Prestress Concrete
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BASIC CONCEPTS
1.1 INTRODUCTION
Concrete is strong in compression, but weak in tension: its tensile strength varies from 8 to 14 percent of its compressive strength. Due to such a low tensile capacity, flexural cracks develop at early stages of loading. In order to reduce or prevent such cracks from developing, a concentric or eccentric force is imposed in the longitudinal direction of the structural element. This force prevents the cracks from developing by eliminating or considerably reducing the tensile stresses at the critical midspan and support sections at service load, thereby raising the bending, shear, and torsional capacities of the sections. The sections are then able to behave elastically, and almost the full capacity of the concrete in compression can be efficiently utilized across the entire depth of the concrete sections when all loads act on the structure.
Such an imposed longitudinal force is called a prestressing force, i.e., a compressive force that prestresses the sections along the span of the structural element prior to the application of the transverse gravity dead and live loads or transient horizontal live loads.
The type of prestressing force involved, together with its magnitude, are determined mainly on the basis of the type of system to be constructed and the span length and slenderness desired. Since the prestressing force is applied longitudinally along or parallel to
The Diamond Baseball Stadium, Richmond, Virginia. Situ cast and precast post-tensioned prestressed structure. (Courtesy, Prestressed Concrete Institute.)
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Chapter 1
Basic Concepts
Figure 1.1 Prestressing principle in linear and circular prestressing. (a) Linear prestressing of a series of blocks to form a beam. (b) Compressive stress on midspan section C and end section A or B. (c) Circular prestressing of a wooden barrel by
6/3/09
09:38 AM
Page 1
1
BASIC CONCEPTS
1.1 INTRODUCTION
Concrete is strong in compression, but weak in tension: its tensile strength varies from 8 to 14 percent of its compressive strength. Due to such a low tensile capacity, flexural cracks develop at early stages of loading. In order to reduce or prevent such cracks from developing, a concentric or eccentric force is imposed in the longitudinal direction of the structural element. This force prevents the cracks from developing by eliminating or considerably reducing the tensile stresses at the critical midspan and support sections at service load, thereby raising the bending, shear, and torsional capacities of the sections. The sections are then able to behave elastically, and almost the full capacity of the concrete in compression can be efficiently utilized across the entire depth of the concrete sections when all loads act on the structure.
Such an imposed longitudinal force is called a prestressing force, i.e., a compressive force that prestresses the sections along the span of the structural element prior to the application of the transverse gravity dead and live loads or transient horizontal live loads.
The type of prestressing force involved, together with its magnitude, are determined mainly on the basis of the type of system to be constructed and the span length and slenderness desired. Since the prestressing force is applied longitudinally along or parallel to
The Diamond Baseball Stadium, Richmond, Virginia. Situ cast and precast post-tensioned prestressed structure. (Courtesy, Prestressed Concrete Institute.)
1
7166M01.qxd_lb
6/3/09
09:38 AM
Page 2
2
Chapter 1
Basic Concepts
Figure 1.1 Prestressing principle in linear and circular prestressing. (a) Linear prestressing of a series of blocks to form a beam. (b) Compressive stress on midspan section C and end section A or B. (c) Circular prestressing of a wooden barrel by
References: 1.1 Freyssinet, E. The Birth of Prestressing. London: Public Translation, Cement and Concrete Association, 1954. 1.2 Guyon, Y. Limit State Design of Prestressed Concrete, vol. 1. Halsted-Wiley, New York, 1972. 1.3 Gerwick, B. C., Jr. Construction of Prestressed Concrete Structures. Wiley-Interscience, New York, 1993, 591 p. 1.4 Lin, T. Y., and Burns, N. H. Design of Prestressed Concrete Structures. 3d ed. John Wiley & Sons, New York, 1981. 1.6 Dobell, C. “Patents and Code Relating to Prestressed Concrete.” Journal of the American Concrete Institute 46, 1950, 713–724. 1.7 Naaman, A. E. Prestressed Concrete Analysis and Design. McGraw Hill, New York, 1982. 1.8 Dill, R. E. “Some Experience with Prestressed Steel in Small Concrete Units.” Journal of the American Concrete Institute 38, 1942, 165–168. 1.10 Magnel, G. Prestressed Concrete. London: Cement and Concrete Association, 1948. 1.11 Abeles, P. W., and Bardhan-Roy, B. K. Prestressed Concrete Designer’s Handbook. 3d ed. Viewpoint Publications, London, 1981. 1.12 Nawy, E. G., Fundamentals of High Performance Concrete, 2nd ed. John Wiley & Sons, New York, 2001, pp 1.13 Nawy, E. G., editor-in-chief, Concrete Construction Engineering Handbook, 2nd ed., CRC Press, Boca Raton, FL, 2008, 1560 p.