Ultimate limit state analysis of externally post-tensioned structures.

The UK Department of Transport (DoT) has recently encouraged the use of externally
post-tensioned structures for bridge construction. This is due to the durability
problems encountered with the conventional internally bonded post-tensioned
structures. However, due to the lack of bond between the concrete and the external
tendons, the ultimate strength of these structures cannot be determined by just
performing a sectional analysis at the section of maximum moment. Although several
recommendations have been made for the ultimate analysis of these structures, none
were considered satisfactory here. Hence the purpose of this investigation is to study
the flexural behaviour of these structures of all stages up to collapse, and to propose a
practical methodology for estimating their ultimate strength.
The study introduces eight non-linear analytical models developed for the prediction
of the moment vs. deflection response of simply supported externally post-tensioned
beams (with and without deviators located along their spans) up to ultimate. The
models employ an iterative procedure that involves the application of loads to the
structure in increments up to the collapse condition, where the curvature distribution
predicted at each loading sequence is used to estimate the stress increase and variation
in eccentricity in the external tendons. Second-order effects due to variation in
eccentricity of external tendons and frictional behaviour of tendons at the deviators
are both taken into account in these models. The eight models were then verified by
comparing the results derived from them with reported experimental data, whereby
good correlation was obtained. An extensive parametric study was subsequently
conducted using the proposed models applied to the various parameters that influence
the ultimate behaviour of externally prestressed structures Finally, the
recommendations in the codes of practice for the ultimate design of these structures
were also investigated here