Limit State Design Of Rcc Bridge Superstructure

Abstract

Concrete is the second most widely used material after water on Earth and it would not be an overstatement to call this era as “Concrete Age''. Concrete has become an intrinsic part of modern Infrastructure development around the world. These developments have become backbone of today's economies and will continue to do so in future. But the question is "what is the life expectancy of these infrastructure?" These infrastructure is built at large costs and are expected to remain in service for long periods - 50 to 100 years or even more. And it's not just the service life but also the safety that has become an important aspect. A failure of large dam, bridge or high-rise can be catastrophic and will result in unimaginable losses at all fronts. Therefore it is of utmost importance to control and monitor these constructions by creating and enforcing guidelines in form of codes of practices without restricting the use of improved and better ways of designing and constructing structures. Past half century has seen tremendous growth of knowledge in the field of concrete as a material and its design process. Limit State philosophy a more realistic and comprehensive over Working Stress philosophy has found its way to almost all countries’ design standards. It should be obvious that the practices in any country should remain more or less in line with the international developments, without any country lagging too much behind, or following a totally different path. Need for ‘Internationalization’ is obvious. Not so obvious is the need for ‘Rationalization’ of design practices or the need for revamping existing codes and developing ‘New Generation’ codes. Unlike western countries India has separate codes and formation committee for concrete design as a general (BIS) and bridge design (IRC). IRC is the latest committee to publish a code on basis of Limit State Design Philosophy (IRC 112:2011). Its predecessor IRC 21:2000 based on Working Stress Method was in practice for more than four decades. Therefore availability of large amount of trusted and tested expertise has hindered widespread use of IRC 112 aka Limit State Design philosophy. In line with international practice, IRC 112 also divides limit state into two groups Ultimate Limit State (ULS) and Serviceability Limit State (SLS). To mention some of major facets: section 5 of code provides a detailed explanation of “Basis of Design” which provides a transparent view of codal recommendations, applicability and limitations. Section 6 covers "material properties and their design values". Section 7 of “Analysis” covers classical methods of analysis, modern methods such as non-linear analysis, plastic analysis, effects of torsion \& time dependent properties of concrete. Preceding sections 8 to 11 covers “ULS” for flexure, axial, shear, torsional and induced deformations. Section 12 covers “SLS” for cracking and deflection. Section 13 covers various aspects related to "Prestressing". Section 14 covers “Durability” requirements. Next three sections 15 to 17 covers detailing requirements as a general and for seismic resistance separately. Lastly section 18 covers the requirement of Quality control and workmanship. Code allows design using working stress method as an alternative for verification of ULS and accordingly annexure A-4 covers the same. Present study, limited to RCC , has been divided into two parts : 1. Theoretical part covering comparison of IRC 112 : 2011 with IRC 21 : 2000 and EUROCODE-2 (considered to be major source of influence for IRC 112:2012) \& preparation of explanatory commentary for IRC 112 : 2011 and 2. Analytical part covering conversion (design by LSM) of standard sections from publication “Standard Plans for RCC T-beam and Slab Superstructure – Span from 10 m to 24 m with 12 m carriageway width,1991'', as published by MOST (now known as MoRTH) for parametric study. Explanatory commentary is limited to the RCC part of IRC 112:2011 covering clauses related to design of T-Beam type Bridge Superstructure. In detail study is carried out on supporting sections of Scope, design philosophy, Material (i.e. Concrete and Reinforcement) and Analysis. Effect of time and temperature on concrete strength are covered along with examples. Strain compatibility method is explained along with example and further standard equation are derived frequently used RCC sections (i.e. rectangular and T). Concept of average stress is used while deriving these equations. Because of which same equations are applicable to all three stress-strain idealizations provided by IRC 112:2011. A combined approach of shear + torsion calculation and their effect on design is explained. For serviceability limit state detailed study is carried out for crack width effect and its calculations. A complete example in form of procedural flow-chart is prepared covering Ultimate Limit State and Serviceability Limit State of RCC T-Beam design. For analysis grillage analogy has been adopted in aid with STAAD.Pro software. Further for parametric study a revised standard section considering current practice has been proposed. The study is carried out for span ranging from 10m to 24m as in original literature. Concrete Grades of M25, M30, M35 \& M40 are considered, while reinforcement grade is kept constant as Fe500. Depth variation is considered to obtain most economical depth for each grade of concrete for all spans. Results of these study is presented in form of Table for easy comparison and understanding. Span/Depth ratio chart is prepared to obtain most economical depth for all spans ranging from 10m to 24m. Further charts are provided to obtain longitudinal reinforcement at mid-span and transverse reinforcement at End span / support (i.e. maximum required at the said location respectively). Standard detailing drawings are prepared for all spans covering Beam, Slab and Cross diaphragm. Detailing is carried out as per guidelines of IRC 112:2011 provisions. It is important is note here is that these results are applicable to the revised section proposed. Excel sheets are prepared for design of all of above mentioned components.