Nadeem A. Siddiqui

Fatigue and fracture reliability of TLP tethers under random loading

Tension leg platform (TLP), a deep water oil exploration offshore compliant system, is vulnerable to considerable fatigue damage over its design life period due to the dynamic excitations caused by the oscillating waves and wind. In the presence of random loading like, that produced by the wind and waves, reliability against fatigue and fracture failure becomes important. The reliability assessment against fatigue failure considers the uncertainties associated with the parameters and procedures employed for the fatigue damage estimation. This fatigue estimate requires a dynamic response analysis under various environmental loadings. In the present study a non-linear dynamic analysis of the platform has been carried out for response calculations. The response histories so obtained have been employed for the study of fatigue reliability analysis of TLP tethers under long crested random sea and associated wind. Fatigue damage estimation of tether joints is made using Palmgren–Miners rule (S–N curve approach) and fracture mechanics approaches. The stress ranges employed are described by Rayleigh distribution. Non-linear limit state functions based on above two approaches are derived in terms of random variables. First order reliability method (FORM) and Monte Carlo simulation technique have been employed for reliability estimation. The influence of various random variables on overall probability of failure has been studied through sensitivity analysis.

Textile-Reinforced Mortar versus FRP as Strengthening Material for Seismically Deficient RC Beam-Column Joints

In this paper, efficiency and effectiveness of textile-reinforced mortars (TRM) on upgrading the shear strength and ductility of a seismically deficient exterior beam-column joint has been studied. The results are then compared with that of carbon fiber-reinforced polymer (CFRP) and glass fiber-reinforced polymer (GFRP)-strengthened joint specimens. Five as-built joint specimens were constructed with nonoptimal design parameters (inadequate joint shear strength with no transverse reinforcement) representing an extreme case of preseismic code design construction practice of joints and encompassing the vast majority of existing beam-column connections. Out of these five as-built specimens, two specimens were used as baseline specimens (control specimens) and the other three were strengthened with TRM, CFRP, and GFRP sheets, respectively. All five subassemblages were subjected to quasi-static cyclic lateral load histories to provide the equivalent of severe earthquake damage. The response histories of control and strengthened specimens were then compared. The test results demonstrated that TRM can effectively improve both the shear strength and deformation capacity of seismically deficient beam-column joints to an extent that is comparable to the strength and ductility achieved by well-established CFRP, and GFRP-strengthening of joints.

Seismic Response of FRP-Upgraded Exterior RC Beam-Column Joints

Shear failure of exterior beam-column joints is identified as the principal cause of collapse of many moment-resisting frame buildings during recent earthquakes. Effective and economical strengthening techniques to upgrade joint shear resistance and ductility in existing structures are needed. In this paper, efficiency and effectiveness of carbon fiber-reinforced polymer (CFRP) sheets in upgrading the shear strength and ductility of seismically deficient exterior beam-column joints have been studied. Four as-built joints were constructed with nonoptimal design parameters (inadequate joint shear strength with no transverse reinforcement) representing preseismic code design construction practice of joints and encompassing most of existing beam-column connections. Out of these four as-built specimens, two specimens were used as baseline specimens (control specimens) and other two were strengthened with CFRP sheets under two different schemes (strengthened specimens). In the first scheme, CFRP sheets were epo...

Reliability analysis against progressive failure of TLP tethers in extreme tension

Abstract The tension leg platform (TLP) is a moored floating structure whose buoyancy is more than its weight. The mooring system of TLP consists of number of tensioned tethers connected to the columns at the top and anchored to the seabed at the bottom. These tethers are vulnerable to failure due to extreme (maximum and minimum) tensions. The reliability study of these tethers is an important subject of study. The reliability may further deteriorate after the failure or removal of any of these tethers. In the present study, reliability analysis of an intact TLP and TLP with one tether missing has been carried out. A Von-Mises failure criterion has been adopted to define the failure of tether against maximum tension. The minimum tension failure occurs when the tethers slack due to loss of tension. The limit state functions for maximum and minimum tension cases have been derived. The computationally efficient algorithm First order reliability method (FORM) has been adopted for reliability calculations. Results are presented in terms of reliability indices and probabilities of failure for each sea state. The probabilities of failure so obtained for different sea states have been used for the assessment of annual and life time probability of failures. Reliability of the TLP with one tether missing has been compared with that of the corresponding intact TLP. Effect of wind has also been studied on the progressive failure of TLP tethers and the results have been compared with that of an intact TLP.

