Effects Of Joint Flexibility Research Articles

Effect of Joint Flexibility on Seismic Performance of a Reinforced Concrete Ductile Moment-Resisting Frame

Currently, the seismic collapse risk of modern code-conforming reinforced concrete (RC) frame structures is often evaluated without considering the behavior of the beam-column (BC) joints since joints designed to meet modern concrete design codes are assumed to be sufficiently rigid. BC joints in RC ductile moment frames may undergo a significant amount of inclined cracking during strong earthquakes. Consequently, the stiffness, strength, and dynamic characteristics of the RC moment frames may vary. Hence, the rigid joint assumption may be unsuitable for assessing the seismic performances of RC ductile moment frames because the local strength and ductility demands of the constituent members can be misinterpreted by structural analyses based on the assumption of a rigid joint. In this study, nonlinear static and dynamic analyses are conducted to quantify the seismic response variations of low- to midrise RC-frame structures under different joint modeling assumptions. In total, six 2D continuum finite element models of four- and eight-story frames with rigid and flexible joints were constructed by simulating the actual structures as realistically as possible. The results indicate that the interior joints in the considered frames suffer light-to-moderate damage, and the effect of joint cracking is found to be significant in a four-story structure whose column-to-beam flexural strength ratio and column-to-beam area ratio are below 1.5. Such joint behavior demonstrates that current code provisions cannot ensure joint behavior compatible with the rigid joint assumption that is used in the practical structural analysis of RC-frame buildings.

Comparison of effects of joint flexibility on the lumbo-pelvic rhythm in healthy university students while bending the trunk forward.

[Purpose] To clarify the influence of flexibilities of the hip and lumbar spine joints on bending the trunk forward. [Participants and Methods] We assessed the joint flexibility of 47 healthy university students using the Beighton hypermobility score and assigned them to the group of normal or poor flexibility. We performed electromyography to acquire kinematic data and analyzed the three-dimensional motion while the students bent their trunks forward. Further, we compared the groups based on angular displacements of the hip joint and lumbar spine in each phase of the movement. Offset of the erector spinae and hip extensor muscle activity was calculated as a percentage (%) of the maximum range of motion. [Results] The lumbo-pelvic rhythm differed between participants with and without poor flexibility of the hip joint in the second half of the forward bending task. Participants with poor flexibility of the hip joint showed activation of the erector spinae and biceps femoris for a longer period compared to those with normal flexibility. Notably, flexion–relaxation responses were not found in the biceps femoris of 30% of the participants. [Conclusion] Poor hip joint flexibility may cause low back pain. Measuring the lumbo-pelvic rhythm might help identify individuals at a high risk of low back pain while they are still healthy.

Fidelity of computational modelling of offshore jacket platforms

Fixed offshore jackets are the most commonly adopted rigs for exploration of offshore oil and gas resources. The present paper focuses on the modelling of a 4-legged X-braced jacket type platform using nonlinear finite element analysis. The nonlinear effects of joint flexibility, piled foundations and geometrical imperfections on the platform behaviour are investigated. Joint flexibility is modelled by introducing rigid or flexible joints into the jacket beam element model. A hybrid model, with the critically-loaded leg and connected joints built using shell elements, is applied for the investigation of localised effects on increasing joint flexibility. The soil-pile interaction is modelled using sets of decoupled springs distributed along the piles. The geometrical imperfections, generated based on the failure modes of the platform, are introduced in the compression legs of the jacket. The platform is loaded by operational and environmental loads. The environmental loads are gradually increased until platform failure occurs. The findings of the paper indicate that incorporation of joint flexibility and piled foundations result in the reduction of platform yielding and ultimate strength. The piled foundation affects platform stiffness severely. The imperfections increase platform deformability in the elastic rage and lead to dramatic reduction of jacket base shear capacity.

