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M.Ing. (Civil Engineering) / Variable EI, post tensioned continuous bridge decks frequently occur in practice. The design of such a deck is essentially a process of trial and error. The aim of this thesis is to facilitate the design process by eliminating the sting from the otherwise complex flexural analysis by the development of suitable PC programs. This thesis will focus on the following three main issues: 1 SECTION PROPERTIES The program PROP presents a minimum input approach for the determination of uni-axial section properties. Special enP1aSis is placed on the section efficiency factor, being a direct quantitative measure of a sections flexural ability, ranging from 0 to 1. The effect of this factor on partial prestressing is also discussed. 2 THE MAGNEL DIAGRAM The program MAGNEL presents a linear programming optimisation aid for the design of uneracked sections (i.e. Classes 1 and 2 prestressing). The following is catered for: • variable prestress loss • both sagging and hogging moments • composite construction • two stage stressing • secondary moments due to prestressing The effects of the section efficiency factor, area redistribution and secondary moments on the Magnel diagram is also evaluated. 3 VARIABLE EI CONTINUOUS SPAN ANALYSIS The program BEAM presents a transparent finite element approach for the evaluation of bending moments, shears and deflections for variable EI continuous spans. The following features are available: • for a single span, supports may be fixed, supported or free • up to a six span continuous structure may be analysed • loading may consist of point loads, moments, uniformly distributed loads, support deflection or prestressing The direct prestressing input feature is compared against the equivalent load method for a varying parabolIe tendon force as developed by Catchick. The formulation of the Catchick method is also simplified by the introduction of additional parameters.

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published_or_final_version / Civil Engineering / Master / Master of Science in Engineering

Thesis. 1975. M.S.--Massachusetts Institute of Technology. Dept. of Civil Engineering. / Bibliography: leaves 53-54. / M.S.

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Multi-span precast concrete segmental bridges are commonly constructed using the balanced cantilever method, which essentially involves sequentially extending precast segments outwards from each pier in a balanced manner. A gap of 100 to 200 mm wide is usually provided around the mid-span location between the last two approaching segments to facilitate erection. In-situ concrete is then cast to ‘stitch’ the segments together, thus making the bridge deck continuous. In the current practice, the in-situ concrete stitches are usually designed to be capable of sustaining considerable sagging moment but only minimal hogging moment. Failure of stitches may occur under exceptional circumstances that may potentially trigger a progressive collapse. However, relatively little research in this area has been carried out. In view of this, the author is motivated to undertake an extensive study of the behaviour of in-situ concrete stitches and the effects of their performance on the robustness of typical segmental bridges. Experimental study is carried out to examine the behaviour of in-situ stitches under different combinations of internal forces. Series of stitch specimens of different configurations are tested. Subsequent parametric studies are conducted numerically to examine the effects of various parameters on the load-displacement characteristics of the stitches. Formulae for strength estimation are proposed based on the results. A study of robustness involves analyzing the collapse behaviour of a structure in an extreme event and the analysis should be carried out up to and then well beyond the state of peak strength of structural members. A finite element programme for post-peak analysis is therefore developed for the present study. As the ability of a member section to sustain large inelastic deformation can ultimately affect the robustness of a structure, an investigation is conducted to examine the effects of steel content, yield strength and prestressing level on the ductility and deformability of prestressed concrete sections. Using the programme developed, the formation of collapsing mechanisms of a multi-span segmental bridge deck in an extreme event is examined. A typical bridge deck is subject to prescribed accidental load on its span in order to analyze the sequence of failure. Substantial redistribution of internal forces along the deck is observed as failures initiate, thus causing subsequent failures of other deck sections even though they have been designed to resist the internal forces at the ultimate limit state. The results indicate that any span of a multispan bridge may become a temporary end-span in the event of collapse of an adjacent span and the strength of the sections must be designed accordingly to prevent progressive failure. As a span becomes a temporary end-span, the in-situ concrete stitches may experience substantial moment and shear, and their failure could potentially trigger progressive collapse of the entire bridge deck. Towards the end of the thesis, important design considerations that can enhance the performance of in-situ concrete stitches and robustness of precast concrete segmental bridges are presented. / published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy

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