Strand Debonding for Pretensioned Girders (2017) / Chapter Skim
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1 C h a p t e r 1 1.1 Introduction The initial prestressing force in pretensioned girders can produce large extreme-fiber stresses particularly near the ends of the girders where span effects (primarily dead load moment) are minimal.
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2 Strand Debonding for pretensioned Girders The AASHTO LRFD Bridge Design Specifications provides requirements for strand transfer and development length calculations in Article 5.11.4.2 and for partially debonded strands in Article 5.11.4.3. At any section, the number of debonded strands should be limited to 25% of the total number of strands (note that the AASHTO wording is non-mandatory in this instance: "should," not "shall")
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Background 3 1.3.1.1 AASHTO LRFD Article 5.8.3.5: Longitudinal Reinforcement The presence of shear in a section results in a tensile force generated in the primary longitudinal reinforcement that is in addition to that caused by flexure. The free-body diagram of a diagonally cracked member is conceptually depicted in Figure 1.1.
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4 Strand Debonding for pretensioned Girders Equation 1.4 must account for the possible lack of full development of the strand and/or reinforcing steel at the critical section. Additionally, only bonded prestressing strand contributes to the required longitudinal capacity; thus, greater debonding will require additional nonprestressed reinforcement as described by Eq.
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Background 5 were tested over a very short shear span, and failure, in all cases, was characterized by insufficient anchorage of the prestressing strand regardless of the degree of debonding. Russell B
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6 Strand Debonding for pretensioned Girders capacity. In one specimen, fully bonded straight strands were used at one end, and 38% of the straight strands were partially debonded at the other end.
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Background 7 the effects of partial debonding on Vcw. Again the beams with 25% and 50% debonding performed similarly to the beam with 0% debonding and the beam with 75% debonding showed a significant reduction in capacity.
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8 Strand Debonding for pretensioned Girders 1.3.3.1 Synthesis of Past Analytical Studies Past studies have not adequately addressed issues associated with modeling of bond and local effects in the vicinity of strands. Furthermore, neither 3D nonlinear finite element models nor simpler techniques, such as modified compression field theory or STMs, have been used to investigate the effects of partial debonding on shear capacity.
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Background 9 • A minimum of one-half the number of debonded strands shall have a debonded length equal to one-half times the maximum debonded length. • Strands extended from a beam to develop positive moment resistance at pier locations shall not be debonded strands.
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10 Strand Debonding for pretensioned Girders g. The number of states using different permitted percentages of partial debonding is as follows: (a)
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