STUDIES IN TRANSFER BEAMS FOR HIGH-RISE BUILDINGS

Abstract

To provide for functional requirement of large column-free space in High-rise buildings, the RC columns are placed at the periphery of the built-up plan area. With a view to developing high flexural and torsional stiffness, these columns are very closely spaced and connected through very stiff beams, called as Spandrel beams. These closely spaced columns at the periphery, however, pose hindrance to the movement of people and the goods at the ground floor and basement levels. To fulfill this requirement, the columns at these floor levels have to be placed at larger spacing. As a result, an interface has to be provided between the closely spaced columns of the upper floor and the widely spaced columns at the ground/basement level. This interface hasto be a horizontal RC element and hence is referred to as Beam. Conventionally, a beam is taken to be a flexural member of the structural system. The above mentioned interface beam, however, does not behave as a flexural member since it gets sandwiched between closely spaced upper columns and a little widely spaced supporting columns below it. Also to transfer the high magnitude of loads collected from all the upper floors of a high-rise building, the depth of the interface beam has to be kept much higher than the conventional beams, ranging from 1 m to 4.5 m. As a result of this, the load transfer mechanism through this beam becomes altogether different than the conventional mechanism of the flexure. Such a beam is also referred to as Transfer Beam. Transfer beams transfer heavy concentrated gravity loads from columns predominantly through shear and very little through flexure. As a result, failure of such beams takes place in shear. Unlike flexure, shear failure is sudden, violent and hence treacherous. This mode of failure has to be, therefore, transformed in to a ductile mode of failure, wherever possible. In a beam, vertical steel called stirrups are provided to build-up shear resisting capacity. These stirrups are discrete members and hence do not provide a continuous medium in the body of concrete. Failure of concrete due to shear generated tension, therefore, occurs in brittle mode. This mode of failure, being treacherous, should be avoided through appropriate measures, if possible. Inducing ductility into the body of concrete, thus, becomes aprimary requirement. Ductility can be induced in concrete by introducing steel bars in a continuous mode as against steel stirrups which are non-continuous steel elements embedded into the body ofconcrete. It is, therefore, considered imperative to study the growth of shear resistance through longitudinal steel along with its influence on achieving a little ductile mode of failure. As a result of this concept, shear resistance needs to be developed by a suitable combination of vertical stirrups (efficient but brittle failure) and longitudinal steel (less efficient but ductile failure). Experimental studies need to be carried-out intensively with varying distribution of longitudinal and vertical steel towards development of shear resistance. The primary objective of the research is to build-up shear resisting capacity in RC beams of high depth/s through a suitable combination of horizontal and vertical steel bars which would impart both substantial ductility as well as high shear capacity. In the present research study, an extensive experiments incorporating the strength of concrete, percent longitudinal steel, percent vertical steel and varying shear span-to-depth ratio have been carried-out. These have resulted into testing about 340 beams yielding alarge set of relevant and reliable data. The span of the beam has been kept constant at lm with 0.1m overhang on either side of the supports. The spacing between the top two Point-Loads has been kept at 200mm. The depth of the beam has been varied at 150, 200, 250, 300, 350 and 400mm. The study on beams related to varying shear span-to-depth ratio (a/d) has been carried-out by varying the depth (d) and not by varying the shear span( a). This has been consciously done to achieve the flow of applied load through the entire depth of the body of concrete. This allows the concrete to develop stresses over the full depth of the beam which varies non-linearly across the depth. This aspect is very distinct, important and relevant from viewpoints of li the structural response of the beams. It is to be particularly noted that the same value of a/d can be attained by varying the shear span (a ) which is easy to implement since it simply demands shifting of the top loading points towards the supports. This way of varying the shear span-to-depth ratio does not result in the true structural response as in a deep beam to be used as a Transfer beam. The failure patterns are significantly different in the two set of beams having the same a/d ratio but one obtained by varying the depth( d ) and the second varying the shear-span( a ). Based on the actual observations on the structural behavior of the beams, it is recommended with all the emphasis at command that no attempt should be made to interpret the results of beams wherein the shear arm(a) has been varied to obtain the variation of a/d. The deep beam model of Arch-Strut-and-Tie never comes into action when the shear arm(a) is varied keeping the depth constant. It may be easy to carry-out experiments with varying the shear arm (a) while keeping the depth (d) constant but it fails to structurally simulate a deep beam. The results from the experiments have been processed suitably to come out with empirical expressions for estimating the shear capacity of beams incorporating variables such as compressive strength of concrete, percentage of longitudinal and vertical steel/s, depth of beam in terms of shear span-to-depth ratio. These empirical expressions will henceforth be referred to as proposed expression/s for shearcapacity. Further, the comparisons of shear design provisions of five National codes viz.: (i) IS 456-2000, (ii) Euro code EC2-2002, (iii) BS 8110-1997, (iv) ACI 318-2002, (v) CIRIA Guide-2, for the prediction of shear strength of Normal beam/s and Transfer beam/s ( shear span-to-depth ratio < 1.8 ), have been made with a view to seeing their goodness of fit against the experimental values. in