DOUBLY REINFORCE SECTIONS FOR RCC STRUCTURES

DOUBLY REINFORCE SECTIONS

INTRODUCTION

A doubly reinforced concrete section is reinforced in both compression and tension regions. The section of the beam or slab may be a rectangle, T and L section. The necessity of using steel in the compression region arises due to two main reasons:

(I) When depth of the section is restricted, the strength available from a singly reinforced section is inadequate.

(II) At a support of a continuous beam or slab where bending moment changes sign. Such a situation may also arise in the design of a beam circular in plan.

TYPES OF PROBLEM

There are two of problem in doubly reinforced beams just as in the singly reinforced beams:

(i) Analysis of section, and

(ii) Design of section.

The analysis of doubly reinforced section involves the determination of the amount of moment of resistance knowing breadth, depth, area of tension and compression steels and their covers. The analysis is similar to that for the singly reinforced sections except that the compressive force consists of two parts: in steel and in concrete. The stress in compression steel at the collapse limit state may be the yield stress or some what less depending on the position of the neutral axis. The stress in compression steel must be compatible with the strain in the compression steel.

When a section. of a given dimension is required to carry a given favored bending moment in excess of what it can carry as a singly reinforced section, compression reinforcement becomes necessary. Certain additional tension reinforcement will also be required. However, the total tension reinforcement cannot exceed the maximum permissible tension reinforcement.

The design of a doubly reinforced sector involves the determination of cross sectional dimensions and areas of tension and compression reinforcements. The first step is to determine whether compression steel is required. This may be done by comparing the factored bending moment with the limiting moment of resistance of a singly reinforced section. The amount of compression steel may be obtained by writing the equations of equilibrium.

ACCURACY OF COMPUTATIONS and TYPES OF CONSTRUCTION

ACCURACY OF COMPUTATIONS

Digital calculators and computers can carryout analysis and design of reinforced concrete structures involving calculations to 16 or more significant digits. But it will be noticed that there are lots of variations in practically every aspect of a reinforced concrete structure, vizanalysis, design, construction, loads, material strengths. dimensions and measurements etc. Thus it is clear that too much accuracy in computations is undesirable, it is recommended that computations with three significant digits may be carried out in each step of design. The designer should place more care in determining proper locations and lengths of statement to carry the tensile and compressive forces.

TYPE OF CONSTRUCTION

The nature of construction of buildings in India can be cla.ifi in two categories:

(1) Brick masonry constructions, and.

(2) Reinforced concrete framed construction.

The brick masonry construction mainly consists of brick columns, walls and strip foundations. The brick columns and walls are load bearing components. It is usual -to provide reinforced concrete beams slabs and staircases. Thud, ft is a mixed construction.

The dead and live loads are resisted by reinforced concrete slabs and beams, and brick columns and walls, while wind and earthquake loads are resisted by all brick walls acting as a unified structural system. Such a construction can be economically used for up to three or four storeys Now-adàys, concrete block masonry construction is also becoming popular due to better workmanship and strength of concrete block masonry construction is also becoming popular due to better workmanship and strength of concrete blocks than those of bricks.

Beyond four storeys, it is advisable to go in for reinforced concrete frame& construction. The lateral forces due to wind and earthquake become quit significant. The gravity as well as lateral loads can be conveniently resisted by the framed structure consisting of slabs, beams and columns. The walls are used as partition elements or non-load bearing elements. During earthquakes, the brick partition walls do offer significant mass and lateral stiffness. In case they &re not connected properly with the framed structure, damage to the whole structure is imminent thus; first fifteen chapters of this book are relevant to either type of construction. The later chapters are applicable to concrete framed construction.

BUILDING CODE FOR RCC STRUCTURES

BUILDING CODE

A reinforced concrete structure should conform to certain minimum specifications with regard to design and construction. These specifications are being constantly revised and improved as results of years of analytical and experimental research pour in. The Bu: au of Indian Standards issues building code requirements form time to time. The most recent is the Code of practice for Practice for Plain and Reinforced .Concrete (IS:45t-2OOO), hereafter referred to as the Code.

A building code specifies minimum requirements with regard to a safe structure. It should be clearly understood that a building code is not intended to replace basic engineering principles, knowledge, judgment and experience; lt should be quite easier and quicker to understand the use of designer of the responsibility for having a safe and economical structure.

It must be emphasized that text book does not provide design tables and graphs. Once the basic principles are clearly understood, it should be quite easier and quicker to understand the use of design table and graphs which can greatly speed up the design process. Reference may be made to Design Aids for Reinforced Concrete to IS: 456-1978 published may be made to Design Aids for Reinforced Concrete to charts and design tables.

Limit State method VS Working Stress Method for rc structures

The design of reinforced concrete structural members involves the knowledge of loads, material properties and factor of safety. The working stress method is refered to as deterministic because it is presumed that loads, permissible stresses and factor of safety are known accurately. In physical situations, however, these parameters are not always known and the analysis must be based on predictions. The prediction of such parameters is not always known and the analysis must be based on predictions. The prediction of such parameters is normally based on experience or on field data. The design of a structural member depends on how closely an actual situation is predicted. Parameters that involve the element of prediction are refered to as non deterministic and there is no guarantee that they will actually occur. This forms the basis of limit state design.

In the limit state design method these parameters are deterministic based on observations taken over a period of time. These parameters will thus be influenced by chance or random effect not fast at a single instant but throughout the entire period of time or the sequence of time that is being considered. Such a process is refered to as a stochastic process. In a rough sense a stochastic process is an phenomenon that varies to some degree unpredictably as time goes on. It is also refered to as a Random process.

