Masonry Magazine October 2003 Page. 22
POST-TENSION MASONRY
Courtesy of Ledy Design Gro
Figure 13
Courtesy of Ledy Design Group
Figure 14
Tensioning
THE TIGHTENING of the tendons is not difficult. It is done either with a wrench (Figure 13) or a small, hydraulic jack torque wrenches are not commonly used. Tension indicator washers are available to check the tension if a wrench is used. Figure 14 shows a feeler gauge being used to check one of these washers. Dye filled washers will be avail- able soon to eliminate the need for the gauges. As the tendons are stressed, they stretch; their elongation must be measured to verify the full length is being tensioned.
Since tendons are designed to accommodate 25 to 35 percent losses, it is usually not necessary to retighten the tendons. However, the design should specify the tensioning sequence. If none is given, each tendon is tensioned fully one at a time. However, other procedures are possible. Some engineers may require partial tensioning of all tendons followed by a final stressing. Others may require the tendons to be stressed in a patterned sequence. The intent is to eliminate some of the losses inherent in the tensioning process.
What's Coming?
PRESTRESSED MASONRY is evolving quickly. This evolution is due to both new research and new material and system developments. Companies are now developing mortarless systems reinforced with post-tensioning, and new masonry units specifically made to facilitate tendon installation are currently being developed. Prefabricated walls, beams and columns are possible.
Research on the subject of prestressed masonry is also being conducted more frequently. In July 2003, the 9th North American Masonry Conference (9NAMC) dedicat- ed an entire session on prestressed masonry.
The MSJC code will once again be updated in 2005. This update is a direct reflection of the research and expe- rience derived over the past ten years. The combination of new research and advancements in material and system development will lead to the greater use of prestressed masonry, especially in seismic areas. One of the outgrowths of this work will be that non-metallic tendons and hard- ware made from composites will likely be included in the codes.
As you can see, prestressed masonry has come quite a ways over the years and there is much advancement still to come!
David T. Biggs, P.E., is a structural engineer with Ryan-Biggs Associates, P.C. in Troy, N.Y. He chairs the Prestressed Masonry Subcommittee of the Masonry Standards Joint Committee (ACI 530). He was selected 2003 Engineer of the Year by both the New York State Society of Professional Engineers and the New York Association of Consulting Engineers.
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20 Masonry
October 2003
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