Slender concrete deep beams : behaviour, serviceability and strength

Abstract

Reinforced concrete deep beams have useful applications in construction. However, their design is not yet covered by the British Standard BS 8110: 1985 which explicitly states that "for the design of deep beams, reference should be made to specialist literature". A selection of literature on deep beams is considered. First, the major works that have led to design recommendations are reviewed. Then, the current major codes and manuals covering deep beams, namely the CIRIA Guide, the European CEB-FIP model code, the American ACI(318-83) (revised 1986) code and the Canadian CAN3-A22.3-MB4 code are outlined; worked examples are given in order to illustrate their practical applications and compare their different approaches to deep beam design. The purpose of this literature review was to define the deep beam problem and identify the major questions still remaining unanswered together with the limitations of the present design documents on the subject. The nature of diagonal cracking in slender deep beams has recently raised a question as to the application of the shear-strength equation in cl.3.4.2 of the CIRIA Deep Beam Guide. The effectiveness of web reinforcement on serviceability and strength of deep beams in general is also an area where strong disagreement exists. A testing programme, consisting of 15 beams of height/thickness ratios ranging from 20 to 50 and grouped in 3 different series, was performed to provide information on these two areas. The main variables were the height/thickness ratio and the quantity and arrangement of web steel. The beams were tested under concentrically applied two point-loads. Based on the test results and observations, modifications are given for the CIRIA equation and other formulae derived from stocky deep beam tests to be used in slender ones for analysis and design purposes. A new formula is also proposed for the prediction of the ultimate shear capacity. The stability of deep beams is another area which has received less attention in the past by researchers and designers who often avoided the problem by opting for stocky sections. To quote from the CIRIA Guide "as a possible criterion of failure, buckling can not be disregarded". However, information on such topic is very scarce in the literature. Currently, the only documents that provide design guidelines for buckling are the CIRIA Guide and the Portland Cement Association Design Aid, both of which are based on theoretical studies and engineering judgement. An experimental testing programme, consisting of 7 large scale beam-panels with height/thickness ratios in the range of 20 to 70 and a constant span/depth ratio of 1.0, provided buckling data against which the reliability of the two design documents was assessed. These tests confirmed that both documents offer a safe buckling design with the CIRIA Guide being too conservative. Although deep beams are frequently continuous over several spans, very little published data exist for such beams. For this purpose, 12 two-span continuous concrete deep beams with span/depth ratios less than 1.0 and having different quantities and arrangements of web reinforcement were tested under two point-loads. The specimens were heavily instrumented to obtain as much information as possible about the behaviour of the beams at each stage of loading. Applied loads and reactions were among the measurements made and enabled the actual bending moment distribution to be determined and compared to that of corresponding continuous shallow beams. Based on the test results and observations and in the light of other published work, recommendations are given for the bearing, shear and flexural design of continuous deep beams.