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 so that a positive feedback occurs between the aerodynamic forces and natural vibration of the bridge. In essence, the vibration movements of the bridge increase the aerodynamic load which in turns cause further movement of the bridge. Consequently, it results in large magnitude of movement and may cause bridge failure. 7. How does deck equipment (median dividers and parapets) affect the aerodynamic response of long-span bridges? Bridge parapets raise the overall level of bluffness of long-span bridges. When the solidity ratio of barriers increases, the effect of increasing the bluffness also becomes more significant. The principal effects of deck equipment such as median dividers and parapets is that it enhances an increase in drag forces and a reduction in average value of lift force. 8. In wind tunnel test, why are similarity of Reynolds Number between real bridge and model is often neglected? Wind tunnel test is often conducted to check aerodynamic stability of long-span bridges. To properly conduct wind tunnel test, aerodynamic similarity conditions should be made equal between the proposed bridge and the model. Reynolds Number is one of these conditions and is defined as ratio of inertial force to viscous force of wind fluid. With equality of Froude Number, it is difficult to achieve equality in Reynolds Number. For instance, for a model scale of 1/40 to 1/150, the ratio of Reynolds Number between the bridge and the model varies from 252 to 1837 with a difference of order from 100 to 1000. As such, similarity of Reynolds Number between real bridge and model is often neglected. 56
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU Level Two (Advanced FAQs) Part III: Prestressing Works 1. What are parasitic forces for prestressing? In statically determinate structures, prestressing forces would cause the concrete structures to bend upwards. Hence, precambering is normally carried out to counteract such effect and make it more pleasant in appearance. However, for statically indeterminate structures the deformation of concrete members are restrained by the supports and consequently parasitic forces are developed by the prestressing force in addition to the bending moment generated by eccentricity of prestressing tendons [53]. The developed forces at the support modify the reactions of concrete members subjected to external loads and produces secondary moments (or parasitic moments) in the structure. 2. Why type of prestressing is better, external prestressing or internal prestressing? (PT1) At several locations in the span (i.e. third or quarter points) the tendons are deviated to the correct tendon profile by concrete deviators in external prestressing. The advantages of external prestressing are listed below: (i) Owing the absence of bond between the tendon and structure, external prestressing allows the removal and replacement of one or two tendon at one time so that the bridge could be retrofitted in the event of deterioration and their capacity could be increased easily. This is essential for bridges in urban areas where traffic disruption is undesirable. (ii) It usually allows easy access to anchorages and provides the ease of inspection. (iii) It allows the adjustment and control of tendon forces. (iv) It permits the designer more freedom in selecting the shape of cross section of bridges. (v) Webs could be made thinner so that there is a reduction of dead load. (vi) It enhances a reduction of friction loss because the unintentional angular change like wobble is eliminated. Moreover, the use of polyethylene sheathing with external prestressing has lower friction coefficient than corrugated metal ducts in internal prestressing. 57
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A Self-Learning Manual - Institution of Engineers Mauritius

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