Rcc design by working stress method

Rcc design by working stress method

1. RCC DESIGN BY WORKING STRESS METHOD By- Jyoti ranjan nayak

2. Overview of building design

3. Introduction The design of a structure may be regarded as the process of selecting proper materials and proportioned elements of the structure, according to the art, engineering science and technology. In order to fulfill its purpose, the structure must meet its conditions of safety, serviceability, economy and functionality.

4. Terms of RCC design   Neutral Axis (n) Neutral axis lies at the centre of gravity of the section.  It is defined as that axis at which the stresses are zero.  It divides the section into tension and compression zone. The position of the neutral axis depends upon the shape (dimensions) of the section and the amount of steel provided.

5. Lever arm- Lever arm is the distance between the resultant compressive force and the resultant tensile force.  Moment of resistance is the resistance offered by the beam against external loads.  As there is no resultant force acting on the beam and the section is in equilibrium, the total compressive force is equal to the total tensile force. These two forces (equal and opposite separated by a distance) will form a couple (and the moment of this couple is equal to the resisting moment or moment of resistance of the section. Moment of Resistance ( M r )

6. WORKING STRESS METHOD Working Stress Method is the traditional method of design not only for Reinforced Concrete but also for structural steel and timber design. The conceptual basis of the WSM assumes that the structural material behaves in a linear elastic manner and that appropriate safety can be ensured by suitably limiting the stresses in the material due to the presumed working loads (service loads) on the structure. WSM also assumes that both the steel reinforcement and concrete act together and are perfectly elastic at all stages, and hence the modular ratio can be used to determine the stresses in steel and concrete. The stresses under the working loads are obtained by applying the methods of ‘strength of materials’ like the simple bending theory. The limitations due to non-linearity and buckling are neglected. The stresses caused by the ‘characteristic’ or service loads are checked against the permissible (allowable) stress, which is a fraction of the ultimate or yield stress. The permissible stress may be defined in terms of a factor of safety, which takes care of the overload or other unknown factors.

7. Assumptions of elastic theory Plane Section before bending will remain plane after bending Bond between steel and concrete is perfect with in elastic limit of steel The steel and concrete behaves as linear elastic material All tensile stresses are taken by reinforcement and none by concrete The stresses in steel and concrete are related by a factor known as “modular ratio The Stress-strain relationship of steel  and concrete is a Straight line under working load

8. Steps to design beam section by WSM Step 1- calculation of design constants. Step 2- calculation of bending moment. Step 3- design of section. Step 4- reinforcement Step 5- check for shear and design of shear reinforcement. Step 6- details of reinforcement.

9. Balanced Beam Section Reinforced concrete beam sections in which the tension steel also reaches yield strain simultaneously as the concrete reaches the failure strain in bending are called  balanced sections.

10. Under-Reinforced Beam Section Reinforced concrete beam sections in which the steel reaches yield strain at loads lower than the load at which the concrete reaches failure strain are called  under-reinforced sections. Every singly reinforced beam should be designed as under-reinforced sections because this section gives enough warning before failure. Yielding of steel in under-reinforced beam section does not mean the structure has failed, as when steel yields, excessive deflection and cracking in beam will occur before failure which gives enough time to occupants to escape before the section fails. The failure in under-reinforced beam section is due to the concrete reaching its ultimate failure strain of 0.0035 before the steel reaches its failure strain which Is much higher 0.20 to 0.25.

11. Over-Reinforced Beam Section Reinforced concrete beam sections in which the failure strain in concrete is reached earlier than the yield strain of steel is reached, are called  over-reinforced beam sections. If over-reinforced beam is designed and loaded to full capacity then the steel in tension zone will not yield much before the concrete reaches its ultimate strain of 0.0035. This due to little yielding of steel the deflection and cracking of beam does not occur and does not give enough warning prior to failure. Failures in over-reinforced sections are all of a sudden. This type of design is not recommended in practice of beam design.

12. CONCEPT OF TRANSFORMED OR EQUIVALENT SECTION The bond between steel and concrete is assumed to be perfect so the strains in steel and the surrounding concrete will be equal It means that stress in steel is m times the stress in concrete or load carried by steel is m times the load carried by concrete of equal area. Using Eqns. ( i ) and (ii) (ii) ( i )

13. Limitations of working stress method The assumptions of linear elastic behaviour and control of stresses within specially defined permissible stresses are unrealistic due to several reasons viz., creep, shrinkage and other long term effects, stress concentration and other secondary effects Different types of load acting simultaneously have different degrees of uncertainties. This cannot be taken into account in the working stress method The actual factor of safety is not known in this method of design. The partial safety factors in the limit state method is more realistic than the concept of permissible stresses in the working stress method to have factor of safety in the design.

14. Shrinkage Creep

15. Limit State Method The stresses are obtained from design loads and compared with design strength. In this method, it follows linear strain relationship but not linear  stress relationship (one of the major difference between the two  methods of design). The ultimate stresses of materials itself are used as allowable stresses. The material capabilities are not under estimated as much as they  are in working stress method. Partial safety factors are used in limit  state method.

16. Accordingly, the working stress method is gradually replaced by the limit state method. The Indian code IS 456 has given working stress method in Annex B to give greater emphasis to limit state design. Moreover, cl. 18.2.1 of IS 456 specifically mentions of using limit state method normally for structures and structural elements. However, cl.18.2.2 recommends the use of working stress method where the limit state method cannot be conveniently adopted. Due to its simplicity in the concept and applications, better structural performance in service state and conservative design, working stress method is still being used for the design of reinforced concrete bridges, water tanks and chimneys. In fact, design of tension structures and liquid retaining structures are not included in IS 456 for the design guidelines in the limit state method of design Calculation alone do not produce safe, serviceable and durable structures. Suitable material quality control adequate detailing and good supervision are equally important.

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