Abstract
The 1998 draft of the AASHTO Guide Specifications for Segmental Bridges restricts the use of external posttensioning tendons in regions of severe seismic activity, Seismic Performance Category (SPC) C and SPC D, by requiring that at least 50 percent of the tendons be internal to the concrete. This requirement has also been applied to selected projects in SPC B designed before or during the development of the 1998 draft. A case study is presented that substantiates the use of purely externally posttensioned structures in SPC B and indicates that the requirement may not even be necessary for SPCs C and D. The case study concerns a project in which precast segmental superstructures carry a light-rail access system to the John F. Kennedy International Airport in New York. Two different precast segmental superstructures with external tendons are modeled using a multimode spectral analysis. The goal is to determine the stress levels under the worst possible seismic conditions for SPC B and, in particular, to establish whether the joints would open or crack and the external tendons would suffer any distress. For both models, results show conclusively that the forces, moments, and stresses in the superstructure are less severe than under normal service level loads. Thus, there is no risk of cracking or opening of joints, and the external tendons experience no increase in stress over their normal service conditions. In addition, research shows that there is no significant difference in behavior between external and internal tendons for loads within ultimate conditions.
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