Masonry Magazine February 2000 Page. 10
Prestressed Masonry
Continued from page 9
Building Code Requirements for Masonry Structures'. While the chapter allows the use of a number of prestressing alternatives, the one which seems to have the highest potential for application in the States is the use of unbonded tendons in the open cells of masonry units.
After many years of application in Europe and over 10 years of application on the West Coast the system has proven itself as a viable alternative design method.
It is important to understand that prestressed masonry is another form of reinforced masonry and, as such, will have comparable structural properties and safety to reinforced masonry when designed according to the Code.
Key Features
Prestressed masonry has a number of features, which separate it from plain or reinforced masonry and concrete.
As mentioned earlier, prestressed masonry utilizes relatively low levels of precompression. This results in systems that are much less complicated than concrete prestressing systems.
No special tools are needed for the field application of the prestress load. All the work can be accomplished with an open-end wrench or a deep throat socket wrench.
Since tendons do not have to rely on grout to transfer their tension to the masonry, the system can eliminate virtually all the grout used in comparably reinforced masonry walls. Horizontal bond beams, which are installed for a number of reasons beside compressive strength, should still be used to tie walls together.
Major Benefits
Prestressing has all of the benefits over plain masonry as reinforced masonry and a number of benefits over the use of reinforced masonry. To list a few:
Vs. Plain Masonry
Increases vertical bending strength
Increases out-of-plane ductility
Eliminates the debate over the use of masonry cement mortars versus portland cement or mortar cement mortars.
Vs. Reinforced Masonry
Saves cost of grout material and labor
Speeds construction
Can eliminate horizontal cracking
Reduces the weight of a wall resulting in smaller footings and lower seismic forces.
Can provide a method for horizontal bracing
Can simplify the construction of interior in-fill partitions in seismic areas
Applications
Prestressed masonry can be applied in every application for which plain or reinforced masonry is used. However, there seem to be certain applications that stand out as being particularly suited to take advantage of this new design approach. By the same token there are some applications where prestressed design economies can not be realized. These applications are those where code provisions for high seismic areas currently prescribe arbitrary minimum levels of grouted reinforcement.
Below are some of the applications which have a high potential for change.
Non-load bearing
and lightly loaded exterior walls
Warehouses, commercial buildings and schools stand out as prime candidates. The design of these walls is almost always controlled by tensile bond strength. As a result more and more reinforced masonry is used for this application. By eliminating the need for vertical grouting, this method improves cost and construction time.
Interior partitions and exterior
in-fill walls are also prime candidates
In many low seismic areas, vertical reinforcement is indicated to resist horizontal loads. It is virtually impossible to install grout and reinforcing in these situations. With Continund on page 12
One option for installing the tendon in the foundation is to place the three-foot assembly Includes the tendon, coupler and foundation anchor) into the footer when it is poured. The length of the assembly helps to keep it straight.
10 MASONRY- JANUARY/FEBRUARY, 2000
Inspection ports are required at all coupler locations along the height of the tendon. This provides the installer with a method for making sure that tendons lowered down from the top of the wall are property seated in the coupler