Post-tensioning application part 1
nowadays post-tensioning system used in every large structure such as bridges and high-rise building. the applications of the post-tensioning system are not limited to deck slab. it can be used for piers, pier cap, and piling. the following represent some of the post-tensioning applications:
cconstruction management: concrete construction course
- Cast-in-place bridges on falsework: the superstructures for this type of bridges have a solid or cellular shape. box girder bridge is an example of this type of bridges. formwork constructed above the falsework. the formwork will be used to support the superstructure and form the shape of the structures. reinforcement and post-tensioning ducts placed on formwork. then concrete poured and once the concrete attains the desired strength. strands installed in ducts. then the concrete prestressed. The post-tensioning will be used in a longitudinal direction and it may be used in the transverse direction. the direction, number, and profile of ducts and prestressing force will be determined by the designer.
- Post-tensioning AASHTO, Bulb-T, and spliced girder: these girders are pre-tensioned and they have the ability to support their self-weight. they are placed above piers and then connected to each other. a cast-in-place joints are cast transversely. these girders are cast and pre-tensioned in the factory. prior casting post-tensioning ducts are placed in girder web. these ducts are connected at the cast-in-place joints. the ducts will go up above the pier support and the profile of ducts shall be placed smoothly as per the design. tendons will be installed and post-tensioned for part of these ducts prior to casting of the deck. following the deck casting and once the concrete attains the required strength. the remaining ducts are post-tensioning. this will be used to make simply supported span a continuous span. Long span can be built and spliced using similar techniques. for variable depth span cantilevering above the piers can be spliced with constant depth using similar techniques. first, the variable depth girders are placed above the piers and aligned with girder of side span(middle part). the end of girders shall have protruding mild steel and ducts. so it will be connected to each other. then the cast-in-place joint will be cast. the girders shall be supported as shown in figure no:3. once all cast-in-place joints have been cast and attained the desired strength. strands are installed and stressed.
- precast segmental balanced cantilever bridge: This type of bridges involved symmetrical erection of precast segments around the supporting piers. the first segment is lifted and the epoxy coat is used to connect the precast segments faces. also, a temporary post-tensioning is used to hold the segment in place. once the next segment on the other side of the pier is placed. the permanent post-tensioning cables are installed and prestressed. Figure no:4 showing two different methods of installing segmental cantilever bridge.
- Cast-in-Place Segmental Balanced Cantilever Bridges: this system is similar to the previous system. in this system, the segments are cast at the field. traveler form used to support the segment until it attains the required strength to be post-tensioned. figure no:5 showing cast-in-place segmental balanced cantilever bridge.
- precast span by span bridges: in this system precast segments are installed for a bridge span. the epoxy coat will be used for closure joint for segments. segments will be supported by erection truss or overhead erection gantry. tendons for post-tensioning system can be external or partially external and internal. figure no:6 showing the schematic erection of precast span by span bridge.
- Precast Segmental Progressive Cantilever Bridges: this system used when access to the site is restricted. and the delivery of the structural components is possible at one end only. in this case, the first span between abutment and pier are placed in falsework and the remaining are erected progressively. once the mide span reached. temporary support will be added and the erection will continue until reaching the next pier. the process will be repeated until the completion of the superstructure.