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 deformation by diffusion of nitrogen in steel. Hence, there is limitation stated in some design codes to restrict the nitrogen content of steel to 0.012%. 4. Is fatigue more serious in large-diameter reinforcing steel? (SR4) Indeed past research showed that large-diameter reinforcing steel appeared to be weaker under fatigue loading conditions. Therefore, in some standards the stress range for testing fatigue of steel bars are reduced for increasing bar size for the same reason. Moreover, fatigue performance of steel bars is also governed by stress concentrations at the root of ribs. Bar failure usually initiates from the root of ribs under fatigue loading. In this connection, any damage to ribs during bending process can lead to failure by fatigue. 5. How can heating assist in rebending of steel reinforcement? It is not uncommon that starter bars are bent up within the formwork as a measure of temporary protection. Later, after the concrete is placed and formwork is removed, the steel bar reinforcement would be pulled out and straightened. For rebending of bars, it is preferably be implemented for small diameter bars with mild steel. Moreover, rebending of steel bars should not be carried out below 5 o C owing to brittle fracture. Heating could be adopted to assist rebending process. However, heating should be applied to a good length of a bar instead of a concentrated location because of the possible occurrence of overheating. Moreover, after heating the cooler adjacent part of the steel bar may experience fracture when the bars are stressed in case concentrated heating is applied to steel bars. 6. Why does square hollow section become more popular than circular hollow section in steelworks? (S1) Circular hollow section was available for many years until in 1960s for the approval of square hollows section and rectangular hollow section. From technical point of view, circular hollow section is the most efficient form of strut when compared with square hollows section and rectangular hollow section. However, nowadays, the use of square hollows section and rectangular hollow section is ever on the rise because of the ease of connections between individual struts. 394
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU 7. What is the vibration mechanism caused by driving sheetpiles? (S2) There are generally three main vibration mechanisms caused by driving sheetpiles: (v) When the sheetpiles are impacted by a hammer, a compressive wave would be formed and it travels down to the toe of sheetpiles. A large amount of energy would be used to cause downward movement of sheetpiles while some of the energy would be reflected back up to the sheetpiles. The remaining energy would be transmitted to soils which expand outward as a spherical wavefront called “P” waves. (vi) The impact action of hammer causes temporary lateral deformation of sheetpiles. A surface wave is then established which travels outward from pile shaft circumferentially. (vii) The downward motion of sheetpiles arising from hammering action induces vertically polarized shear waves which propagate outward in cylindrical wavefront. Fig. Vibration waves by driving 8. Should wide or narrow sheetpiles be adopted in temporary work? (S3) In general, wide and deep sheetpiles tend to be more cost-effective than narrow sections because they provide the same bending strength with a lower weight per square foot. As such, with increasing width of sheetpiles sections, fewer sheetpiles are required to cover a certain length of piling 395
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A Self-Learning Manual - Institution of Engineers Mauritius

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