STUDIES IN BEAMLESS RC STRUCTURAL FLOOR SYSTEMS FOR MEDIUM FLOOR-PLANS

Abstract

The control ofdeflection and punching shear govern the design ofRC beamless floors unlike the other two way slabs supported by beams. Since the thin slab is supported directly by columns, the time dependent measured deflections are more when compared with the calculated deflections. Though the building may be safe against the strength aspects, the excessive deflections lead to lack of feeling of safety in the occupants of the building that may even question the proficiency of the designer. Another main problem in these beamless slabs is the brittle punching shear failure. Existing research on flat plates has shown extensive study on either eliminating j the punching shear failure or to change the mode of brittle failure to flexural failure j mode, by using different arrangements of reinforcement. Prefabricated systems are also available to address the punching shear problems. But in many of those systems, either the depth of slab is to be increased or skilled labour is needed to fix them in position. Hence the main aim ofthis research was concentrated in improving the stiffness of slab without increasing the thickness of slab. To address the above problems effectively, the concept of providing the concealed beam in the slab was considered for this research. The concealed beam is an arrangement of reinforcement having equal number of longitudinal reinforcement in both tension and compression face of the slab for afixed width tied together by means of shear stirrups along the transverse direction. Since the column strip plays amajor role in transmitting the load, the stiffness of the column strip must be improved. To utilise the benefits of beamless slabs without much compromise in safety and serviceability, the concealed beam can best be placed within the column strip. This study was aimed to give atheoretical model for the concealed beam and to study the effect of the same in terms of deflection, moment and shear carrying capacity of slab. The modelling of the concealed beam was made, by using the normal reinforced concrete theory for transformed section for flexure. For torsion, the concealed beam wasmodelled as a thin equivalent concrete box. 11 y^\\ The Equivalent Frame method was used to study the stiffness of the flat plate and flat slab with column head using concealed beam. Since the depth of the slab is the main object, the required depth based on long-term deflection was to be found. For this, ikar^ite provisions in the codes ACI: 318-2002, BS: 8110-1997, EC: 2-2002 and IS: 456- 2000 were studied and the available empirical beam formulas have been used with suitable modifications by applying the Bransonapproach. *-^ A parametric study was conducted by using the software developed for the modified empirical formulas as per the above codes. A modification to the multiplying factor in the Branson formula was proposed based on this study. The results using the proposed formula was verified with the experimental results available in the literature and found that there was good agreement between them. in