Strut-and-tie actions in pile cap analysis elastic analysis

Plate analysis under the 'plane remains plane' assumption has been used extensively for design of pile cap structures in Hong Kong, though it is well known that the 'plane remains plane' phenomenon is not correct for those structures where the thickness is comparable to the spacing of supports and loads. Strut-and-tie model (STM) is another alternative in which the structure is idealised as a series of struts and ties in locations and directions of the anticipated stress paths. However, the authors find that the assumed struts and ties may not be distinctly formed within the structures under working load condition, as revealed by finite element analysis on worked examples. So great care must be exercised in using the STM method. In fact, with the extensive use of the finite element method nowadays, stresses within these structures can be conveniently analysed and design be properly performed. The finite element method should be considered as a more reliable tool. Reinforcement design to stresses as analysed by the flnite element method is not common at present. The works by Clarke [3] is limited to 2-dimensional problem and care should be exercised in extending the principles to 3-dimensional problem. Nevertheless, the authors consider that this is the way ahead by which more accurate and economical design can be achieved. The objectives of the paper are to alert engineers that thick cap structures may not exhibit distinct strut and tie behaviours as assumed in the STM method and to advocate that the establishment of user friendly methods for reinforced concrete design against stresses obtained from finite element analysis for deep caps should be the right way forward.