Dynamic behavior and parameter sensitivity of the free-floating base for space manipulator system considering joint flexibility and clearance

The joint flexibility and clearance greatly affect the dynamic behavior of the free-floating base when space manipulators operate due to the dynamic coupling between the manipulators and the free-floating base. Based on the Lagrange–Euler method and momentum conservation, the dynamic model of a free-floating space manipulator system with joint flexibility and clearance is obtained. With this model, the coupling effects of joint flexibility and clearance on the dynamic response of the free-floating base are analyzed and some case studies are conducted to investigate the effects of different parameters of joint flexibility and clearance on the free-floating base. Further, the parameter sensitivity of the free-floating base for space manipulator system with joint flexibility and clearance is analyzed using the improved response surface method. According to the analysis of response surface method, we can increase the joint clearance size or decrease the dynamic friction coefficient to improve the stability and motion accuracy of the free-floating base position and attitude. This work points out the direction for reducing the base disturbance and provides a basis for the optimization design for free-floating space manipulator system.

Joint stress based deflection limits for transmission line towers

Experimental investigations have revealed significant mismatches between analytical estimates and experimentally measured deflections of transmission towers. These are attributed to bolt slip and joint flexibility. This study focuses on effects of joint flexibility on tower deflections and proposes criterions for permissible deflection limits based on the stresses in joints. The objective has been framed given that guidelines are not available in the codes of practices for transmission towers with regard to the permissible limits of deflection. The analysis procedure is geometric and material nonlinear with consideration of joint flexibility in the form of extension or contraction of the cover plates. The deflections due to bolt slip are included in the study by scaling up the deflections obtained from analysis by a factor. Using the results of the analysis, deflection limits for the towers are proposed by limiting the stresses in the joints. The obtained limits are then applied to a new full scale tower to demonstrate the application of the current study.

Effect of joint flexibility on vibration characteristics of a composite box manipulator

Flexibility of links and joints affects dynamic behavior of manipulators. In this work, the vibration characteristics of a one-link carbon-fiber box manipulator with a rectangular cross-section and a [0/90/0/90] lay-up are studied to examine the effects of the link and joint flexibility. The theoretical results supported with experimental studies are presented to realize the mentioned examinations. An experimental system, which consists of a composite box manipulator driven by a geared servo motor and a wireless measurement system, is introduced. Some parameters of the driver are manually set in the experiments. Vibration tests of the composite manipulator are realized for the stationary and nonstationary cases. Experimental vibration responses are both evaluated for auto and manual tune modes of the servo driver. Theoretical results obtained via the finite element vibration analysis compared with the experimental results. The changes of the natural frequencies and damping properties are presented for the stationary and nonstationary cases. The experimental vibration responses of the composite box manipulator with various payloads match well with the theoretical ones by tuning servo motor parameters manually.

Transient response analysis of tapered FRP poles with flexible joints by an efficient one-dimensional FE model

This research develops a finite element code for the transient dynamic analysis of tapered fiber reinforced polymer (FRP) poles with hollow circular cross-section and flexible joints used in power transmission lines. The FRP poles are modeled by tapered beam elements and their flexible joints by a rotational spring. To solve the time equations of transient dynamic analysis, precise time integration method is utilized. In order to verify the utilized formulations, a typical jointed FRP pole under step, triangular and sine pulses is analyzed by the developed finite element code and also ANSYS commercial finite element software for comparison. Thereafter, the effect of joint flexibility on its dynamic behavior is investigated. It is observed that by increasing the joint stiffness, the amplitude of the pole tip deflection history decreases, and the time of occurrence of the maximum deflection is earlier.

Effect of stressed skin action on the behaviour of cold-formed steel portal frames

This paper describes a series of six full-scale laboratory tests conducted on cold-formed steel portal frame buildings in order to investigate the effects of joint flexibility and stressed skin diaphragm action. The frames used for the laboratory tests were of 6m span, 3m height, 10° pitch and the frame spacing was 3m. Vertical loading was applied in two tests, and horizontal loading was applied in another four tests. The laboratory test set-up represented a building having two gable frames and two internal frames. Tests were conducted on frames having two joint types, both with and without roof sheeting. It was shown that as a result of stressed skin action, the internal frame with roof sheeting resisted approximately three times more horizontal load than the bare frame and the deflection of the internal frame was reduced by 90% relative to the bare frame. When the difference in loads between 2D (bare frame model) and 3D (stressed skin model) were considered, it was shown that the joint flexibility of the frame has a significant effect on the load transfer between frames through the roof sheeting. It was found that the ‘true’ loads transferred to the gable frames are between three and seven times higher than the loads deriving from tributary area. By using stressed skin analysis, it is possible to assess the shear force in the roof sheeting so that damage to the fixings is prevented and a more economical design is possible.

