ABSTRACT
The distribution of forces resembles that which is usually assumed to occur in a deep column footing (Figure 10.1), and the distribution of stresses is qualitatively similar; the analogy is by no means exact, however. Figure 10.2 shows the distribution of stress and the directions of the principal tensile and compressive stresses which actually occur, and which should be allowed for in design. It is of interest to note that this diagram was published by Coker in 1921 in connection with the effects of concentrated loads^. It is obvious that the
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more closely the distribution of the prestressing force approaches the resultant distribution of stress, the smaller are the local tensile stresses in the anchorage zone. From the stress diagram (Figure 10.2) it will be noted that there are large transverse compressive stresses close to the point of loading and transverse tensile stresses at a greater depth. The compressive stresses tend to cause bursting of the concrete and helical reinforcement and/or welded mats are required to resist this tendency; the tensile stresses may cause cracks along the axis of loading, and straight transverse reinforcement (usually in the form of stirrups) should be provided to control this cracking. It should be noted also that cracking cannot always be completely eliminated since the stress diagram shows that the presence of tensile stresses (and hence in some cases of micro cracks) is unavoidable. The transverse reinforcement should therefore be sufficient to ensure that no visible cracks develop.
