LECTURE NOTES ON PRESTRESSED CONCRETE STRUCTURES Prepared By Assistant Professor
CONTENTS UNIT I Introduction: Historic development- General principles of prestressing pretensioning and post tensioning- Advantages and limitations of Prestressed concrete- General principles of PSC- Classification and types of prestressing M aterials- high strength concrete and high tensile steel their characteristics.Methods and Systems of prestressing: Pretensioning and Posttensioning methods and systems of prestressing like Hoyer system, Magnel Blaton system, Freyssinet system and Gifford- Udall System- Lee McCall system. UNIT II Losses of Prestress: Loss of prestress in pretensioned and post-tesnioned members due to various causes like elastic shortage of concrete, shrinkage of concrete, creep of concrete, relaxation of stress in steel, slip in anchorage, frictional losses. UNIT III Flexure: Analysis of sections for flexure- beams prestressed with straight, concentric, eccentric, bent and parabolic tendons- stress diagrams- Elastic design of PSC beams of rectangular and I sections- Kern line — Cable profile and cable layout. Shear: General Considerations- Principal tension and compression- Improving shear resistance of concrete by horizontal and vertical prestressing and by using inclined or parabolic cables- Analysis of rectangular and I beams for shear — Design of shear reinforcements- Bureau of Indian Standards (BIS) Code provisions. UNIT IV Transfer of Prestress in Pretensioned Members : Transmission of prestressing force by bond — Transmission length — Flexural bond stresses — IS code provisions — Anchorage zone stresses in post tensioned members — stress distribution in End block —Analysis by Guyon, Magnel, Zielinski and Rowe’s methods — Anchorage zone reinforcement- BIS Provisions UNIT V Composite Beams: Different Types- Propped and Unpropped- stress distributionDifferential shrinkage- Analysis of composite beams- General design considerations. Deflections: Importance of control of deflections- Factors influencing deflections — Short term deflections of uncracked beams- prediction of long. time deflections- BIS code requirements.
Definition of Prestress: Prestress is defined as a method of applying pre-compression to control the stresses resulting due to external loads below the neutral axis of the beam tension developed due to external load which is more than the permissible limits of the plain concrete. The pre-compression applied (may be axial or eccentric) will induce the compressive stress below the neutral axis or as a whole of the beam c/s. Resulting either no tension or compression. Basic Concept Prestressed concrete is basically concrete in which internal stresses of a suitable magnitude and distribution are introduced so that the stresses resulting from the external loads are counteracted to a desired degree. Terminology 1. Tendon: A stretched element used in a concrete member of structure to impart prestress to the concrete. 2. Anchorage: A device generally used to enable the tendon to impart and maintain prestress in concrete. 3. Pretensioning: A method of prestressing concrete in which the tendons are tensioned before the concrete is placed. In this method, the concrete is introduced by bond between steel & concrete. 4. Post-tensioning: A method of prestressing concrete by tensioning the tendons against hardened concrete. In this method, the prestress is imparted to concrete by bearing. Materials for prestress concrete members: 1. Cement: The cement used should be any of the following (a) Ordinary Portland cement conforming to IS269 (b) Portland slag cement conforming to IS455. But the slag content should not be more than 50%. (c) Rapid hardening Portland cement conforming to IS8041. (d) High strength ordinary Portland cement conforming to IS8112. 2. Concrete: Prestress concrete requires concrete, which has a high compressive strength reasonably early age with comparatively higher tensile strength than ordinary concrete. The concrete for the members shall be air-entrained concrete composed of Portland cement, fine and coarse aggregates, admixtures and water. The air-entraining feature may be obtained by the use of either airentraining Portland cement or an approved air-entraining admixture. The entrained air content shall be not less than 4 percent or more than 6 percent. 3 Minimum cement content of 300 to 360 kg/m is prescribed for the durability requirement. The water content should be as low as possible.
3. Steel:- High tensile steel , tendons , strands or cables The steel used in prestress shall be any one of the following:(a) Plain hard-drawn steel wire conforming to IS1785 (Part-I & Part-III) (b) Cold drawn indented wire conforming to IS6003 (c) High tensile steel wire bar conforming to IS2090 (d) Uncoated stress relived strand conforming to IS6006 High strength steel contains: 0.7 to 0.8% carbons, 0.6% manganese, 0.1% silica Durability, Fire Resistance & Cover Requirements For P.S.C Members:According to IS: 1343-1980 20 mm cover for pretensioned members 30 mm or size of the cable which ever is bigger for post tensioned members. If the prestress members are exposed to an aggressive environment, these covers are increased by another 10 mm. Necessity of high grade of concrete & steel: Higher the grade of concrete higher the bond strength which is vital in pretensioned concrete, Also higher bearing strength which is vital in post-tensioned concrete. Further creep & shrinkage losses are minimum with high-grade concrete. Generally minimum M30 grade concrete is used for post-tensioned & M40 grade concrete is used for pretensioned members. The losses in prestress members due to various reasons are generally in the range of 250 N/mm2 to 400 N/mm2. If mild steel or deformed steel is used the residual stresses after losses is either zero or negligible. Hence high tensile steel wires are used which varies from 1600 to 2000 N/mm2. Advantage of Prestressed Concrete 1. The use of high strength concrete and steel in prestressed members results in lighter and slender members than is possible with RC members. 2. In fully prestressed members the member is free from tensile stresses under working loads, thus whole of the section is effective. 3. In prestressed members, dead loads may be counter-balanced by eccentric prestressing. 4. Prestressed concrete member posses better resistance to shear forces due to effect of compressive stresses presence or eccentric cable profile. 5. Use of high strength concrete and freedom from cracks, contribute to improve durability under aggressive environmental conditions. 6. Long span structures are possible so that saving in weight is significant & thus it will be economic. 7. Factory products are possible. 8. Prestressed members are tested before use. 9. Prestressed concrete structure deflects appreciably before ultimate failure, thus giving ample warning before collapse.