Masonry Magazine June 1998 Page. 28
No One Connector is Perfect... Careful Study of the Connectors Shown Below
Will Help to Identify the Benefits and Weaknesses of Some Typical Connectors
Adjustable
Veneer Tie Assembly
Column Anchors
Joint Reinforcement
Fig. 1 (a) One piece anchors provide the required out-of-plane stiffness and in-plane adjustability.
This anchor, (fig. 1a) is popular for veneers and other multiwythe walls. It is free to move up and down to accommodate vertical expansion and contraction and has a continuous horizontal slot to accommodate horizontal movement. It is important to note that large movements do not have to be accommodated. Horizontal movements greater than about 1/4" can be handled by many connectors but cannot be accommodated by the masonry itself. Note that this connector design loses stiffness as the vertical misalignment between the pintle and the plate increases. When large eccentricities are expected a sheet metal pintle (Figure 1b) is the best.
Fig. 2(a) Traditional weld-on anchor has a basic flaw since a hinge is formed at the column.
This anchor is the "workhorse" for connection to steel columns. For many years, the weld-on triangle (Figure 2a) was the most popular because the triangle tie attached to the clip was assumed to have the best pullout strength. On close in spection it can be seen that this assembly acts like a hinge and as a result allows the triangle tie to rotate under load. This significantly reduced the out-of-plane compression stiffness and load capacity of the anchor. A better solution is to use a single piece anchor (Figure 2b). Tests on this anchor verify that its pullout capacity is more than adequate for normal loads.
Fig. 3 (a) When C/M veneer is used it is essential that joint reinforcement be installed for crack control in the veneer wythe. Truss is best when there is no insulation in the cavity
There are so many different configurations of joint reinvorcement that it is sometimes difficult to determine which is the best for a particular application. A good rule of thumb is to provide one longitudinal wire for each mortar bed joint. When both wythes of a wall are laid simultaneously (Figure 3a), one piece joint reinforcement works well. It is important to note that, when a concrete masonry veneer is used, at least one longitudinal wire should be provided in the veneer for crack control. If the veneer is made from clay masonry the extra wire is not needed except in high seismic applications. Only ladder type joint reinforcement should cross an insulated cavity. When the two wythes are built separately, adjustable joint reinforcement/connectors are recommended (Figure 3b). If the backup wythe is vertically reinforced, ladder type is recommended. If not, truss type is recommended for improved crack control.
Fig. 1 (b) Two screws provide stability even when Fig. 2 (b) Sheet metal thickness can be designed to
one screw is overdriven or corroded.
provide desired stiffness in high load situations..
Fig. 3 (b) When veneer and backup is laid separately
adjustable systems are the best. Ladder type is
used in the backup when vertical reinforcement is
used.
SUMMARY
Even though there have been many different types of connectors and joint reinforcement configurations developed over the years, a careful analysis will help designers and contractors narrow down the selection alternatives to select the best for the applications at hand. One can only remember the age-old adage that says, "If we do what we did we will get the results we have always gotten". Improved performance of masonry requires that we all pay attention to the specific needs of a project. If a contractor sees plans without proper details from a designer, it would be a good idea to remind that designer of the code requirements.
References
Anchors And Ties For Masonry NCMA TEK 12-1
National Concrete Masonry Association, 1995
The Structural Role Of Joint Reinforcement in
Concrete Masonry NCMA TEK 12-2 National
Concrete Masonry Association 1997
Joint Reinforcement And Selection Guide Tech.
Bulletin 92-1 DUR-O-WAL, Inc. 1992
28 MASONRY-MAY/JUNE, 1995
Continued on next page-