Masonry Magazine April 1995 Page. 13
Heavy Duty Joint
Reinforcement
In order for building owners to get the best possible performance from their masonry walls and the joint reinforcement in them, standard gage reinforcement should be used whenever possible.
By MARIO J. CATINI, P.E.
President, Dur-0-Wal Incorporated
VER THE LAST few years, there's been a marked increase in the amount of extra heavy (7/16" diameter side rods) and super heavy (is" diameter side rods and cross rods) joint reinforcement specified and used in masonry work. If not designed and detailed carefully, this material can be difficult to work with and can actually reduce a mason's productivity.
For this reason it's important for both designers and masons to understand the conditions under which extra heavy or super heavy material should be specified and, if it is specified, to know of ways to make substitutions that could improve its cost effectiveness.
There are three fundamental points to consider when trying to decide on the use of extra heavy joint reinforcement. They are the amount of reinforcement actually needed; the effect of mortar strength on the development of the strength of the reinforcement and the problems associated with interference resulting from tolerances allowed for masonry joints.
Amount of reinforcement needed
Joint reinforcement can be used in three applications namely: for crack control (pure tension in the side wires and diagonal cross wires), to connect wythes of a multiwythe wall (mainly tension in the cross wires), and as part or all of the horizontal reinforcement in a reinforced masonry wall (mainly flexure or tension).
When joint reinforcement is used for crack control the amount of steel used should be such that any cracks formed as a result of the drying shrinkage of the masonry will be kept closed. This is accomplished by keeping the stress in the steel low.
When standard gage joint reinforcement is used at a vertical spacing of 16" on center, the tensile stress in the steel is kept below 50% of its working stress level and, as a result, cracks in the masonry are distributed along the steel and kept invisible. Tests have documented that the use of standard gage joint reinforcement reduces crack widths by 90%.
Traditionally joint reinforcement has been used in another crack control function. Standard 9 gage joint reinforcement at a 16" vertical spacing is used to make a masonry wall with a random vertical crack as strong as an uncracked plain masonry wall.
The use of ladur type joint reinforcement provides less steel in the wall, and as such increases the tensile stress in the steel. Standard truss reinforcement is 39% stronger than ladur type reinforcement. This means that the potential crack widths for ladur type reinforcement will be greater but experience shows that ladur reinforcement is still adequate, especially when control joints are spaced no more than 20' apart. Based on the above, it can be seen
MASONRY-MARCH/APRIL, 1995 13