ABSTRACT
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MOTIVATION AND ACTUALITY OF THE PROBLEM
The engineering practice shows that the main cause for earthquake failure is the inadequate
behavior of the columns of buildings and bridge structures when the detailing of the longitudinal
and transverse reinforcement doesn’t assure enough element ductility and strength.
Horizontal effects (e.g. according to wind or seismic actions) yield considerable values of
bending moments M, axial forces N and shear (lateral) forces V in the isolated columns and
columns part of unbraced frames. Their interaction could cause collapse with a form, different to
those typical for combined bending moment and axial force M+N or dominant shear.
Indicative of the relevance and complexity of the shear phenomena of columns are the
numerous of published classical, standardized, normed, modified, improved, clarified, expanded
and simplified models and the applied theories and empirical dependencies on the item.
Regardless, the traditional standards and even many of the modern norms and design codes
mainly suggest models and procedures for separated determining each of the capacities for
combined M+N and shear V rather than accounting the simultaneity of these actions.
It can be stated (with a few exceptions) that theoretical and experimental researches on shear
in reinforced concrete elements have not been done for more than 40 years in our country.
Meanwhile, there are intensive and dynamic researches about the problem in many European and
other countries in this period. Still it is a paradox that here and now in Bulgaria we work
according to standard released back in 1987 and this standard do use a shear model which is
introduced with the first Regulations for the design of reinforced concrete structures by the limit
states method almost 60 years ago, in 1957.
The tendency of the contemporary theories and codes for design is to study the strength of
regions and even entire elements, instead of traditional critical normal and inclined sections. This
requires us to apply modern and adequate models which do take into account the simultaneous
action of various internal forces.
In general, these models represent one-dimensional (1D) or two-dimensional (2D) concrete
and reinforced concrete stress fields or a combination thereof:
- With 1D-fields modeled longitudinal effects (bending and longitudinal forces) and
- With 2D-fields (membranes) – shear phenomena.
Present-day approaches for analysis of the critical regions capacity of reinforced concrete
columns subjected to shear are based on theories of RC compression fields and have a significant
contribution to the development of the applied theory of reinforced concrete. Essentially they
analyze the limit state in regions of reinforced concrete elements as in other approaches using the
most logical physical parameters (effects) – strain. Rather than analyzing the stress state known as
"internal forces" like in other summaries.
Besides their application in automated solution of more complex tasks, compatibility
equations are already present (though incomplete and implicitly) in "everyday design" codes. This
trend is also obvious in the newest Model Code 2010 [71], which traces the future development of
code for the design of reinforced concrete structures Eurocode 2 [1].
AIMS AND OBJECTIVES OF THE THESIS
Main objective of this thesis is to specify the application procedures for complex
calculation/verification of capacity in critical regions of reinforced concrete columns with
rectangular and circular cross-section in the combined static action of bending, compression and
shear.
And also to classify, grade and assess the main parameters that influence the ultimate limit
state of those elements such as:
(1) Rigidity of columns and shear slenderness.
(2) Interaction of the components of the combined actions
(3) Effective width of the sections.
Shear Limit State of RC columns – Konstantin Velinov
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(4) Efficiency of longitudinal reinforcement.
(5) Inclination of critical cracks.
(6) Impact of the transverse reinforcement of various shapes.
(7) Shear concrete contribution.
Following activities were conducted to achieve credible clarifications and proposals
(8) Experimental tests reaching failure of 6 columns with circular cross section subjected to
a complete combination of loads.
(9) Quasi-natural (numerical) testing on adequate advanced models for shear strength.
(10) Comparison between the results of the experiments and data from other experiments,
where shear failure is observed, with the proposed procedures.
OUTLINE OF THE THESIS
Chapter 1 presents analysis on current models and theories for shear, which are reflected in
the leading global standards and codes (total 37 models). Additionally, 11 more other proposals
relating to the characteristics of the loading and the shape of the cross-sections of the columns are
examined as well.
Chapter 2 outlines geometric parameters of the critical regions of the columns and defines
new categories: D-columns and B-columns. Subject of the dissertation are relatively slender Bcolumns.
Chapter 3 presents complex capacity calculation of columns with rectangular cross-sections
with the proposed interactions.
Chapter 4 focuses on the detailing and preparation of the specimens, experimental setup,
used test equipment and measuring instruments and the program of experimental research.
Chapter 5 presents the substantial results from the experiments which are related to shear
strength of the specimes and are necessary for the purposes of this research.
Chapter 6 includes a summary of all analyses, comparison and proposals referring to
columns with circular cross-sections
The final findings and conclusions of the research work are outlined in Chapter 7. It
summarizes recommendations for future research and analysis on the subject, and as well the
author claims for scientific and applied contributions in the present thesis.
Shear Limit State of RC columns – Konstantin Velinov
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