Bursting and Spalling in Pretensioned U-Beams - Ferguson ...

(e.g. others for the same project) would have some (or more) strands debonded at thebeam end.TxDOT design specifications permit up to 75% debonding at the beam end,though fewer strands are typically debonded in U-beams (20 to 40%). While debondingmay improve end-region serviceability, it is done to the detriment of shear strength.Under shear loading, bonded prestressing strands work as a tension tie near supports: thefewer bonded strands provided, the less the tie capacity.As presently directed on the standard drawings, debonding in U-beams proceedsfrom the outermost strands inward, skipping adjacent strands. This order makes sense inI-beams, in which the strands near the center are both in the web and above the bearingpad. The last strands to be debonded in this out-to-in pattern are those most crucial to theshear response of the member. For U-beams, the strands beneath the webs and above thebearing pad are still important in shear, but are in different locations. The standarddebonding order should be changed to reflect this fact.The practice of debonding strands fundamentally changes the behavior of aprestressed concrete beam. No longer is the prestressing force applied entirely at thebeam end, so it follows that the bursting forces are no longer confined to the end region.Instead, bursting reinforcement should be provided at each section in which a newportion of the prestressing force is applied, in quantities sufficient to resist the transverseforce associated with that portion of the prestressing force. An example of this designconcept is shown in Figure 4.29.Similar to the situation of beams with debonded strands is that of beams withdapped ends. In the dapped-end case, no strands are bonded at the beam end. It is clearthat bursting reinforcement should be provided beyond the dap rather than at the memberend.185