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Title: Analysis and design of FRP-reinforced indeterminate structures
Author: Zhang, Chao
ISNI:       0000 0004 2695 1795
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2010
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There has been limited research on statically indeterminate fibre reinforced polymer (FRP) - strengthened structures. Indeterminacy can give rise to self-equilibrating moments that influence first yield and failure loads, the latter due to potential FRP buckling and end-peel on account of offsets between points of contraflexure and nearby FRP curtailments. For thin-walled metallic structures, buckling failure is also a concern. The nature and extents of these complex effects have not thus far been investigated. In this PhD study, closed-form algebraic expressions are presented for the selfequilibrating moments, due to differential settlement between supports, developed in 2-span continuous structures. The expressions were written to clearly show how the moments depend on the support-to-member stiffness ratios and on the distribution of stiffness along the member for service and ultimate limit states. Four 2-span carbon FRP (CFRP)-plated steel beam specimens with varying layouts of plates and plate thicknesses have been fabricated and loaded to failure. The instrumentation (strain gauge and displacement transducer) layouts have enabled investigation of several important aspects related to structural indeterminacy and those related to FRP-plated thin-walled metallic structures. One novelty is that the steel stiffeners have been adhesively bonded (rather than welded) to the steel main beams. This proved successful in inhibiting web/flange buckling. In addition, a computational technique was developed for time-efficient prediction of ultimate loads of ductility-deficient indeterminate structures, which was then incorporated into an iterative procedure to create an analysis-design process for indeterminate structures of varying ductility levels. The key findings of the research include: • There exists a 'sensitive zone' for support stiffnesses, within which the moments change rapidly with support stiffnesses and particular care should be taken for test setup in the lab and also for bridges in practice; • Adhesive bonding the steel stiffeners to the test beam has worked very efficiently to prevent local flange/web buckling. Local buckling was prevented up to a load of at least 36% higher than the failure load for the beam of similar section but without any stiffeners. This technique is an alternative to welding and allows for safe application of strain gauging at the section of stiffening; • The general nonlinear stiffness variation can be approximated in a piecewise linear manner. The choice of the number of linear stiffness segments depends on how rapidly the stiffness changes and the precision level specified. A 2-to-5-piece linear stiffness approximation generally gives sufficient representation of the nonlinear stiffness variation; • This technique has been illustrated by way of examples, which cover a wide range of structural ductility under different types of loading. Convergence was attained after 4-8 iterations in all cases, suggesting that standard structural analysis software for linear beam analysis may well be used confidently for analysing nonlinear problems; • An iterative procedure has been developed entailing multiple detailing and ultimate analysis loops for the structure to create an analysis-design process for ductilitydeficient indeterminate structures. The new analysis-design process has been illustrated by way of examples including a FRP-plated continuous RC beam, a traditional RC continuous beam and a three-storey three-span plane frame. Quick attainment of reinforcement details in all cases suggest that this may well constitute a design procedure applicable to structures of varying levels of ductility. Design based on constant EI may lead to structures with a load deficiency of 12.9% and 9.2% for Examples 1 and 3, respectively; and • Through thickness strain variation and/or local bending of plates are important in the evaluation of the average shear bond stresses, especially for thick plates. Using outer surface strains only (which is common in the research environment) could lead to unreasonable prediction of interfacial shear stresses. It is concluded that the self-equilibrating moments should be properly accounted for in designing and analysing FRP-plated indeterminate structures, and that the rigid bond section analysis and piecewise linear stiffness variation can be incorporated into the proposed iterative procedures to construct a reliable analysis and design approach for indeterminate structures of varying ductility levels.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available