Masonry Magazine May 2010 Page. 17
Most brick ties have been developed to comply with a stiffness criterion of 2,000 pounds per inch as encouraged by the Brick Industry Association and the Western States Clay Products Association. This is a critical standard, due to the veneer reaction to wind pressure. The wall tie system must be capable to transfer the live load to the parent structure with minimal veneer deflection. Insufficient and/or inadequate ties and fasteners can exacerbate the wall deflection and allow for deficiencies in the performance-based expectations of the veneer, compromising water tightness and air permeability. An unanticipated weakness of a wall tie configuration can be the result of a less then adequate fastener. The analogy of "you're only as strong as your weakest link" plays true in this scenario.
The veneer can be attached to a host of building elements, which can include concrete, masonry, cold formed metal stud or wood stud, and structural steel the task is to select the appropriate fastener. Keep in mind that once the veneer is constructed, the fastener connection is not serviceable, leading to multiple questions that need to be answered prior to construction. What type of fastening system should be used? What cautions should the engineer or installer be aware of regarding various fastener types? How effective is the fastener for the application? Is the fastener's stiffness as good as the tie selected? How sensitive to installation challenges are the various types of anchors or fasteners?
Tie selection
ADJUSTABLE TIES ARE PREFERRED. They can be a base plate with a "v" or triangular tie, a plate and two-leg pintle, or a
Brick ties connect with concrete, metal stud and masonry structures
single post and tie system. The selection is a function of the wall make-up, and whether waterproofing/flashing and insulation has been applied. Ideally, the base plate should make firm contact with the concrete, masonry, or sheathing in order to assure both compression load transfer and that a secure connection is made. Extended legs for piercing the insulation are taxing to maintain in a perpendicular installation. Also note that two-screw installations required by manufacturers are not for redundancy, but performance. A loss of one screw in a two-screw set renders the tie compromised, and remediation is required. Also, the tie system must have holes in the base plate large enough to accommodate the selected anchor. For instance, a 5/16-inch diameter hole would be the required hole size for a 1/4-inch anchor.
The connection methods are numerous, but any anchorage must resist the veneer loads. The first step is to quantify the order of magnitude of loads for the fastener to resist. Adjustable ties spaced at 1 per 2.67 square feet are expected to resist a tension and compression load. The basic live load as calculated can be as low as 21 psf for a non-essential structure based on 90 mph wind speed. The resulting force is 56 pounds tension and compression. Applying a 4:1 safety factor, an anchor allowable load greater than 225 pounds would be useful and marginally acceptable. If a seismic condition exists, the live load can be 70 pounds or greater and the allowable load for the fastener can be 280 pounds.
Concrete and masonry fastening applications
BRICK TIE PRODUCTS typically specified are dovetail slots and dovetail anchors or hook and eye reinforcement for the concrete and masonry applications respectively. If typical product placement and serviceability are effected by installation issues, or a veneer is to be added to an existing concrete or masonry wall, an alternate veneer anchor method will be required. Once selected, how is it fastened to the concrete or masonry?