Masonry Magazine February 2000 Page. 9
Prestressed Masonry
Offers New Opportunities to Keep Masonry Competitive
by Mario Catani
Prestressed design concepts have been used for concrete for over 50 years. It is the design method of choice for a number of building types and has helped the concrete industry stay competitive and increase its market share. Now the masonry industry has an opportunity to use this design approach to improve its market position particularly for walls with small vertical loads.
The design of most lightly loaded walls is controlled by the very low tensile bond strength of mortar. For years the industry has tried to improve this bond strength in order to take better advantage of the high compressive strength of masonry and to reduce the constructed cost of a wall. Prestressed masonry design accomplishes this goal.
Figure 1 above shows a non-reinforced non-load bearing wall subjected to wind loads. As a result of the wind load, the wind side of the wall is subjected to compression and the lee side of the wall is subjected to tension. Since masonry is relatively weak in tension, the design is controlled by this property. The strong compressive strength of masonry is not utilized. When the wall is precompressed by means of prestressing tendons, the tension on the lee side of the wall can be eliminated. In the process the compression on the wind side of the wall is increased, but masonry is very strong in compression so it can be designed to resist the loads more efficiently. Since the amount of tension that has to be overcome is usually small, the amount of prestressing is also small. This makes the process of prestressing masonry much simpler than that needed for concrete.
New Code Provisions
A design procedure for the use of prestressed masonry has been published by the Masonry Standards Joint Committee. It appears as a separate chapter in its 1999 edition of the
The use of the post-tensioned tendons in ungrouted walls enhances speed of construction of the wall because masons can continue work without the delays associated with pouring grout.