Seismic Behavior of As-Built, ACI-Complying, and CFRP-Repaired Exterior RC Beam-Column Joints

In this paper, the efficiency and effectiveness of carbon-fiber-reinforced polymer (CFRP) sheets for upgrading the shear strength and ductility of a seismically deficient exterior beam-column joint were studied and compared with an American Concrete Institute (ACI)-based design joint specimen. One as-built joint specimen, representing the preseismic code design and construction practice for joints and one ACI-based design joint specimen, satisfying the seismic design requirements of the current code of practice were cast. The as-built specimen was used as baseline (control) specimen. These two specimens (i.e., the as-built control and the ACI-based specimens) were subjected to cyclic lateral load histories to induce damage equivalent to damage expected from a severe earthquake. The damaged control specimen was then repaired by filling its cracks with epoxy and externally bonding CFRP sheets to the joint, the beam, and part of the column regions. This specimen was identified as the repaired specimen. The repaired specimen was subjected to a similar cyclic lateral load history, and its response history was recorded. The response histories of the as-built control, the repaired, and the ACI-based design specimen were then compared. The test results demonstrated that externally bonded CFRP sheets can effectively improve both the shear strength and the deformation capacity of seismically deficient and damaged beam-column joints to a state comparable to the ACI-based design joint.

Reliability analysis of nuclear containment without metallic liners against jet aircraft crash

The present study presents a methodology for detailed reliability analysis of nuclear containment without metallic liners against aircraft crash. For this purpose, a nonlinear limit state function has been derived using violation of tolerable crack width as failure criterion. This criterion has been considered as failure criterion because radioactive radiations may come out if size of crack becomes more than the tolerable crack width. The derived limit state uses the response of containment that has been obtained from a detailed dynamic analysis of nuclear containment under an impact of a large size Boeing jet aircraft. Using this response in conjunction with limit state function, the reliabilities and probabilities of failures are obtained at a number of vulnerable locations employing an efficient first-order reliability method (FORM). These values of reliability and probability of failure at various vulnerable locations are then used for the estimation of conditional and annual reliabilities of nuclear containment as a function of its location from the airport. To study the influence of the various random variables on containment reliability the sensitivity analysis has been performed. Some parametric studies have also been included to obtain the results of field and academic interest.

Numerical Investigations on the Seismic Behavior of FRP and TRM Upgraded RC Exterior Beam-Column Joints

AbstractIn this paper, a detailed procedure for nonlinear finite-element analysis of fiber reinforced polymer (FRP) and textile reinforced mortar (TRM) upgraded reinforced concrete (RC) beam-column exterior joints is presented for predicting their seismic performance under simulated earthquake loading. The finite-element (FE) model was developed using a smeared cracking approach for concrete and three-dimensional layered elements for the FRP and TRM-composites. The results obtained from FE analysis were compared with the test results. The tests were conducted on four as-built exterior beam-column joint specimens under simulated seismic loads. Out of these four specimens, one specimen was tested as a control specimen and the other three were tested after strengthening with TRM, carbon FRP, and glass FRP sheets, respectively. The FE results were compared with the test results through load-displacement behavior, ultimate loads, and crack pattern. Comparison of FE results with the experimentally observed resp...

Reliability analysis of TLP tethers under impulsive loading

In the present study, reliability assessment of Tension Leg Platform (TLP) tethers against maximum tension (i.e. tension exceeding yield) has been carried out under combined action of extreme wave and impulsive forces. For this purpose, a nonlinear dynamic analysis of TLP has been carried out in time domain. A limit state function for maximum tension (i.e. tension exceeding yield) has been derived employing Von-Mises theory of failure. Using this derived limit state function and responses obtained after dynamic analysis under sinusoidal, half-triangular and triangular impulsive forces, reliability assessment of the TLP tethers has been carried out. Design point, important for probabilistic design of tethers, has been located on the failure surface after solving a constrained optimization problem. To study the influence of various random variables on tether reliability, sensitivity analysis has been carried out. Effects of angle of impact; effect of variable submergence; and effect of material yield strength on tether reliability have also been studied on parametric basis. Effect of uncertainty on overall tether reliability has also been discussed to show the importance of quality control in the various design parameters.

Reliability analysis of projectile penetration into geological targets

Abstract The present study is dedicated to the reliability analysis of projectile penetration into a buried concrete target. The expressions for depths of penetration in the buried target have been derived for crater and tunnel regions separately using the methodology proposed by earlier investigators. These penetration depths have then been employed for subsequent reliability analysis using first-order reliability method. Design points, important for probabilistic design, have been located on the failure surface. Sensitivity analysis has been carried out to study the influence of various random variables on projectile reliability. Some parametric studies have also been included to obtain some results of field interest.

Reliability analysis of a buried concrete target under missile impact

This paper presents the methodology for reliability analysis of a buried concrete target against normal missile impact. The expressions for the depths of penetration in buried target have been derived using the methodology proposed in the literature. These equations have then been employed for reliability estimation. Design points, important for the probabilistic design, have been located on the failure surface. Sensitivity analysis has been carried out to study the influence of various random variables on target safety. Some parametric studies have also been included to obtain the results of field interest.