Article 7.7 Contingency Tendons

by Dean Van Landuyt

Contingency Tendons

Three separate phenomena sometimes encountered in segmental construction warrant the installation of additional ducts and post-tensioning hardware. During the construction of a span-by-span or balanced cantilever bridge, it is possible that the post-tensioning force provided by the designed tendons is insufficient because of higher than anticipated friction and wobble losses. For balanced cantilever construction, where top slab tendons are grouted in stages, it is possible that grout may cross over from a duct with a tendon into an empty duct making it impossible to install a tendon at a later construction stage. Also in balanced cantilever construction, particularly cast-in-place, deflections may be larger than anticipated.

The AASHTO Guide Specification for Design and Construction of Segmental Concrete Bridges 2nd Edition, 1999 (Sections 15.2) requires that bridges with internal tendons be outfitted with a 5% excess of duct and anchorage capacity in the positive and negative moment regions. The ducts shall be located symmetrically about the centerline of the box and there shall be at least one for each web.

There are two different ways to meet the requirements of Section 15.2 for the negative (top slab) tendons for balanced cantilever construction. Given that it is not known at exactly which stage the extra force is going to be required, the preferred solution is to use oversized ducts and anchorages for all top slab tendons. If more force is required, additional strands can be added in each of the next few segments. For cast-in-place balanced cantilever construction, it is possible to add one extra duct per side. If additional post-tensioning force is required, the additional ducts can simply be deviated horizontally towards the webs and anchorages installed before casting.

Future Tendons

It is possible that sometime during the life of a segmental bridge, there would be a need for additional post-tensioning force: the bridge could crack or deflect more than anticipated, additional dead load such as additional overlays could be added or tendons could become corroded. Section 15.3 requires that provisions to permit future external tendons be incorporated into the original structure. There should be a way to attach future anchorages and deviation blocks and a clear path for the tendons to run. There should also be access and clearance for stressing rams. Future tendon force capacity shall be 10% of the positive moment and negative moment original post-tensioning forces.

Submitted by:

Dean Van Landuyt, P.E.
Senior Design Engineer
TxDOT - Bridge Division