Prestressing is the deliberate and controlled introduction of forces into a structural element to counteract loads that will be imposed on it in service. Post-tensioning is a form of prestressing that works well for masonry. Steel tendons are placed within cores, cells, or cavities in a masonry wall, anchored at each end, then tensioned to a specified percentage of their capacity. This has the effect of increasing the compressive load on the masonry, holding it together tightly enough that it can resist strong lateral loads without failing in tension.
Post-tensioned masonry requires a detailed engineering analysis, but manufacturers of post-tensioning systems generally will provide detailed design criteria and guidelines or will perform the analysis and provide a suitable design.
The basic system consists of 4-, 6-, or 8-inch concrete block designed to interlock at head joints. The block are laid on "dip" footings of reinforced concrete. Threaded tension rods are embedded in the footings, hooked under horizontal steel reinforcement, and spaced according to the design wind loads.
Post-Tensioned Wall System
The system uses open-ended, H-shaped block; anchor bolts with attached couplers embedded in the building foundation; 7/16-inch steel tendons; and special fixture blocks in the top course that accommodate both steel plate washers and special washers indicating the tension on the tendons.
Ducted Tendon System
A third post-tensioning system now commercially available in the United States was introduced in Switzerland in 1988. Unlike those described above, the Swiss system does not require the use of a particular type of unit. The other principal difference is that the tendons are enclosed by galvanized steel or plastic ducts within the wall.