Rational design of simply supported and continuous deep beams

Abstract

Deep beam is a very important structural element in various types of concrete structures such as pile cap, transfer girder, panel beams, foundation walls of rectangular tanks and bins, shear walls, folded plate roof structures, etc. Analysis of deep beams under shear is quite complex and because of the complexities involved, exact theoretical basis for the analysis of deep beams in shear has not yet developed. Current design rules for deep beams are largely based on the provisions of American Concrete Institute (ACI) Code, Construction Industry Research and Information Association (CIRIA) Guide, and other codes and design methods. The present-day design provisions for deep beams are usually based on test results conducted on simply supported model deep beams, although deep beams are often continuous. Again, some of the provisions for the design of deep beams are essentially those for slender beams and are not applicable to deep beams. In the absence of an exact and simplified basis for the design of deep beams, several new lines of thinking have developed in the recent past in order to unify the design of various structural concrete members. One of these new approaches is the concept of Compressive Force rath (CFr). However, a very limited number of tests have been conducted to verify the applicability of this method in the design of deep beams. One of the primary purposes of this study is to try to understand the behaviour of deep beams with the help of tests conducted by various researchers. Again, available test results have been used, in conjunction with results from the limited tests conducted by the investigator on simply supported as well as continuous deep beams, in finding out the suitability of CFr method in the design of deep beams. In the present study, test data of about 175 deep beams have been collected, analysed and employed in understanding the behaviour and predicting the strength of such members using ACI, CFr and CIRIA Guide methods. In addition, five simply supported and five two span deep beams were designed, fabricated and load tested to failure in an effort to enrich the data base of deep beam test results as well as to ascertain the applicability of CFr method in the design of deep beams. From the investigation it has been found that shear failure is common in all the cases of deep beams. Whereas, clear span-to-effective depth ratio has demonstrated insignificant effect, shear span-to-effective depth ratio has been found to be a governing parameter in controlling the shear strength. Vertical web reinforcement, strength of concrete and amount of flexural steel have shown some influence on the shear strength. It also appeared that the effect of horizontal steel is much less than what is usually considered. Again, inclined cracking strength has little influence on the shear strength of such type of beams. The ACI code and CIRIA Guide have failed in predicting the ultimate strength of deep beams. Although, in most cases, the CFr method predicted such strengths of deep beams more closely, as the shear span-to-depth ratio gradually decreases from 1.0, this method becomes progressively conservative. Among other findings, it has been gathered that the results obtained from tests of simply supported beams may not be applied to continuous deep beams and that further tests on multi-span beams are essential.