Contents Preface 7 iv Design of Reinforced Cement Concrete Structures 233 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 233 245 258 263 271 279 285 290 292 Concrete – the material Principle of reinforced concrete design Design norms for reinforced concrete beams Design of reinforced concrete slab Design of reinforced concrete foundations Design of axially loaded columns Pre-stressed concrete – an introduction Multistoried structures Conclusion
Preface Construction is the largest industry in the world and of course, anything that is constructed needs to be designed first. Structural Engineering deals with analysis and design aspects, the basic purpose of which is to ensure a safe, functional and economical structure. Throughout the designing process, the designer constantly interacts with specialists like architects, operational managers, etc. Once the design is finalized, the implementation requires the involvement of people to handle aspects such as statutory approvals, planning, quality assurance, material procurement, etc. The entire exercise can be undertaken in a highly coordinated way if everyone involved understands the ‘project language’, which is a combination of designs and specifications. To understand the language fully, it is necessary to appreciate the principles of structural analysis and design, and a book on this topic comes in handy here. Reading this book will help you gain the basic knowledge of structural engineering that includes principles of analysis of structures and their application, behavior of materials under loading, selection of construction materials and design fundamentals for RCC and steel structures. The emphasis has been kept on the determination of the nature and amount of stress developed under loads, and the way structures offer resistance to it. Being the most widely used construction materials, RCC and steel have been covered in detail, though masonry and timber have been described briefly as well. This manual is suitable for anyone associated with the construction industry. In view of the vastness of the sector, the following personnel would typically be able to gain immediate benefit out of the course. • Building Inspectors • Project Managers • Construction Supervisors • Municipal Officials • Architects • Quantity Surveyors • Insurance Surveyors • Concrete Technologists • Reinforcement Detailers • Structural Fabricators • Building Maintenance Personnel • Structural Rehabilitation Staff It is expected that this book will enable you to: • Fully understand the role of a structural engineer • Comprehend the behavior of structural members under loading • Understand the concept of stress functions like tension, compression, shear and bending • Use the basic concepts for analysis of statically determinate and indeterminate structures • Analyze deformation of members under loading • Understand the significance of material properties in design • Undertake basic design of reinforced cement concrete structures • Undertake basic design of steel structures • Undertake basic design of masonry and timber structural members
7 Design of Reinforced Cement Concrete Structures This chapter will enable the reader to understand the behavior of reinforced cement concrete as a composite structural material. The reader also would be able to understand the design process of various types of structures made of this material, using the previously understood principles of structural analysis and design. Learning objectives After completion of study of this chapter, one will be able to: • Fully understand the behavior of concrete as a structural material and the factors that influence this behavior • Understand the principles of design of this composite material • Understand design techniques, based on working stress theory, used in a number of structures commonly constructed with reinforced cement concrete • Obtain a reasonable degree of familiarity with the principles of pre-stressed concrete construction Reinforced Cement Concrete (RCC) is undoubtedly the most widely used construction material. It is, perhaps, the most versatile of all construction materials. The advantages of using RCC for construction are manifold. It can be produced at any location; can be designed to adopt the locally available constituents for its production and gives great flexibility regarding component size and shape to the designer and constructor. Besides, concrete is a well-researched material and its properties are quite well understood. 7.1 Concrete – the material Concrete is a word of Latin origin. It refers to a material that hardens under water. In construction practice, concrete is a mix of cement, aggregate, water and occasional admixtures, which remains plastic at the time of manufacturing. Subsequently, as a result of chemical action between cement and water, the mixture looses its plasticity and gains strength. The process of losing plasticity (termed setting) is quite rapid and takes from a few minutes to few hours depending on various factors. The process of gaining strength (termed hardening) is a prolonged one and continues for a very long period after the setting of the concrete. These properties of concrete, and the associated property of not being prone to any chemical reaction by water in the later periods of its life, are very important and have contributed to the widespread use of concrete as a building material.
234 Structural Design for Non-structural Engineers The relationship between the constituents of a concrete mix can be described in more than one way. It is possible to view hardened cement as the main component of formation and aggregate as the filler or dilutant. Another way to look at it is by viewing the aggregate as the main structural component, something like masonry, bonded together by cement. Research and analysis has established that if one tries to use cement alone as a construction material, the disadvantages are aplenty. Cement has certain negative characteristics such as excessive shrinkage, creep, heat generation during hydration, etc which are brought down in magnitude due to the presence of aggregate. Hence, aggregate in concrete has a role larger than that of mere filler. At the same time, aggregate can hardly be considered as the main component in concrete. Therefore, it is sensible to view concrete as a material consisting of two phases, the hydrated cement phase and the aggregate phase. The overall properties of concrete are dependent on the combination of the properties of these two. After referring to concrete as a two-phase material, now we would discuss its characteristics further with special reference to the modulus of elasticity of the composite product. We know that the value of modulus of elasticity is one of the most important aspects for structural design. Concrete, being a composite material, cannot be assumed to have a constant value. Due to the two-phased structure, concrete can be viewed to have two possible forms, which are fundamentally different from each other. The first is of an ideal composite hard material, which has a continuous matrix of an elastic phase with a high modulus of elasticity and embedded particles of a lower modulus. The second possible form can be that of an ideal composite soft material, which consists of elastic particles with a high modulus of elasticity, embedded in a continuous matrix phase with a lower modulus. The difference between the calculated modulus of elasticity in both the cases can be large. In the case of a composite hard material, it is assumed that the strain is constant over any cross-section while the stresses in the phases are proportional to their respective moduli. While in the case of composite soft material, the modulus of elasticity is based on the assumption that the stress is constant over any cross-section and the strain in the phases is inversely proportional to their respective moduli. For practical purposes, a fairly good approximation is derived based on assumptions for composite soft material for mixes made with normal aggregates. We now will observe the properties and contribution of each ingredient of the concrete. As stated earlier, concrete has the following ingredients: • Cement • Aggregate (large and small) • Water • Admixtures 7.1.1 Cement A product with binding characteristics is called cement. For concrete, the most commonly used cement is Portland cement. Portland cement is the name given to cement obtained by intimately mixing together calcareous and argillaceous materials, alumina, and iron oxide-bearing materials, burning them at a high temperature (called clinkering temperature) and finally grinding the resulting clinker. Normally no material other than gypsum, water and grinding aids may be added after burning. From the description of Portland cement given above, it can be seen that it is made primarily from a combination of a calcareous material, such as limestone or chalk, and of silica and alumina found as clay. The manufacturing process, described briefly below, consists of grinding the raw materials into a very fine powder, mixing them intimately in predetermined proportions and then burning the mix in a large rotary kiln at a temperature of about 1350 °C. In the kiln, the material fuses partially and forms a clinker. The clinker is cooled and ground to a fine powder with some gypsum added, to produce commercial Portland cement. A typical composition limit for Portland cement has been shown in the table below: