04-12-2012, 12:46 PM
ANALYTICAL INVESTIGATION OF PRESTRESSED CONCRETE STRUCTURES INCORPORATING COMBINED POST-TENSIONED AND POST-COMPRESSED REINFORCEMENTS
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ABSTRACT
Although strengthening of structures by post-tensioning is much more common in practice, the authors did locate
innovative literature on the theory and applications of post-compression which form the basis for this study. This work
analytically assesses the behaviour and criteria for optimal performance of prestressed concrete structures under the
combined actions of post-tensioning and post-compression. Magnel type diagrams are also developed to show the feasible
solution regions for different loading conditions. The findings revealed that the prestressing forces in the post-tensioned
tendons are independent of the eccentricity ratio, but increases perfectly curvilinearly up to prestressing ratio of 0.7 beyond
which the technique becomes impracticable. On this basis, there is a realistic reduction of 50 percent in the eccentricity and
possibly the overall depth of the concrete section over the conventional technique. This qualifies this structural innovation
as a reliable candidate for long span structures.
INTRODUCTION
Prestressed concrete structures are an attractive
alternative for long-span bridges, and have been used
worldwide since the 1950s. Savings in materials and
maintenance and life-cycle costs are additional
advantages. The extensive application of prestressed
concrete (PC) to buildings, military constructions and civil
infrastructure has been generally enhanced by awareness
with its principles, practice and further advances in its
design and construction [1, 2]. Sriskandan [3] and Smyth
[4] provided excellent reviews on research advances,
developments and achievements of PC bridges in the
United Kingdom in comparison with practices in the
United States and Europe. The performance of PC
highway bridges in the USA was also investigated by
Dunker and Rabbat [5]. Recent bridge inventories in the
Western world and some Asian countries have shown that
such structures constitute more than 50 percent of existing
bridges, and are rapidly replacing existing structural steel
bridges. Furthermore, the use of external prestressing
techniques has been growing rapidly since its evolution in
the early 1950s and many investigations have been
reported on externally PC structures [6]. Indeed the
amount of literature related to advances in PC structures is
quite extensive.
General assumptions and sign conventions
The analysis of stresses developed in the
prestressed concrete structures under gravity loads and
combined action post-tensioned tendon and postcompression
reinforcing system is based on the
assumptions that strain distribution across the depth of the
member is linear, concrete is a homogeneous elastic
material, and both concrete and steel behave elastically
within the range of working stresses notwithstanding the
small amount of creep which occurs in both materials
under sustained loading. The sign conventions of positive
sagging moments and negative hogging moments, and
positive compressive forces and negative tensile forces are
applied in this analysis. In addition, eccentricity of posttensioned
tendons below the concrete centroid and
eccentricity of post-compressed bars above the concrete
centroid are positive.
Preliminary section sizing
From several inequalities that have to be satisfied
at any cross-section, it is possible to separate out the
design of the cross-section from the design of prestress.
By considering pairs of stress limits on the same fibre, but
for different load cases, the effects of prestress can be
eliminated. The preliminary sizing is more easily achieved
by introducing some simplifying assumptions. Assuming
little or negligible loss of prestress such that the residual
prestress ratios, μ and α, are approximately equal to unity,
the top and bottom elastic section moduli can be obtained
from Equations (6) and (7).
CONCLUSIONS
A general overview of the historical background,
fundamental principles and previous applications of postcompression
techniques in prestressed concrete structures
has been covered in this paper. Basic equations and
inequalities are also derived, and Magnel type diagrams
constructed for general analysis and design of prestressed
concrete structures with both post-tensioned tendons and
post-compressed bars. In the analytical study of a simple
beam, the prestressing forces in the post-tensioned tendons
are found to be independent of the eccentricity ratio, but
increase quadratically up to a prestressing ratio of 0.7 after
which they become asymptotic as the post-compression
force equals the post-tension force. On the other hand, the
prestressing force ratio and the eccentricity ratio have
direct effects on the eccentricities. There is reduction in
eccentricities as the post-compression force and
eccentricity of post-compressed bar increase.