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 Level Two (Advanced FAQs) Part III: Steel Reinforcement 1. What is the difference in application between open stirrups and closed stirrups in concrete beams? Open stirrups are provided principally to resist shear forces in concrete beams and they are applied in locations in which the effect of torsion is insignificant. U-shaped stirrups are placed in the tension side of concrete beams in which shear cracks would occur. However, when concrete beams are designed to resist a substantial amount of torsion, closed stirrups should be used instead. 2. What are the reasons for establishing minimum distance between bars and maximum distance between bars? (SR1) In some codes, a minimum distance between bars is specified to allow for sufficient space to accommodate internal vibrators during compaction. On the other hand, the restriction of maximum bar spacing is mainly for controlling crack width [49]. For a given area of tension steel areas, the distribution of steel reinforcement affects the pattern of crack formation. It is preferable to have smaller bars at closer spacing rather than larger bars at larger spacing to be effective in controlling cracks. Hence, the limitation of bar spacing beyond a certain value (i.e. maximum distance between bars) aims at better control of crack widths. 3. What are the reasons of establishing minimum area of reinforcement and maximum area of reinforcement? (SR2) Beams may be designed to be larger than required for strength consideration owing to aesthetics or other reasons. As such, the corresponding steel ratio is very low and the moment capacity of pure concrete section based on the modulus of rupture is higher than its ultimate moment of resistance. As a result, reinforcement yields first and extremely wide cracks will be formed. A minimum area of reinforcement is specified to avoid the formation of wide cracks [49]. On the other hand, a maximum area of reinforcement is specified to enable the placing and compaction of fresh concrete to take place easily. 110
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU 4. Why is tension anchorage length generally longer than compression anchorage length? (SR3) Tension anchorage length of steel reinforcement in concrete depends on bond strength. When steel reinforcement is anchored to concrete and is subjected to compressive forces, the resistance is provided by the bond strength between concrete and steel and the bearing pressure at the reinforcement end. Tension lap length is generally longer than compression lap length. In some design codes, instead of permitting the use of bearing pressure at reinforcement ends, the allowable ultimate bond stress is increased when calculating compression anchorage length. 5. Why does lap length generally greater than anchorage length? (SR3) In some structural codes, the lap length of reinforcement is simplified to be a certain percentage (e.g. 25%) higher than the anchorage length. This requirement is to cater for stress concentrations at the end of lap bars. A smaller load when compared with the load to pull out an anchored bar in concrete triggers the splitting of concrete along the bar because of the effect of stress concentration. A higher value of lap length is adopted in design code to provide for this phenomenon. 6. Does longitudinal steel serve as an enhancement of shear strength? In addition to shear resistance provided by shear reinforcement, shear forces in a concrete section is also resisted by concrete compression force (compressive forces enhances higher shear strength), dowel actions and aggregate interlocking. The presence of longitudinal steel contributes to the enhancement of shear strength of concrete section in the following ways [46]: (i) The dowelling action performed by longitudinal reinforcement directly contributes significantly to the shear capacity. (ii) The provision of longitudinal reinforcement also indirectly controls the crack widths of concrete section which consequently affects the degree of interlock between aggregates. 7. Why are longer tension lap lengths designed at the corners and at the top of concrete structures? (SR3) 111
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A Self-Learning Manual - Institution of Engineers Mauritius

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