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 22. Why are dimples present in golf balls? The golf balls are subject to lift and drag forces when struck. The drag force is a retarding forces acing in a direction opposite to the direction of flight path of the golf ball. Separation occurs behind the ball which forms a low-pressure wake. The pressure drag is the difference in pressure between the front and back of the ball. With dimples on the ball, the formation of separation is delayed so that it narrows the size of wake. Consequently, less pressure is pulling on the back of the ball so that the golf ball could travel longer in distance. 164
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU Level Two (Advanced FAQs) Part II: Cracking and Tests 1. What are the possible causes of longitudinal and circumferential cracking in concrete pipes? (C1) Concrete pipes are designed to crack in tensile zone so that steel reinforcement could take up the tensile stress. Flexural stresses are developed at the top and bottom inside surfaces and on the outside surface at the sides. As such, they are the potential locations of longitudinal cracks. Longitudinal cracks are formed as a result of excessive soil and traffic loads or inadequate pipe bedding. Visible longitudinal cracks observed at top and invert inside the pipe should be more severe than those on the outside because tensile stress occur at top and invert portion of the inside of the pipes. On the contrary, longitudinal cracks formed outside the pipe at the sides of concrete pipe should be more severe than those on the inside. Multiple longitudinal cracks with small crack width (e.g. 0.15mm) is acceptable which indicate effective transfer of stress from concrete to steel. Care should be taken when discovering a single wide longitudinal crack. Circumferential cracks may occur owing to loads imposed during construction and uneven bedding. It may also be caused by the relative movement of another drainage structure connecting to the concrete pipe. Circumferential cracks do not generally affect the load-carrying capacity of concrete pipes. 2. What is the importance of uniform support for precast concrete pipes? (C1) Concrete pipes are designed to be uniformly supported along the length to carry vertical loads on its top. They are normally not intended to serve as a beam to carry loads in longitudinal direction under poor ground supports (i.e. high and low spots in bedding). Under cantilever beam action and simply support beam action as shown in the diagram below, circumferential cracks would develop in concrete pipes. Circumferential cracking develops in concrete pipes only when pipe bedding becomes non-uniform. Therefore in unstable ground conditions such as soft spot and hard foundations, care should be taken to provide firm and even support to concrete pipes. 165
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A Self-Learning Manual - Institution of Engineers Mauritius

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