Three-dimensional joint flexibility element for modeling of tubular offshore connections

Intubular jointsofoffshoreplatforms, jointsmay exhibitconsiderableflexibilitysincethematerialsinthechord members near the intersection zone of brace-chord behave as shell-like structures. The resulting flexibility may have sig- nificant effects on the local and overall behavior of offshore platforms. This study describes the derivation of a three-di- mensionaljointflexibilityelement whichisbasedonphysical interpretationofbehaviorofatubularjointupondeformation. Theelementisdevelopedonthebasisofflexibilitymatrixand implemented in a finite element program to account for local joint flexibility effects in numerical models of jacket-type offshore platforms. Results are validated by more sophisti- cated multi-purpose software and compared against conven- tional rigid-joint models which are widely employed in practice. According to the results, the accuracy of response predictionsisimprovedwhentheeffectsofjointflexibilityare considered. This model can be used for predicting more ac- curate response of jacket-type offshore platforms.

Effects of joint flexibility on lateral response of reinforced concrete frames

It is known that shear deformation of beam-column joints have a significant contribution to lateral response of reinforced concrete structures. Most of the available analysis programs assume rigid joints regardless of reinforcement details in the joint region. Slip of the beam longitudinal reinforcement in the joint and shear deformation of joint panel significantly decrease rigidity of the joint, especially in non-seismically detailed beam-column joints. In the current study, these effects were modelled in the OpenSees software framework and a modified joint element for analysis of multi-storey frames was used. The model was verified by experimental results and the verified model was used to analyze five and ten storey frames with various joint details using nonlinear static and incremental dynamic analyses. The results of frames with flexible joints were compared with those of frames with rigid joints assumption. The analytical results confirmed that the modified joint element successfully predicted experimental cyclic behaviour of beam-column joint specimens and also was found that existing RC structures with deficient beam-column joints are very vulnerable when subjected to severe earthquakes and the rigid joint assumption was not appropriate when assessing the behaviour of existing non-seismically detailed structures. The results also showed that the effect of joint deformation on lateral response of RC frames depends on the peak acceleration values of the ground excitation.

Elastic tubular joint element for modelling of multi-brace, uni-planar tubular connections

Traditionally, an analytical model of an offshore structure is composed of beam-column elements rigidly interconnected. However, considerable flexibility exists at the joints where chord outer wall deforms locally, and produces flexibility. In this paper, a new elastic joint element is introduced to consider the local flexibilities for TY- and K-type tubular joints. The element is developed on the basis of flexibility matrix and it is implemented in a finite-element programme to account for local joint flexibility effects in numerical models of steel structures made from tubular members. The element is suitable for modelling of multi-brace, uni-planar tubular joints. The results are validated by more sophisticated model using multi-purpose programmes and compared against conventional rigid-joint models. Based on the results of this study, it is concluded that incorporating the effects of joint flexibility results in more realistic response prediction of offshore structures.

Individual Stiffness of 3D Space Frame Thin Walled Structural Joint Considering Local Buckling Effect

Thin walled beams are widely used to build vehicle structure. Thin walled structure offers high stiffness-to-weight ratio to the vehicle for better handling and fuel consumption. Despite these advantages, thin walled structures will expose to the buckling and joint flexibility effects. For a vehicle structure, 3D space frame is used a lot in designing vehicle structure. However, the stiffness of thin joint is difficult to predict accurately by numerical and analytical model due to buckling effect and the complexity of the joint. The essence of this research work is to determine individual stiffness of 3D space frame members using reduction member method. By knowing the individual stiffness of all members, the stiffness of 3D space frame can be predicted easily. This study will help design engineer to optimize vehicle structure with more efficient way rather than trial and error approach. As a case study, structure around bulkhead area which consists of A pillar, sidesill and cross member will be analyzed.

Nonlinear Adaptive Output Feedback Control of Flexible-Joint Space Manipulators with Joint Stiffness Uncertainties

Growing research interest in space robotic systems capable of accurately performing autonomous manipulation tasks within an acceptable execution time has led to an increased demand for lightweight materials and mechanisms. As a result, joint flexibility effects become important and represent the main limitation to achieving satisfactory trajectory-tracking performance. This paper addresses the nonlinear adaptive output feedback control problem for flexible-joint space manipulators. Composite control schemes in which decentralized simple adaptive control-based adaptation mechanisms to control the quasi-steady-state robot model are added to a linear correction term to stabilize the boundary-layer model are proposed. An almost strictly passivity-based approach is adopted to guarantee closed-loop stability of the quasi-steady-state model. Simulation results are included to highlight the performance and robustness of the proposed adaptive composite control methodologies to parametric and dynamics modeling uncertainties.

Joints flexibility effect on the dynamic performance of robots

SUMMARYThis paper studies the effect of joint flexibility on the dynamic performance of a serial spatial robot arm of rigid links. Three models are developed in this paper. The first and the third models are developed using the multibody dynamics approach, while the second using the classical robotics approach. A numerical algorithm and an experimental test-rig are developed to test the final model. The links' inertial parameters are estimated numerically. Empirical formulae with assumption models are used to estimate the flexibility coefficients. The simulation results show that the joint damping is a major source of inaccuracies, causing trajectory error without a proper feedback controller.

Local joint flexibility element for offshore plateforms structures

A large number of offshore platforms of various types have been installed in deep or shallow waters throughout the world. These structures are mainly made of tubular members which are interconnected by using tubular joints. In tubular frames, joints may exhibit considerable flexibility in both elastic and plastic range of response. The resulting flexibility may have marked effects on the overall behavior of offshore platforms.This paper investigates the effects of joint flexibility on local and global behavior of tubular framed structures in linear range of response. A new joint flexibility element is developed on the basis of flexibility matrix and implemented in a finite-element program to account for local joint flexibility effects in analytical models of tubular framed structures. The element formulation is considerably easy and straightforward in comparison with other existing tubular joint elements. It was concluded that developed flexible joint model produces accurate results comparing to sophisticated multi-axial finite element joint models.

Modeling and verification of a missile launcher system

The launcher models used in most of the previous studies have few degrees of freedom that cannot possibly capture many important motion characteristics. In this work, a flexible multibody model of a missile launcher system is built. The fixed, azimuth and elevation platforms and the launching pod are modeled as flexible bodies. Both the normal vibration modes and constraint modes are considered. Modes that do not contribute significantly to the total strain energy are eliminated to reduce simulation time. The joint flexibility effects are also included to the model by spring-damper elements. For the plume force and the motor thrust, experimental data are utilized. The forces transmitted to the base of the launcher system and the azimuth platform displacements during the launch are determined. Launching tests show good conformity with the simulation results, thus revealing that the developed model provides a realistic representation of the system.

Complete dynamic modeling and approximate state space equations of the flexible link manipulator

This work treats the problem of dynamic modeling and state space approximation for robotic manipulators with flexibility. Case studies are planar manipulators with a single flexible link together with clamped-free ends and tip mass conditions. In this paper, complete dynamic modeling of the flexible beam without premature linearization in the formulation of the dynamics equations is developed, whereby this model is capable of reproducing nonlinear dynamic effects, such as the beam stiffening due to the centrifugal and the Coriolis forces induced by rotation of the joints, giving it the capability to predict reliable dynamic behavior. On the other hand, in order to show the joint flexibility effects on the model dynamic behaviors, manipulator with structural and joint flexibility is considered. Thus, a reliable model for flexible beam is then presented. The model is founded on two basic assumptions: inextensibility of the neutral fiber and moderate rotations of the cross sections in order to account for the foreshortening of the beam due to bending. To achieve flexible manipulator control, the standard form of state space equations for a flexible manipulator system (flexible link and actuator) is very important. In this study, finite difference method for discretization of the dynamic equations is used and the state space equations of the flexible link with tip mass considering complete dynamic of the system are obtained. Simulation results indicated substantial improvements on dynamic behavior and it is shown that the joint flexibility has a considerable effect on the dynamic behavior of rotating flexible arm that should not be simply neglected. The effects of tip mass is proved to be increasing the elastic deformations’ amplitudes and increasing stability.

