Vibration characterisation of aluminium pedestrian bridges

Despite several full-scale applications in Canada, the vibrational characteristics and performance of aluminium pedestrian bridges have not been studied comprehensively in the literature. There is a large degree of variability between design codes and standards, particularly in North America and Europe. This is in part due to a lack of comprehensive experimental test data on full-scale pedestrian bridges. This is compounded by a lack of agreement between researchers on the characterization of pedestrian induced loads and the interaction between loads and the structure. This thesis aims to bridge this gap by building and testing full-scale aluminum pedestrian bridges in a controlled laboratory test program. Results from the experimental program are presented, discussed in detail, and used to estimate the vibration characteristics of an aluminium pedestrian bridge of various lengths. These characteristics include the modal properties -- natural frequency, damping ratio, and mode shapes -- and human-structure interactions measured using accelerometers, load cells, and strain gauges. Using multiple signal processing techniques, these characteristics were extracted from the data. The results from the pedestrian loading tests were then used to assess the bridge specimens through the above-mentioned design codes. Finite element models of each specimen were built and used for parameter studies and model verification. These data from full-scale pedestrian bridges are likely to shed new light on their vibrational behaviour and performance, and allow aluminium bridge designers to create competitive alternatives to bridges constructed with conventional materials. It is also anticipated that these tests will form a foundation for future research in the area of pedestrian bridge load modelling

Joint Friction during Deployment of a Near-Full-Scale Tensegrity Footbridge

Most deployable structures, such as operable roofs and masts, move over one degree of freedom. This paper describes a structure that involves loosely coupled movement over several degrees of freedom. Analysis models of these structures are typically inaccurate. A source of inaccuracy is joint friction. Static and kinetic friction are studied experimentally and analytically. Simulations have been modified to account for these effects, and two methods are used to quantify friction effects. Friction has a significant effect on the movement of the tensegrity structure. Of two candidate parameters, cable tension and interior cable angle, cable angle is the factor that best characterizes friction effects. Values of static and kinetic friction coefficients are not significantly different in this context, and this leads to a reduction in the complexity of the friction model for simulation. Including friction effects in analysis decreases the difference between simulations and tests. Lastly, strut elements of the tensegrity structure are most critically affected by friction. (C) 2017 American Society of Civil Engineers

Deployment and Shape Change of a Tensegrity Structure Using Path-Planning and Feedback Control

Tensegrity structures are pin-jointed assemblies of struts and cables that are held together in a stable state of stress. Shape control is a combination of control-commands with measurements to achieve a desired form. Applying shape control to a near-full-scale deployable tensegrity structure presents a rare opportunity to analytically and experimentally study and control the effects of large shape changes on a closely coupled multi-element system. Simulated cable-length changes provide an initial activation plan to reach an effective sequence for self-stress. Controlling internal forces is more sensitive than controlling movements through cable-length changes; internal force-control is thus a better objective than movement-control for small adjustments to the structure. The deployment of a tensegrity structure in previous work was carried out using predetermined commands. In this paper, two deployment methods and a method for self-stress are presented. The first method uses feedback cycles to increase speed of deployment compared with implementation of empirically predetermined control-commands. The second method consists of three parts starting with a path-planning algorithm that generates search trees at the initial point and the target point using a greedy algorithm to create a deployment trajectory. Collision and overstress avoidance for the deployment trajectory involve checks of boundaries defined by positions of struts and cables. Even actuator deployment followed by commands obtained from a search algorithm results in the successful connection of the structure at midspan. Once deployment at midspan is achieved by either method, a self-stress algorithm is implemented to correct the position and element forces in the structure to the design configuration prior to in-service loading. Modification of deployment control-commands using the feedback method (with twenty cycles) compared with empirically predetermined control-commands successfully provides a more efficient deployment trajectory prior to midspan connection with up to 50% reduction in deployment time. The path-planning method successfully enables deployment and connection at midspan with a further time reduction of 68% compared with the feedback method (with twenty cycles). The feedback control, the path-planning method and the soft-constraint algorithm successfully lead to efficient deployment and preparation for service loading. Advanced computing algorithms have potential to improve the efficiency of complex deployment challenges

