Masonry Magazine April 1995 Page. 14
Joint Reinforcement
That extra heavy or super heavy reinforcement in a wall, the cross sectional area, and hence the diameter of the side wires is a function of the design loads imposed. These loads are determined by the engineer or architect of record. Given a tension force on the joint reinforcement, a designer can choose a large diameter wire at a wide spacing or a small diameter wire at a close spacing. The cross reinforcement is not needed for crack control.
When using joint reinforcement as a wall tie to connect wythes of masonry together, standard gage cross wires are almost universally allowed. The only exception to this rule is in the Uniform Building Code where 16" diameter cross wires are required when connecting wythes of grouted multiwythe masonry. Codes do not address the diameter of side wires of joint reinforcement for this application.
The Masonry Standards Joint Committee Standard, MSJC, (Sec. 5.8.1.5) allows the use of 9 ga. (W1.7) cross wires spaced at one wire for each 2.67 square feet of wall area. The cross wires of standard ladur at 16" on center provides a 9 ga. wire for every 1.66 square feet and truss provides even more steel area.
Based on the above, it can be seen that 16" cross wires are seldom needed for connecting multiwythe walls and 3/16" side wires are never a consideration in this application.
When joint reinforcement is used for all or part of the structural reinforcement, that extra heavy or super heavy reinforcement in a wall, the cross sectional area, and hence the diameter of the side wires is a function of the design loads imposed. These loads are determined by the engineer or architect of record. Given a tension force on the joint reinforcement, a designer can choose a large diameter wire at a wide spacing or a small diameter wire at a close spacing.
For example: A design requirement of 0.052 in. per ft. in an 8" wall can be met by a number of combinations of joint reinforcing type, wire diameter, and vertical spacing. This flexibility applies to regular joint reinforcement as well as to adjustable assemblies such as Dur-O-Eye and Ladur-Eye.
In the Uniform Building Code there is a requirement that the cross sectional area of horizontal reinforcement in reinforced masonry walls be a certain minimum. Often, extra heavy reinforcement is used to meet this requirement. This does not have to be the case. In fact the best way to handle this situation is to provide the required steel area with a combination of a small amount of rebar in a bond beam at the top of a wall and the balance of steel in the form of standard joint reinforcement at 16" o.c.
For example, in a 14' high 8" wall, the minimum steel requirements can be met with standard joint reinforcement at 8" o.c. and 1-#5 (0.31 in.) bar or equivalent in the top bond beam. If extra heavy were used at 16" o.c. the bond beam steel would be 1-#6 (44 in.) bar.
Based on the above it can be seen that whenever extra heavy reinforcement is specified at 16" spacing, another type of reinforcement can be substituted at 8" spacing.
Bond Strength
Bond strength and development of steel tests have shown that Type M or S mortar is needed in order to fully utilize the strength of 316' wire in joint reinforcement. These tests showed that with Type N mortar, the strength of a wall did not improve beyond that attained with standard joint reinforcement at 16" o.c. regardless of the amount of steel used. With Types M or S mortar, the strength of a wall improved as more steel was added.
Based on the above it can be seen that, when extra heavy side wires are used, Type M or S mortar must be specified also. With Type N mortar the extra heavy steel can not be fully utilized.
Construction considerations
Most mortar joints are specified to be 36" thick. According to the MSJC Standard, bed joint thickness can vary 18" in either direction so that joints can actually vary in thickness from 1/4" to 12". The use of extra heavy joint reinforcement becomes questionable when the joints are at their minimum allowable thickness (14").
When 16" wire is used, there is very little clearance between the wire and block. Wire tolerances allow 16" wire to be as big as 190 in. in diameter. When you add the thickness of hot dipped galvanizing the diameter can be as high as 0.193 in. This leaves only .057 in. clear between the wire and block... less than Vis".
This clearance is much less than the diameter of the aggregate in mortar and can result in "rocking" when a mason tries to lay a unit. In addition, when a joint gets "crowded" with steel there is less protective cover from mortar and a greater potential for corrosion and leakage.
For a given amount of steel required, small diameter reinforcement to closer spacing is better than large diameter steel at wide spacing.
Based on the above it can be seen that, for a given amount of steel required, small diameter reinforcement at a closer spacing is better than large diameter steel at wide spacing.
Summary
In order for building owners to get the best possible performance from their masonry walls and the joint reinforcement in them, standard gage joint reinforcement should be used whenever possible even if this means that it will be used at 8" centers. Below are some of the reasons for this recommendation:
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