Masonry Magazine October 2003 Page. 20
POST-TENSION MASONRY
Figure 9
c) Planning the layout of the tendons and bottom anchors.
d) Setting the top anchors.
e) Sequencing and tensioning the tendons.
System Selection
THE CHOICE of which prestressing system to use is based upon whether you want to use bar tendons or strands. Bars are less expensive, more readily available, and come in stock lengths. Couplers are used to connect the bar tendons together. The threaded ends must be fully engaged in the couplers. Strands are used full-length without couplers.
The tendon size is a function of the design. However, as with reinforced masonry, larger tendons at a wider spacing tend to be more economical. On the other hand, larger tendons will cause locally higher stresses, whereas using smaller tendons makes installation of each tendon easier. The decision as to which tendon size to use has to balance both concerns. The final selection is usually based upon economics.
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If corrosion protection is important, strands offer the best corrosion protection because they are covered with a plastic sheath and are placed in a duct (Figure 8, on the previous page, shows a duct). The duct is installed with the masonry and the tendon is installed full height after the wall is constructed. Bars are usually hot-dipped galvanized or epoxy coated and left exposed in the masonry core.
Suitability of the Design
THIS IS OFTEN a function of the layout of openings and piers, as well as the size of the building. Small, thin wall areas or piers are not suited to prestressing due to the high compression introduced by the tendons. If there are a few of these, they can be constructed using reinforced masonry. If there are several instances of these conditions, it may make any prestressing system uneconomical.
Multi-story construction must also consider the alignment of openings from floor to floor. While alignment is preferred for all masonry to avoid distributing loads over openings, for post-tensioning the offsets also add additional expenses since the tendons cannot be installed continuously. Also, there is added cost in labor and hardware to install two one-story tendons versus one two-story tendon. The left side of Figure 9 shows the effect of offset openings. The preferred alignment is shown on the right.
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