Design of Liquid Retaining Concrete Structures_طراحی بتن نگهدارنده مایع سازه ها

Design of Liquid Retaining Concrete Structures

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It is common practice to use reinforced or prestressed concrete structures for the storage of water and other aqueous liquids. Similar design methods may also be used to design basements in buildings where groundwater must be excluded. For such purposes as these, concrete is generally the most economical material of construction and, when correctly designed and constructed, will provide long life and low maintenance costs. The design methods given in this book are appropriate for the following types of structure (all of which are in-line with the scope of Part 3 of Eurocode 2, BS EN 1992-3, 2006): storage tanks, reservoirs, swimming pools, elevated tanks (not the tower supporting the tank), ponds, settlement tanks, base ment walls, and similar structures (Figures 1.1 and 1.2). Specifi cally excluded are: dams, structures subjected to dynamic forces, and pipelines, aqueducts or other types of structure for the conveyance of liquids.It is convenient to discuss designs for the retention of water, but the principles apply equally to the retention of other aqueous liquids. In particular, sewage tanks are included. The pressures on a structure may have to be calculated using a specifi gravity greater than unity, where the stored liquid is of greater density than water. Throughout this book it is assumed that water is the retained liquid unless any other qualifi cation is made. The term ‘structure’ is used in the book to describe the vessel or container that retains or excludes the liquid. The design of structures to retain oil, petrol and other penetrating liquids is not included (the code (BS EN 1992-3, 2006) recommends reference to specialist literature) but the principles may still apply. Likewise, the design of tanks to contain hot liquids (> 200°C) is not discussed. A structure that is designed to retain liquids must fulfi l the requirements for normal structures in having adequate strength, durability, and freedom from excessive cracking or defl ection. In addition, it must be designed so that the liquid is not allowed
to leak or percolate through the concrete structure. In the design of normal building structures, the most critical aspect of the design is to ensure that the structure retains its stability under the applied (permanent and variable) actions. In the design of structures to retain liquids, it is usual to fi nd that if the structure has been proportioned and reinforced so that the liquid is retained without leakage (i.e. satisfying the Serviceability Limit State, SLS), then the strength (the Ultimate Limit State, ULS requirements) Historically, the design of structural concrete was based on elastic theory, with specifi ed maximum design stresses in the materials at working loads. In the 1980s, limit state philosophy was introduced in the UK, providing a more logical basis for determining
factors of safety. 2011 has seen the introduction of the new Eurocodes; BS 8110 and BS 8007 have been withdrawn, and in their place is a suite of new codes, including specifi cally BS EN 1992-1-1:2004 (Eurocode 2 Part 1 or EC2) and BS EN 1992-3: 2006 (Eurocode 2 Part 3 or EC2 Part 3) and their respective National Annexes. The new Eurocodes continue to adopt the limit state design approach. In ultimate design, the working or characteristic actions are enhanced by being multiplied by partial safety factors. The enhanced or ultimate actions are then used with the failure strengths of the materials, which are themselves modifi ed by their own partial factors of safety, to design the structure. Limit state design methods enable the possible modes of failure of a structure to be identifi ed and investigated so that a particular premature form of failure may be prevented. Limit states may be ‘ultimate’ (where ultimate actions are used) or ‘serviceability’ (where service actions are used). Previously, when the design of liquid-retaining structures was based on the use of elastic design (BS 5337), the material stresses were so low that no fl exural tensile cracks developed. This led to the use of thick concrete sections with copious quantities
of mild steel reinforcement. The probability of shrinkage and thermal cracking was not dealt with on a satisfactory basis, and nominal quantities of reinforcement were specifi ed in most codes of practice. It was possible to align the design guidance relating to liquid-retaining structures with that of the Limit State code BS 8110 Structural Use of Concrete once analytical procedures had been developed to enable flexural crack widths to be estimated and compared with specifi ed maxima (Base et al., 1966; Beeby, 1979) and a method of calculating the effects of thermal and shrinkage strains had been published (Hughes, 1976).

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