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Question
Let jd be the lever arm between the resultant tensile force T in the reinforcing/ prestressing steel and the resultant compressive force C in concrete of a reinforced/prestressed concrete beam. When subjected to increase in the external bending moment, which one of the following statements is true in reinforced and prestressed concrete beam?
(i) In reinforced concrete beams
(a) T increase at a faster rate than jd
(b) Both T and jd increases at the same rate
(c) jd increases at a faster rate than T
(d) Neither T nor jd increases
(ii) In prestressed concrete beams
(a) T increases at a faster rate than jd
(b) Both T and jd increases at the same rate
(c) jd increases at a faster rate than T
(d) Neither T nor jd increases
(i) In reinforced concrete beams
(a) T increase at a faster rate than jd
(b) Both T and jd increases at the same rate
(c) jd increases at a faster rate than T
(d) Neither T nor jd increases
(ii) In prestressed concrete beams
(a) T increases at a faster rate than jd
(b) Both T and jd increases at the same rate
(c) jd increases at a faster rate than T
(d) Neither T nor jd increases
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Solution
(i) (a) and (ii) (c)
A change in the external moments in the elastic range of a prestressed concrete beam results in a shift of the pressure line rather than in an increase in the resultant force in the beam.
- In reinforced concrete beam section, an increase in the external moment results in a corresponding increase in the tensile force & the compressive force. The increase in the resultant forces are due to a more or less constant lever arm b/w the forces, characterised by the properties of the composite section.
- Basically, the load carrying mechanism is comprised of a constant force with a changing lever-arm, as in the case of prestressed concrete sections, and a changing force with a constant lever arm prevailing in reinforced concrete sections.
- However, if a prestressed concrete member is cracked, it behaves in a manner similar to that of a reinforced concrete section.
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Note: Pressure line :- At any given section of a prestressed concrete beam, the combined effect of the prestressing force & externally applied load will result in a distribution of concrete stresses that can be resolved into a single force. The locus of the points of the application of this resultant force in any structure is termed as the pressure or thrust line.
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A change in the external moments in the elastic range of a prestressed concrete beam results in a shift of the pressure line rather than in an increase in the resultant force in the beam.
- In reinforced concrete beam section, an increase in the external moment results in a corresponding increase in the tensile force & the compressive force. The increase in the resultant forces are due to a more or less constant lever arm b/w the forces, characterised by the properties of the composite section.
- Basically, the load carrying mechanism is comprised of a constant force with a changing lever-arm, as in the case of prestressed concrete sections, and a changing force with a constant lever arm prevailing in reinforced concrete sections.
- However, if a prestressed concrete member is cracked, it behaves in a manner similar to that of a reinforced concrete section.
Note: Pressure line :- At any given section of a prestressed concrete beam, the combined effect of the prestressing force & externally applied load will result in a distribution of concrete stresses that can be resolved into a single force. The locus of the points of the application of this resultant force in any structure is termed as the pressure or thrust line.
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