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2622 results in Civil and geotechnical engineering

Page 28 of 132

The Urban Ocean
  • The Interaction of Cities with Water
  • Alan F. Blumberg, Michael S. Bruno
  • Published online:
    10 October 2018
    Print publication:
    01 November 2018
  • This book introduces the new discipline of urban oceanography, providing a deeper understanding of the physics of the coastal ocean in an urban setting. The authors explore how the coastal ocean impacts with the humans who live, work and play along its shores; and in turn how human activities impact the health and dynamics of the coastal ocean. Fundamental topics covered include: the governing dynamical equations; tidal and circulation processes; variation of salinity and freshwater fluxes; watershed pollutants; observing systems; and climate change. Bridging the gaps between the fields of engineering, physical and social sciences, economics, and policy, this book is for anyone who wishes to learn about the physics, chemistry, and biology of coastal waters. It will support an introductory course on urban oceanography at the advanced undergraduate and graduate level, and will also prove invaluable as a reference text for researchers, professionals, coastal urban planners, and environmental engineers.

  • 5 - Deflection of beams and crack control

  • from Part 1 - Reinforced concrete
  • Yew-Chaye Loo, Griffith University, Queensland, Sanaul Huq Chowdhury, Griffith University, Queensland
  • Book:
    Reinforced and Prestressed Concrete
    Published online:
    12 December 2018
    Print publication:
    26 July 2018, pp 94-122

  • Summary

    Introduction

    As discussed in Section 1.2, in addition to strength, durability and fire resistance, serviceability is a design requirement specified in the Standard. Practical recommendations are given in Clauses 8.5, 8.6 and 8.9 of the Standard, for the treatments of beam deflection, crack control and slenderness limits for beams, respectively. The Standard also touches very briefly on the vibration of beams, stating qualitatively in Clause 8.7 (and in Clause 9.5 for slabs) that ‘vibration of beams shall be considered and appropriate action taken where necessary to ensure that the vibrations induced by machinery, or vehicular or pedestrian traffic, will not adversely affect the serviceability of the structure’.

    Although vibration topics are not dealt with in this book, the reader may refer to articles published by Chowdhury and Loo (2001, 2006), and by Salzmann, Fragomeni and Loo (2003) for the damping characteristics of simple and continuous concrete beams.

    In this chapter, details are provided on short- and long-term deflection calculations, and on alternative design requirements of maximum span/effective depth ratio, in accordance with the Standard. The analysis of the total deflection of beams under repeated loading is also introduced as an advanced topic. For completeness, crack control of beams is discussed in some detail. All the deflection topics are supplemented by worked out examples. Recommendations for computing effective flange widths have been presented in Section 4.2.2. The reader is referred to the Standard for discussions on slenderness limits and vibrations.

    Deflection formulas, effective span and deflection limits

    Formulas

    The deflection analysis of beams is a topic normally dealt with in courses such as ‘mechanics of materials’ and ‘structural analysis’. Therefore, it is sufficient here to simply summarise the general formulas for computing beam deflections. There are three common types of support conditions for beams:

  • simply supported

  • continuous

  • cantilever.

    • Loading, on the other hand, includes concentrated or point load, distributed load and applied moment.

  • 4 - T-beams and irregular-shaped sections

  • from Part 1 - Reinforced concrete
  • Yew-Chaye Loo, Griffith University, Queensland, Sanaul Huq Chowdhury, Griffith University, Queensland
  • Book:
    Reinforced and Prestressed Concrete
    Published online:
    12 December 2018
    Print publication:
    26 July 2018, pp 61-93

  • Summary

    Introduction

    The ultimate strength analysis and design of T-beams and other flanged sections as well as nonstandard or irregular-shaped sections are dealt with in this chapter. The procedures involved together with derivation of relevant formulas and the application of relevant Standard recommendations are presented, with illustrative examples. The treatment of continuous beams and other statically indeterminate structures is briefly described.

    T-beams and other flanged sections

    Definitions

    Beams with a cross-sectional shape of T, L, I or a box are referred to collectively as flanged beams. Flanged sections are a popular choice for major structures, such as road and railway bridges, due to their structural efficiency. A beam-and-slab building floor system may be considered as an assembly of T-beams. An example is shown in Figure 4.2(1)a.

    The analysis and design of flanged beams may seem to be comparatively more involved than the analysis and design of rectangular beams, due to more complicated geometry. However, using simple assumptions and some approximations, the formulas developed for singly and doubly reinforced rectangular sections can be adapted for use in the analysis and design of flanged beams.

    Effective flange width

    Figure 4.2(1) shows a typical T-beam. A three-dimensional stress analysis would indicate that, depending on the dimensions of the beam, the bending stress distribution over the width of the flange (b) is not uniform as it would be for a rectangular section. Instead, the stress varies from a maximum (f1) near the top of the web to a minimum (f2) at the two edges of the flange, as in Figure 4.2(1)b. However, the stresses f1 and f2 are statically indeterminate.

    To simplify the problem, the actual section in Figure 4.2(1)b may be replaced by an ‘effective’ one, as shown in Figure 4.2(1)c, in which the effective flange width is taken as

    where A is the area of the stress diagram E-A-B-C-F. In the effective section, the maximum stress (f1) is uniformly distributed over the effective width (bef) and can be computed in the usual way.

