We all know that a RC column gets its strength not only by grade of concrete and longitudinal reinforcement but also by the confinement of both concrete and steel together in a loop by means of ties. In that case why don't we consider ties as a part of % of reinforcement than just assuming that the ties take shear only.For example, if 1.2 % is the total % of steel required, then, why don't we take a part of the reinforcement is from ties?????
Answer:
The query carries a fundamental crash in the matter asked.
Ties are for many other purposes including providing confinement to the core concrete. In axial load carrying capacity of column also it influences by means of providing confinement and not as a direct load carrying element like main steel. If you see stress-strain plots for unconfined and confined concrete, you will notice two peculiar differences. First, the peak stress for confined concrete is slightly higher (depending upon level of confinement) than that of unconfined concrete and secondly the ultimate strain value for confined concrete is much higher than that of unconfined concrete. This simply means that confinement has more useful influence on ductility rather than strength.
Further, ties while providing confinement undergoes direct tension in turn due to generation of hoop force (in in-plane radial direction of a cross-section) while the main steel is sharing direct compressive stresses and tension (in normal direction to a cross-section), if any, due to bending moment thus obviously the plane of internal forces are different in both of them. This implies that the cross-sectional area of hoop reinforcement i.e. ties and main steel are tackling different sort of forces. In light of this argument, addition of ties with main steel to make up total required percentage of steel is as good as summing a group of monkeys to a group of donkeys! This actually is the fundamental crash. Also, even in case of main steel, just by it being along the line of action of column force, one should not count its area towards total required steel, unless it is properly anchored within concrete at least to its development length.
Note that efficiencies of ties in rectangular and circular shapes are quite different and in general ties in circular shape are more efficient in generating hoop stress than in rectangular shape. Further, instead of ties, from confinement point of view if you see, externally bonded steel or FRP plates on concrete columns are still better in confining the core concrete (though there is still a thin difference between both the cases, in case of ties they generate active confinement while in case of external plate bonding it generates passive confinement).
Answer:
The query carries a fundamental crash in the matter asked.
Ties are for many other purposes including providing confinement to the core concrete. In axial load carrying capacity of column also it influences by means of providing confinement and not as a direct load carrying element like main steel. If you see stress-strain plots for unconfined and confined concrete, you will notice two peculiar differences. First, the peak stress for confined concrete is slightly higher (depending upon level of confinement) than that of unconfined concrete and secondly the ultimate strain value for confined concrete is much higher than that of unconfined concrete. This simply means that confinement has more useful influence on ductility rather than strength.
Further, ties while providing confinement undergoes direct tension in turn due to generation of hoop force (in in-plane radial direction of a cross-section) while the main steel is sharing direct compressive stresses and tension (in normal direction to a cross-section), if any, due to bending moment thus obviously the plane of internal forces are different in both of them. This implies that the cross-sectional area of hoop reinforcement i.e. ties and main steel are tackling different sort of forces. In light of this argument, addition of ties with main steel to make up total required percentage of steel is as good as summing a group of monkeys to a group of donkeys! This actually is the fundamental crash. Also, even in case of main steel, just by it being along the line of action of column force, one should not count its area towards total required steel, unless it is properly anchored within concrete at least to its development length.
Note that efficiencies of ties in rectangular and circular shapes are quite different and in general ties in circular shape are more efficient in generating hoop stress than in rectangular shape. Further, instead of ties, from confinement point of view if you see, externally bonded steel or FRP plates on concrete columns are still better in confining the core concrete (though there is still a thin difference between both the cases, in case of ties they generate active confinement while in case of external plate bonding it generates passive confinement).
