A STRUT-AND-TIE MODEL FOR DEEP BEAMS SUBJECTED TO COMBINED TOP-AND-BOTTOM LOADING

This paper presents a strut-and-tie approach for the analysis of deep beams subjected to: (i) top loading; (ii) bottom loading, and (iii) combined top-and-bottom loading. The model consists of three groups, ie concrete struts, nodal zones and steel ties from the reinforcement. Incorporated with appropriate criteria for each group, the model gives very good shear strength predictions of deep beams, in comparison with the values from the ACI Building Code, 318-95, the 1984 Canadian Code, and the UK CIRIA Guide 2. Detailed derivation for the iterative strut-and-tie model is outlined here; the equations can be easily implemented in a personal computer. Comparisons of strut-and-tie predictions are given for: (i) 233 top-loaded; (ii) six bottom-loaded and (iii) 12 combined top-and-bottom-loaded deep beams. It is shown that the strut-and-tie model gives consistently good predictions of the ultimate strengths of deep beams irrespective of the loading condition. More importantly, the suggested approach is rational and the failure criteria are explicit. The model can also be extended as a useful tool in the design office as it can indicate the 'weakest link' in the truss; structural engineers can subsequently modify the design, thereby controlling the failure load. (A)