  • 14 - Introduction to prestressed concrete

  • from Part 2 - Prestressed concrete
  • Yew-Chaye Loo, Griffith University, Queensland, Sanaul Huq Chowdhury, Griffith University, Queensland
  • Book:
    Reinforced and Prestressed Concrete
    Published online:
    12 December 2018
    Print publication:
    26 July 2018, pp 403-413

  • Summary

    Introduction

    Prestressing may be seen as an elaborate and active way to reinforce concrete when its tensile capacity is insufficient. Whereas the traditional reinforcement becomes active mainly after the concrete has exceeded its cracking strength, the purpose of prestressing is to prevent cracking from occurring. This is done by introducing compressive stress in the concrete to neutralise the anticipated tensile stress developed under load.

    In a traditional reinforced concrete design, the safety margin can always be increased by providing more reinforcement. The same may not be true in prestressed concrete, as over-prestressing can cause cracking or perhaps failure before even any external loading is applied. As a result, prestressed concrete analysis and design are more complicated and mechanics-based than for reinforced concrete, which relies more on empirical formulas. In practice, prestressed concrete also requires a higher level of technology in its construction.

    By nature, prestressing is more efficient than the traditional reinforcement in that the stress in concrete, either tensile or compressive (caused by self-weight or other forms of dead load), can be neutralised before any additional (live) loading is applied. Consequently, for a given design, the maximum permissible prestressed concrete span can be considerably larger than a reinforced one. Following some fundamentals given in this chapter, Chapter 15 presents the bending theory of fully prestressed concrete beams based on the critical stress state criteria (which ensures that no cracking or overstressing in tension or compression would ever occur throughout the life of the beam under service load). The design of beams in bending using the critical stress state approach is given in Chapter 16. To comply with the Australian Standard (AS) 3600–2009 (the Standard), in practice, all prestressed beams must have the required strength. The bending strength analysis of fully and partially prestressed concrete beams is discussed in Chapter 17. The last chapter, Chapter 18, reviews the analysis and design of end blocks for prestress anchorage.

    Non-engineering examples of prestressing

    Wooden barrel

    Figure 14.2(1) shows an assembly of curved barrel staves with tapered ends that are held together by the metal hoops forced from both ends towards the centre. The forced expansion of the hoops produces the compressive stress between the staves. To enhance efficiency, the hoops may be heated up before installation. It is well known that a well-made wooden barrel cannot only hold itself together, but is also water tight.

  • Appendix A - Elastic neutral axis

  • Yew-Chaye Loo, Griffith University, Queensland, Sanaul Huq Chowdhury, Griffith University, Queensland
  • Book:
    Reinforced and Prestressed Concrete
    Published online:
    12 December 2018
    Print publication:
    26 July 2018, pp 483-484

  • Summary

    The elastic, or working stress method, has been dropped from the various design codes and Standards for many years. However, some of the analysis equations remain useful in computing deflections. In particular, neutral axis (NA) positions are needed in the assessments of the effective moments of inertia.

    The procedure for determining the position of the NA of a fully cracked rectangular section is summarised in this appendix, accompanied by some useful formulas.

  • 2 - Design properties of materials

  • from Part 1 - Reinforced concrete
  • Yew-Chaye Loo, Griffith University, Queensland, Sanaul Huq Chowdhury, Griffith University, Queensland
  • Book:
    Reinforced and Prestressed Concrete
    Published online:
    12 December 2018
    Print publication:
    26 July 2018, pp 13-18

  • Summary

    Introduction

    Selection of materials and a decision on the associated properties constitute an essential first step in any structural design. In this chapter, the relevant information and data required for the design of reinforced and prestressed concrete structures are covered in some detail.

    Concrete

    Characteristic strengths

    The characteristic compressive strength at 28 days is defined as the strength value of a given concrete that is exceeded by 95% of concrete cylinders tested (made from the same concrete). The test method for determining is described in AS 1012.9–2014. The value of is used in all Standard-based design calculations. Thus, in mix design, the strength to aim for (or the 28-day mean in situ compressive strength) is

    where s is the standard deviation of the test data and k varies from 1.25 to 2.5 depending on the grade of concrete and the manufacturing process (see AS 1379–2007 for all aspects of the testing and assessment of concrete or AS 1012.9–2014 for determining the compressive strength of concrete specimens).

    As per Clause 3.1.1.2 of AS 3600–2009 (the Standard), fcm at a given age may be taken as 90% of the mean cylinder strength of the same age or fcmi. Refer to Clause 3.1.1.1 for more recommendations. Various design procedures, such as those described by Ryan and Samarin (1992) and by Murdock and Brook (1982), may be used to determine the proportions of the constituent materials (water, cement, fine and course aggregates and admixtures as appropriate).

    As a function of, the characteristic flexural tensile strength as per Clause 3.1.1.3 of the Standard, may be taken as

    or it may be determined for a given concrete by the laboratory test described in AS 1012.11–2000 (R2014). This value is used mainly in cracking moment and deflection calculations (see Clauses 8.1.6.1 and 8.5.3.1, respectively, in the Standard), and in determining the minimum slab and pad-footing reinforcement (Clause 9.1.1).

    A second tensile strength, the characteristic principal (or splitting) tensile strength, is required mainly in the design for longitudinal shear (Clause 8.4.3 in the Standard).

Page 28 of 132