Modifications suggested in IS: 456 (2000) with respect to maximum under-reinforced section with minimum tension reinforcement in beams
By: Jha, Bijoy Kumar
.
Contributor(s): Bhanja, Santanu.
Publisher: Mumbai ACC LTD 2022Edition: Vol96(10), Oct.Description: 6-17p.Subject(s): Civil EngineeringOnline resources: Click here
In:
Indian Concrete Journal - ICJSummary: As per prescriptive method of design specified by our standards designers have the option of designing a reinforced concrete section in flexure within the range of two extreme conditions i.e. balanced and maximum under-reinforced. No specific recommendation or guideline is available regarding the choice of the section. Many designers are of the opinion that in balanced condition both the materials are optimally used and hence it is better. But the performance of an under-reinforced section is much better than that of a balanced one. The yielding of steel and plastic rotation of the section is maximum for maximum under-reinforced section and minimum for a balanced one. Again, a designer opts for a doubly reinforced section only when the external bending moment exceeds the limiting or balanced moment of resistance as a singly reinforced section. Thus, all doubly reinforced sections are designed as balanced in nature. The performance of a structural element should be properly assessed by the designer up to and beyond design loads i.e., under overloading and it needs to be ensured that failure occurs in flexure only as excepting this mode all other failure modes are brittle. For ensuring ductile failure the total strain at failure should be high and, in this context, ductility plays a very vital role. However, for achieving higher levels of ductility higher plastic rotations are necessary. In the present paper an attempt has been made to quantify the plastic rotations of maximum under-reinforced sections. It has been observed that with the minimum percentage of steel as per IS:456 the plastic rotations of sections are very high and seem to be virtually impossible to be actually mobilized especially for deeper Reinforced Concrete girders. These high rotations might result in actual collapse of the section. The codes do not furnish any information regarding the desirable values of plastic rotations of beams. An attempt has also been made to compare some important features of prescriptive method of design and performance-based design and their implication in design of earthquake resistant structures. In the present paper a number of relationships have been proposed for calculation of the minimum reinforcement in beams which will result in optimum plastic rotations that will not result in significant damage of the sections.
| Item type | Current location | Call number | Status | Date due | Barcode | Item holds |
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Articles Abstract Database
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School of Engineering & Technology (PG) Archieval Section | Not for loan | 2023-0058 |
As per prescriptive method of design specified by our standards designers have the option of designing a reinforced concrete section in flexure within the range of two extreme conditions i.e. balanced and maximum under-reinforced. No specific recommendation or guideline is available regarding the choice of the section. Many designers are of the opinion that in balanced condition both the materials are optimally used and hence it is better. But the performance of an under-reinforced section is much better than that of a balanced one. The yielding of steel and plastic rotation of the section is maximum for maximum under-reinforced section and minimum for a balanced one. Again, a designer opts for a doubly reinforced section only when the external bending moment exceeds the limiting or balanced moment of resistance as a singly reinforced section. Thus, all doubly reinforced sections are designed as balanced in nature. The performance of a structural element should be properly assessed by the designer up to and beyond design loads i.e., under overloading and it needs to be ensured that failure occurs in flexure only as excepting this mode all other failure modes are brittle. For ensuring ductile failure the total strain at failure should be high and, in this context, ductility plays a very vital role. However, for achieving higher levels of ductility higher plastic rotations are necessary. In the present paper an attempt has been made to quantify the plastic rotations of maximum under-reinforced sections. It has been observed that with the minimum percentage of steel as per IS:456 the plastic rotations of sections are very high and seem to be virtually impossible to be actually mobilized especially for deeper Reinforced Concrete girders. These high rotations might result in actual collapse of the section. The codes do not furnish any information regarding the desirable values of plastic rotations of beams. An attempt has also been made to compare some important features of prescriptive method of design and performance-based design and their implication in design of earthquake resistant structures. In the present paper a number of relationships have been proposed for calculation of the minimum reinforcement in beams which will result in optimum plastic rotations that will not result in significant damage of the sections.
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