Masonry is the oldest man-made building material in the world, invented nearly 10,000 years ago. However, its use in modern load-bearing jobs remains behind that of steel and concrete construction despite masonry's advantages, including simplicity, durability, strength, and ability to incorporate architectural elements into its design.
Recent advances in masonry design have developed a post-tensioning technique for this versatile building material. Post-tensioned masonry offers a new structural application characterized by a relatively simple construction technique and competitive cost that can move masonry to the forefront of engineering practice.
Experimental studies were conducted at the University of Minnesota to investigate the strength and stability of this new structural masonry system. This article presents the results of these studies, along with some general information about post tensioning.
The structural success of masonry depends on its ability to overcome its intrinsic low tensile strength, which is controlled by the adhesion between the mortar and masonry unit. A variety of methods are available to conquer this weakness.
An unreinforced masonry system can resist overturning loads with the weight of the masonry itself, but this approach may not be the most economical method for designing multiple story structures. For example, the 16-story Monadnock Building in Chicago was built in the late 19th century with stone and brick walls that are nearly 6-ft thick at the base.
Developments in the early 20th century led to conventional reinforcement in masonry, such as that in concrete design. Reinforcement is incorporated into the design of masonry walls to overcome tensile and shear stresses accompanied by gravity (force of components above), in-plane (wind or earthquake forces), and out-of-plane lateral loads (soil pressure, wind, or earthquake forces), while providing ductility, or the amount of deformation that a component can endure before it collapses.
The general design of a reinforced wall under combined axial and lateral loads combines vertical steel to resist bending and horizontal steel to combat shear forces. Using conventional reinforcement, the top 18 stories of the Excalibur Hotel in Las Vegas are load-bearing masonry.
In the past few decades, advancements were made in the development of prestressed systems that are applicable to masonry design. Prestressed masonry involves a compressive force applied by steel tendons placed within the masonry cells. This force counteracts tensile stresses resulting from service loads.
Two procedures can be used to supply the prestressing force to the steel: pre-tensioning (tendons are tensioned between external abutments before stress transfer to the masonry) or post-tensioning (tendons are tensioned against the masonry after it is placed and achieves target strength). Between these two methods, post-tensioning is preferred for masonry because of its construction ease. In addition, stress loss from elastic deformation during the prestressing process is minimized.
Post-tensioned masonry systems are making their debut in the United States, and have already been used in Europe, Australia, and Canada. The theory behind post tensioning the masonry is to offset the tensile stress experienced under service conditions by introducing a level of precompression into the member.