Masonry Magazine September 2002 Page. 24
Post
# TENSION
The governing criterion for post tensioning forces is the limitation of the crack width, besides the masonry strength in general.
Gero Marzahn, doctor of engineering and instructor at the University of Leipzig, wrote a paper in 1997 that discussed post tensioning as a viable alternative with dry stack masonry. He found that, like solid masonry, dry-stacked walls have a good resistance against compression but are slightly weak against shear, bending or tension. As dry-stacked masonry is not able to resist tensile stresses perpendicular to bed planes, cracks running along the bed planes will occur. Therefore the bed planes become planes of weaknesses under direct tension or flex.
If actions induce shear, bending or tension, additional compression stresses in vertical direction are needed. Obviously, any increase in axial load, either caused by additional dead load-or better by pre-stressing-will induce friction to sustain shear as well as reduce the maximum crack width due to bending or direct tension, respectively.
Therefore an action such as flexure that induces tension requires the dry-stacked masonry to be post-tensioned. Post-tensioning enhances the resistance to flexure as well as shear and tension. The governing criterion for post tensioning forces is the limitation of the crack width, besides the masonry strength in general. The ultimate load level mainly depends on the deformation of the entire wall between anchorages.
Masonry constructed from all types of units may be post-tensioned with tendons running through pockets formed in the masonry or cores in the units themselves. A lack of shear strength or bending resistance and the static system of most of the walls clearly favor the vertical axis of a wall to be the best direction for placing tendons. Commonly the tendons are located at the center of the wall. For special applications, such as basement walls, tendons might be tensioned at a constant eccentricity. For post-tensioned masonry, pre-stressing bars or monostrands are usually used.
For grouted masonry structures, a bonded post-tensioning system might be used similar to concrete structures, but an unbonded system using monostrands offers major advantages in ungrouted and dry-stacked masonry structures both for constructability and durability reasons. Unbonded monostrands are provided now with an excellent double corrosion protection of the pre-stressing steel by grease and plastic ducts or simply by means of plastic coat.
Post-tensioned, dry-stacked walls demonstrate how masonry can be turned into a new structural material that is suitable for a wide range of buildings as well as engineering works like bridges or shells.
Gary Otto, president of Proto-II Wall Systems, West Covina, Calif., presides over one of the major post tension masonry material supply companies in the U.S. The Proto-II system is awaiting final patent registration, indicating its special nature. Otto is quick to explain, "Proto-II employs post tensioning techniques and construction methods that had their beginnings in techniques long used in structural concrete. Applied to a freestanding masonry fence, for example, this design approach has created a system stronger than conventional walls while eliminating or down-sizing heavy and costly materials inside the wall. That means the overall cost is reduced. The combination of higher strength, lower price, system lightness, smaller footings and increased installation speed should make post tensioning very attractive to mason contractors."
Proto-II reports that approximately 700 miles of post tension masonry fencing were installed in the western states in 2001 alone. With about 100 licensed and certified installers in the country, Proto-II claims to have as much as 50 percent of the post-tension masonry market in California.
22 Masonry
September 2002
www.masonryshowcase.com
# Proto-IItm Wall Systems
Post-Tensioned Masonry Fences
Post-Tensioned Masonry Retainers
U.S. Patents issued and Patents Pending
The excellence of traditional masonry
The efficiency of modern techniques
* Extremely cost effective
* Higher strengths
* Fewer materials needed
* Elimination of major material groups
* Designs are per code requirements
For more information contact
Proto-II Walls Systems
Gary Otto
(626)967-9765
garyotto@protoll.com
website: www.protoll.com
ence catastrophic forces such as earthquakes or hurricanes, the joints throughout will 'flex' slightly, radially distributing the force across the whole surface, and then lock-up as a single, solid interconnected mass."