Masonry Magazine February 2000 Page. 12
Prestressed Masonry
Continued from page 10
Prestressing, the partitions can be designed as vertical cantilevers or can be reinforced with multiple sections of prestress tendons coupled together inside the wall. With prestressed masonry, the interior wythe of an in-fill cavity wall can be eliminated. Interior cantilever demising walls with steel stud and gypsum board on top can be designed to effectively resist warehouse loads.
Sound barrier
walls offer a unique opportunity. With prestressing it is possible to capture all of the savings compared to reinforced designs and also to prefabricate the walls off-site. The elimination of grout greatly reduces shipping weight and cost.
Basement walls
In areas where empirical design is not used for basement walls, prestressing can reduce costs. Tendons can be offset to increase the precompression applied to the tension side of the wall to resist earth pressures more effectively. The elimination of grout and the labor associated with it helps in this highly competitive market.
Systems Approach
Several proprietary systems have been used in the States over the last 10 years or so. These systems use special block shapes and design parameters. Another system that does not utilize special block shapes and is designed around the MSJC code has been used on a number of projects over the last two years. This system will be described below. Descriptions of the other systems can be obtained from their sponsors who are listed in the references at the end of this article.
Basically prestress hardware consists of tendons and couplers, anchorage devices at the top and bottom of the wall and prestress load indicating devices. In some cases, tendon restraint devices are needed.
Tendons are made from high strength, low relaxation, threaded steel rods. While regular structural steel rods have been used in some applications, their mechanical properties are not adequate to develop the economies needed to make the system viable. Relaxation is a new term to the masonry industry but is common in prestressing. It is a material property that is analogous to masonry creep but in the opposite direction. Steel, held under a constant high degree of tension, will tend to stretch over time without the application of additional load. This is called relaxation. In order to control the loss of initial prestress in the wall, it is important to limit the amount of relaxation in the steel. For this reason manufacturers must report the actual relaxation properties of the steel used based on test results.
Couplers have to be designed and manufactured to be capable of resisting 150% of the yield strength of the tendons. Of equal importance is the need to have the correct thread length to develop the strengths needed. Proper thread engagement is an essential part of constructing prestressed masonry. Since the steel tendon is stressed its highest when the tendon is being tightened down (the Jacking Stage), improper thread engagement will show up as a stripping failure at the coupler. This turns out to be an excellent field quality control test. When tendons have to be coupled near the base of the wall and at intervals up the wall, it is recommended that "inspection ports" be installed to help the contractor engage the couplers properly.
Top and bottom (when used) bearing plates have to bear against a grout plug in the cell of the unit adjacent to the bearing or a bond beam. In order to minimize deflections in the bearing plate that will cause uneven bearing stresses, these bearing plates are casted with stiffening ribs. Where it is desired to embed the bottom anchorage in the foundation, special anchors can be used. These anchors, developed and tested for the concrete industry, provide an excellent means to anchor the wall to the foundation for cantilever applications and other cases where it is desired to develop bending resistance at the foundation.
For threaded tendons the amount of tension applied to the tendons can be measured by the use of torque wrenches or load indicating washers. Load indicating washers seem to be the more reliable since they are not affected by the friction between the nut and tendon. Load indicating washers can be calibrated to indicate a variety of tension values. The washers are installed with their "bumps" pointing upward. A hardened washer is installed above the load-indicating washer followed by a nut. As the nut is tightened, the installer inserts a preset feeler gage between the load indicating washer and the hardened washer. When the feeler gage can no longer fit, the proper tension has been attained. As a secondary check, the operator can measure the total elongation of of the tendons as a result of the stressing and compare this elongation with tables supplied by the manufacturer.
Conclusion
Prestressed masonry can provide a mason contractor with yet another tool to stay competitive with other building materials. While it is a form of reinforced masonry, its design and methods are significantly different. To take full advantage of the benefits of the system a mason contractor should work with his supplier in laying out the project, ordering materials and reviewing techniques unique with the system. History has shown that those who do approach this new method correctly have come back to use it again.
References
1. Building Code Requirements for Masonry Structures (ACI 530-99/ASCE 5-99/TIMS 402-99) available from the Mason Contractors Association of America, Lombard, IL
2. Integra System Superlite Block Co., Phoenix, AZ and VSL Corp. Cedex, France
3. DUR-O-WAL SURE-STRESS, DUR-O-WAL, Inc. Aurora, IL