In the limit state design procedure stresses in an element are obtained from the design loads and compared with design strengths. In the working stress design method the stresses. The main distinction between the two methods lie in the fact that in the former, a member is considered in its limit state, whereas in the later, in its working state. The required more stringent conditions that loading conditions leading to collapse as well as partial safety factors be determined very carefully and accurately in the limit state design.

Type of Constructions for rc structures

The nature of construction of buildings in India can be classified in two categories:

1. Brick Masonry constructions

2. Reinforced concrete framed construction

The brick masonry construction mainly consists of brick columns, walls and strip foundations. The brick columns and walls are load bearing components. It is usual to provide reinforced concrete beams, slabs and staircases. Thus it is a mixed construction.

The dead and live loads are resisted by reinforced concrete slabs and beams, and brick columns and walls. While wind and earth quake loads are resisted by all brick walls acting as a unified structural system. Such a construction can be economically used upto three or four storeys. Now-a-days, concrete block masonry construction is also becoming popular due to better workmanship and strength of concrete block masonry construction is also becoming popular due to better workmanship and strength of concrete blocks than those of bricks.

Beyond four storeys it is advisable to go in for reinforced concrete framed construction. The lateral forces due to wind and earthquake become quit significant. The gravity as well as lateral loads can be conveniently resisted by the framed structure consisting of slabs, beams and columns. The walls are used as partition elements or non-load bearing elements. During earthquakes, the brick partition walls do offer significant mass and lateral stiffness. In case they are not connected properly with a framed structure, damage to the whole structure is imminent. Thus, first fifteen chapters of this book are relevant to either type of construction. The later chapters are applicable to concrete framed construction.

Building codes of RC Structures

A reinforced concrete structure should conform to certain minimum specifications with regard to design and construction. These specifications are being constantly revised and improved as results of years of analytical and experimental research pour it. The Bureau of Indian standards issues building code requirements from time to time. The most recent is the Code of practice for Practice for Plain and Reinforced Concrete, hereafter referred to as the Code.

A building code specifies minimum requirements with regard to a safe structure. It should be clearly understood that a building code is not intended to replace basic engineering principles, Knowledge, judgement and experience. It should be quite easier and quicker to understand the use of designer of the responsibility for having a safe and economical structure.

It must be emphasized that text book does not provide design tables and graphs. Once the basic principles are clearly understood, it should be quite easier and quicker to understand the use of design table and graphs which can greatly speed up the design process. Reference may be made to Design Aids for Reinforced Concrete . Aids for Reinforced Concrete to charts and design tables.

Limit State Method of Rc structures

Limit state design has originated from ultimate or plastic design. The object of design based on the limit state concept is to achieve an achieve an achieve an acceptable probability that a structure will not become unserviceable in its life time for the use for which it is intended, that is ,it will not reach a limit state. A structure with appropriate degrees of reliability should be able to withstand safely all loads that are liable to act on it throughout its life and it should also satisfy the serviceability requirements.

In other words, all relevant limit states must be considered in design to ensure and adequate degree of safety and serviceability

Ultimate Load Method of Rc structures

In ultimate load method, the working loads are increased by suitable factors to obtain ultimate loads. These factors are called load factors. The structure is then designed to resist the desired ultimate loads. This structure is then designed to resist the desired ultimate loads. This method takes into account the non-linear stress-strain behavior of concrete.

The term safety factor has been used in the working stress method to denote the ratio between the yield stress and the permissible stress. It had little meaning as far as the ratio between collapse loads and working load was concerned. The team load factor has been traditionally used to denote the ratio between the collapse or ultimate load to the working load. The knowledge of load factor is more important than the knowledge of factor of safety.

Several ultimate load theories have been proposed in the past. In each of these theories and idealized stress-strain curve has been adopted for concrete which is the only difference among them. This theory is based on the assumption that ultimate strain in concrete is 0.3% and the compressive stress at the extreme edge of the section corresponds to this strain. Whitney replaced the actual parabolic stress diagram by a rectangular stress diagram such that the centre of gravity of both diagrams lies lat the same point and their areas are also equal.

Working Stress Method of Rc Structures

Working Stress method has been the traditional method used for reinforced concrete design where it is assumed that concrete is elastic, steel and concrete sct together elastically, and the relationship between loads and stresses linear right upto the collapse of the structure. The basis of the method is that the permissible stress for concrete and steel are not exceded any where in the structure when it is subjected to the worst combination of bending assuming that both materials obey the Hooke’s law. The elastic theory assumes a linear variation of strain and stress from zero at the neutral axis to a maximum at the extreme fibre.

The permissible stresses are prescribed by a building code to provide suitable factors of safety to allow for uncertainties in the estimation of working loads and variation in properties of materials IS:456-2000 user a factor of safety equal to 3 on the 28 days cube strength to obtain the permissible compressive stress in bending in concrete; and equal to 1.78 on the yield strength of steel in tension to obtain the permissible tensile stress in reinforcement.

Design Philosophies of RC Structures

The object of reinforced concrete design is to achieve a structure that will results in a safe and economical solution. For a given structural system. The design problem consists of the following steps

• Idealization of structure for analysis
• Estimation of loads
• Analysis of idealized structural model to determine axial thrust, shears, bending moments and deflections.
• Design of structural element, and
• Detailed structural drawings and schedule of reinforcing bars.