Biomimetic adaptive control of a deployable tensegrity structure

Biomimetic behavior includes aspects such as learning from previous experience, self-diagnosis, and adaptation. This thesis describes control methodologies that are essential to development towards biomimetic behavior of a complex deployable structure. Simulations of the structure are improved from previous work to include friction effects of cables sliding over joints. Despite improved simulations, testing shows that significant uncertainties in the behavior of the structure remain. Therefore, control is not effective with predetermined actuation movements. Special methodologies for feedback control using simulations and measurements are required. A near-full-scale deployable tensegrity structure is used to test methodologies. Active control methods are proposed for deployment, midspan connection of both halves of the structure, and self-stress. This thesis presents a method using feedback to compare measurements and simulations to modify control commands. Since collision of elements is possible in the folded state and produce undesirable bending stresses, a path-planning algorithm is implemented for the first stage of deployment. Error in nodal positions at midspan is successfully reduced through the use of the path-planning method and deployment time is significantly reduced compared with previous work. Lastly, algorithms for self-stress, involving penalty and rejection constraints on element stress, are useful for correcting nodal positions after deployment. Damage is detected in this thesis using vibration measurements. The method uses dynamic behavior of the structure to determine whether or not the structure is damaged. Using parameters of the structure and a set of candidate locations for the damaged element, candidates are successively excluded until few candidates remain, successfully including the true location of damage. Adaptation and learning are demonstrated by mitigation methods after damage and in-service loading (such as pedestrians). Active control is useful to manipulate the shape of the tensegrity structure to reduce the member stresses and vertical downwards displacement caused by a damaged element. Though the response improves the condition of the structure to respect the serviceability limit for vertical downwards displacement, the tensegrity structure cannot be fully restored to the design configuration. Since correction of end-node coordinates can be grouped by the direction of correction and resulting cable-length changes, case-based reasoning is useful to reduce time of execution and to reduce unnecessary cable-length changes. Single pedestrian and crowd loading configuration is applied analytically and experimentally to the tensegrity structure. Application of mitigation techniques is useful beyond serviceability thresholds for a moving load used to simulate in-service loading. The research question of this thesis: "Is it feasible for a deployable tensegrity structure to improve movement and service performance through behavior biomimetics?". The answer is yes. This work presents methods inspired by those observed in nature for efficient movement that is generalizable for future deployable structures

Element location and classification following a damage event of a near-full-scale deployable tensegrity structure

Current infrastructure is designed and built such that it must resistall possible loads. This leads to overdesigned structures that are inefficient in terms energy and cost. A structure that can self-identify damage, adapt, and learn for future events results from research intothe emerging field of intelligent infrastructure and structural health monitoring.Two halves of a “hollow-rope” tensegrity structure deploy from supports to join at midspan by controlling the length of active cables on each half of the structure. These active cables are continuous throughthe length of the half-structure, guided by intermediary jointswhere cables slide. Springs along the circumference of the structure facilitate deployment due to increasing the diameter of the structure when folding and subsequent decreasing during deployment. Although previous work has addressed damage locationand mitigationof ruptured cableswhen the cables are the load-critical elements of the structure, this work hasnot studiedthe classification of the type of element that is damaged, element locationand damage mitigation.This paper presents work on element classification, detection,and location of damaged elements in a deployable tensegrity footbridge. The footbridge isstudied through monitoring dynamic behavior. Displacement and strain values are measured before, during, and after cable breakage. Natural frequencies inhealthy and damaged states are compared. Free-vibration dynamic behavior of the tensegrity structure are characterized for two situations,deployment and in-service. Examination of ambient vibrations for the half structure and forced vibrations for the full structure successfully led to detection of ruptured cables. Correlation methods using strain measurements also successfullydetect and locate a ruptured cable. Detection of abuckled strut and aruptured cableis successful by observing differences of natural frequencies between healthy and damaged states. Location of a damaged element is successful using nodal-position measurements through excluding possible damage scenarios and using strain measurements to identify elements of significant changes in eigenvector coefficients using principal component analysis. Therefore, excluding scenarios from a population for damageidentification is effective for highly-coupled structures that are capable of large shape changes. These methods reveal the potential for damage identification of complex sensed structures.Classification and location of a damaged element on a complex near-full-scale structure is successful using nodal position measurements through excluding possible damage cases and using strain measurements to identify elements of significant changes in eigenvector coefficients using principal component analysis. Implementing error-domain model falsification to exclude possible scenarios for location of damaged elements successfully reduced the number of probable casesof damage location. Paterns of influence from damaged cables and struts are useful to classify the type of element that is damaged. Therefore, the methodology involving error-domain model falsification (EDMF) for damage location is useful for closely-coupled structures that are capable of large shape changes