On stress concentration factors for tubular Y- and T-Joints in frame structures

This paper is written as a result of some years experience with fatigue analysis of offshore jacket structures where the connections are made as tubular joints. The hot spot stresses at the tubular joints in such analysis are normally derived based on parametric equations for stress concentration factors. These stress concentration factors are normally related to the axial force in the brace. It is observed that the hot spot stresses at the crown positions of the tubular joint in some cases are significantly affected by the local loading on the chord and the bending moment in the chord. In order to use the existing formulae in these cases some engineering effort is required to derive correct hot spot stress. This work can be avoided by using the nominal stress in the chord as the basis for calculating the hot spot stress at the crown position instead of using the axial force in the brace as basis for the analysis. This also extends the validity of the equations for stress concentration factors for T- and Y- joints in design standards. The proposed modification makes it also simpler to include the effect of joint flexibility in a proper way. The basis for a proposed revision of the equations for stress concentration factors for these joints is presented in this paper. It is considered that this modification leads to minor changes of the computer code, but that it will save analysis work for engineers and reduce the possibility of calculating incorrect fatigue lives in tubular frame structures.

Methods of Trajectory Tracking for Flexible Joint Space Manipulators

AbstractThis paper presents a comparative study of four control strategies for a flexible joint space manipulator. The considered control strategies consist in the classical Slotine and Li algorithm, a simple proportional derivative controller, a singular perturbation-based controller and a nonlinear backstepping controller. All control methodologies are assessed in numerical simulations for endpoint positioning while tracking a 12.6 × 12.6 m square trajectory by a two-link flexible joint space robot, thus providing a common framework to compare the respective performance of the considered control schemes. Simulation results indicate that controlling both nonlinearities and joint flexibility effects improve the closed-loop behavior of the space robot where the control of nonlinearities is of greater importance in the sense that it provides the most significant improvements. While each control scheme tracks the trajectory with a different degree of accuracy, the best performance for this particular application is obtained with the nonlinear backstepping control strategy.

Novel reaction control techniques for redundant space manipulators: Theory and simulated microgravity tests

This paper presents two novel redundancy resolution schemes aimed at locally minimizing the reaction torque transferred to the spacecraft during manipulator manoeuvres. The subject is of particular interest in space robotics because reduced reactions result in reduced energy consumption and longer operating life of the attitude control system. The first presented solution is based on a weighted Jacobian pseudoinverse and is derived by using Lagrangian multipliers. The weight matrix is defined by means of the inertia matrix which appears in the spacecraft reaction torque dynamics. The second one is based on a least squares formulation of the minimization problem. In this formulation the linearity of the forward kinematics and of the reaction torque dynamics equations with respect to the joint accelerations is used. A closed-form solution is derived for both the presented methods, and their equivalence is proven analytically. Moreover, the proposed solutions, which are suitable for real-time implementation, are extended in order to take into account the physical limits of the manipulator joints directly inside the solution algorithms. A software simulator has been developed in order to simulate the performance of the presented solutions for the selected test cases. The proposed solutions have then been experimentally tested using a 3D free-flying robot previously tested in an ESA parabolic flight campaign. In the test campaign the 3D robot has been converted in a 2D robot thanks to its modularity in order to perform planar tests, in which the microgravity environment can be simulated without time constraints. Air-bearings are used to sustain the links weight, and a dynamometer is used to measure the reaction torque. The experimental validation of the presented inverse kinematics solutions, with an insight on the effect of joint flexibility on their performance, has been carried out, and the experimental results confirmed the good performance of the proposed methods. In particular, two test cases have been analyzed in order to validate and evaluate the performance of both the unconstrained solution and the solution which takes into account the robot physical limits.