A Self-Learning Manual - Institution of Engineers Mauritius

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A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU account for the effect of pressure bulbs. 8. Which type of pile cap transfers loads equally to piles, flexible pile cap or rigid pile cap? (FD8) Loads from columns transferring to pile cap induce tensile forces at the bottom of the cap. For instance, by using truss analogy to analyze a pile cap sitting on two piles with a column at the centre of the pile cap, the tensile force at the bottom is proportional to the pile spacing and is inversely proportional to depth of pile cap. The bottom reinforcement is designed to resist the tensile stressed generated from loads in columns. Sometimes, reinforcement may be designed at the top of pile caps which serve as compression reinforcement. This type of reinforcement is required in case there is a limitation on the depth of pile caps. Similarly shear reinforcement is introduced to the pile caps in case there is a restriction to the depth of pile caps. Consider that loads are applied at the centre of a pile cap. For the case of rigid pile cap, owing to the effect of interaction of individual piles, the central piles tend to settle more than the edge piles when the pile cap is under loading condition. For the pile cap to be rigid, the local deformation of central piles would not occur. Instead, the stiff pile cap would transfer the loads from the central piles and redistribute them to the outer piles. Therefore, piles at the edge take up a higher fraction of the total loads and are subjected to higher axial and bending loads in case the pile cap is stiff. In the extreme case, the side piles may take up as much as about two to three times the loads in the central piles and this may lead to the failure of these edge piles. For flexible pile cap, load taken up by individual piles are different because the deformation of pile cap enhances non-uniform distribution of loads among piles. The piles closer to the load tend to share more loads when compared with those which are located far away from the loads. The difference of loads induced in piles increase with the flexibility of pile cap. 9. In some pile design, the settlement of piles are not checked. Is it correct? The performance of piles mainly consists of the two elements, namely ultimate bearing capacity and settlement. The local practice of pile design is place emphasis on checking if the bearing capacity of piles would be 226
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU exceeded. Engineers tend to adopt the approach that bored piles are designed to be founded on bedrock while for driven piles they are driven to very stiff stratum with SPT N values greater than 200. Owing to rigid and firm foundation on which the piles are seated, it is therefore assumed that the amount of settlement shall be limited. On the other hand, there is practical difficulty in assessing how much settlements are considered acceptable owing to limited available data. 10. How should the piles be arranged in a pile cap to reduce bending moment induced in piles? Consider that piles are designed to intersect at a single common point in a pile cap. The resultant reactions would pass through the point of intersection in the pile cap. This type of arrangement does not involve any bending moment induced if the horizontal loads pass through this point. However, in real life situation, the piling system is expected to resist a combination of vertical loads, horizontal loads and bending moment. To counteract bending moment, the pile cap about the point of intersection is rotated so that significant amount of bending moment is induced in piles and pure axial forces in piles can hardly generate a counteracting moment based on one single intersection point [53]. However, if the piles are arranged in such a way that there are at least two separated points of intersection in the pile cap, the amount of flexural stresses induced in piles is significantly reduced. Fig. Different arrangement of piles in pile cap 227
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A Self-Learning Manual - Institution of Engineers Mauritius

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