A direct design approach for strengthening simple-span beams with external posttensioning

摘要

>External post-tensioning, in which external tendons are installed on the outside of a beam, is an efficient strengthening method for concrete beams. In such cases, a direct approach to determine the tendon configuration for a desired increase in load-carrying capacity would be useful, and this paper develops just such an approach for simple-span beams. The installation of external tendons causes an increase in the beam capacity in two ways. First, the tendons contribute directly to the load-carrying capacity by balancing part of the increased loads. Second, the concrete compression zone is increased, which indirectly leads to an increase in the beam capacity.Two sets of equationsa€”refined equations and simplified equationsa€”are established for the determination of the increase in load-carrying capacity. The refined equations account for the contribution of both the increased concrete compression zone and the direct contribution of the tendons, while the simplified equations account for the direct contribution of the tendons only. A comparison with 124 simple-span beams from previous investigations showed that the increases in load-carrying capacity predicted by the refined equations are in good agreement with the test data, while those predicted by the simplified equations are conservative.>References>1. Aalami, B. O., and D. T. Swanson. 1988. Innovative Rehabilitation of a Parking Structure. Concrete International, V. 10, No. 2 (February): pp. 30-35. >2. Alkhairi, F. M. 1991. On the Flexural Behavior of Concrete Beams Prestressed with Unbonded Internal and External Tendons. PhD thesis, University of Michigan, Ann Arbor. >3. Naaman, A. E., and F. M. Alkhairi. 1991. target="_blank" title="Stress at Ultimate in Unbonded Post-Tensioning Tendons: Part 2 - Proposed Methodology." href="http://dx.doi.org/10.14359/1288 ">Stress at Ultimate in Unbonded Post-Tensioning Tendons: Part 2 - Proposed Methodology. ACI Structural Journal, V. 88, No. 6 (November-December): pp.  683-692. >4. Naaman, A. E., N. Burns, C. French, W. Gamble, and A. H. Mattock. 2002.target="_blank" title=" Stress in Unbonded Prestressing Tendons at Ultimate: Recommendation" href="http://dx.doi.org/10.14359/12121 "> Stress in Unbonded Prestressing Tendons at Ultimate: Recommendation. ACI Structural Journal, V. 99, No. 4 (July- August): pp. 520-531. >5. American Concrete Institute (ACI) Committee 318. 2008. Building Code Requirements for Structural Concrete (ACI 318-08) and Commentary (ACI 318R- 08). Farmington Hills, MI: ACI. >6. American Association of State and Highway Transportation Officials (AASHTO). 2004. AASHTO LRFD Bridge Design Specifications. 3rd ed. Washington DC: AASHTO. >7. Lin, T. Y. 1963. target="_blank" title="Load-Balancing Method for Design and Analysis of Prestressed Concrete Structures" href="http://dx.doi.org/10.14359/7874 ">Load-Balancing Method for Design and Analysis of Prestressed Concrete Structures. Journal of the American Concrete Institute, V. 60, No. 6 (June): pp. 719-741. >8. Roberts-Wollmann, C. L., M. E. Kreger, D. M. Rogovsky, and J. E. Breen. 2005. target="_blank" title="Stress in External Tendons at Ultimate." href="http://dx.doi.org/10.14359/14271 ">Stress in External Tendons at Ultimate. ACI Structural Journal, V. 102, No. 2 (March-April): pp. 206-213. >9. Roberts-Wollmann, C. L., M. E. Kreger, D. M. Rogovsky, J. E. Breen, J. S. Du, W. L. Lu, and W. Y. Ji. Discussion. 2006. ACI Structural Journal, V. 103,  No. 1 (January-February): pp. 149-154.>10. Tan K. H., and C. K. Ng. 1997.target="_blank" title=" Effects of Deviators and Tendon Configuration on Behavior of Externally Prestressed Beams" href="http://dx.doi.org/10.14359/456 "> Effects of Deviators and Tendon Configuration on Behavior of Externally Prestressed Beams. ACI Structural Journal, V. 94, No. 1 (January-February): pp. 13-22. >11. Tan, K. H., A. E.