the cold bend pipes was considerably lower than a straight pipe and that the presence of internal pressure exhibited a predominant role on the specimens load-deformation behaviors. It was also established that the behavior of the specimens was relatively unaffected by the bending methods that were employed during cold bending.