Masonry Magazine October 2003 Page. 19
Some General Recommendations for Using Post-tensioning
Working with prestressed masonry does require some new skills and thinking by the contractor. Critical elements to the success of the projects include:
a) Selecting a prestressing system along with the anchor types.
b) Evaluating the design for suitability.
Innovations In Masonry
WIRE-BOND Construction from... Level Eye* & Level Tab*
• New single-plane design on all ladder and truss hook-and-eye and tab systems.
• Eliminates "stacked" welds that are too thick.
• Entire welded assembly does not exceed 3/16" height.
*patent pending
HCL-711 Anchoring System
• Installs quickly and easily on exterior walls with brick veneer allowing little or no compressible movement.
• 3 prongs pierce wallboard and abut to steel studs.
• Avoids damage to insulation.
Plumb-Rite
• T-shaped PVC assists in keeping vertical joints in precise alignment where control joints and expansion joints are used in a masonry wall.
• Once installed, not necessary to plumb joint again.
WIRE-BOND® Clip
• Straight-and-cut wire fits into clip designed for adjustable veneer systems in seismic applications.
• WIRE-BOND® Clip may be welded into any system using double hook for adjustment.
Innovation is our job at WIRE-BOND. We constantly search for new products and techniques to save time, labor and money for the masonry industry. WIRE-BOND Contact your WIRE-BOND distributor or call us today at 1-800-849-6722. Visit www.wirebond.com to see our complete catalog.
Figure 7
While all of the projects shown here use prestressing, it's worthy to note that none of them have any outward appearance to indicate there is anything special about their construction.
Materials
IN THE EXAMPLES SHOWN, Prestressing is used with many types of masonry that are readily available. Most hollow core masonry (CMU or clay) will work with prestressing. A- or H-shaped concrete masonry units allow the masonry to be constructed around the tendons. Figure 8 shows a clay unit used in Switzerland. There are also proprietary systems with special shapes specifically for prestressing.
Figure 8
The key components to prestressed masonry are the tendons themselves. Currently the MSJC only allows steel tendons. Manufacturers such as Dur-O-Wal, Williams Form Engineering and VSL provide tendons and hardware for their systems. These products are an outgrowth of the prestressed concrete industry. Bars are more common, but full-length strands are available as well. The bars are partially or fully threaded. Tendons for use in masonry generally vary in diameter from 1/2-inch up to 3/4-inch. Each tendon is stressed to between 7,000 and 16,000 pounds - the actual amount is determined by the design. Similar to concrete design, an allowance is made to account for the tendons losing some of their stress over time.