Masonry Magazine February 2002 Page. 13
ANCHOR REQUIREMENTS
In Seismic Design Category (SDC) C and higher, anchor frequency is increased. The 2.67 sq. ft (0.25 sq. m) of wall area per anchor is reduced to 2.0 sq. ft. (0.19 sq. m); the 3.5 sq. ft (0.33 sq. m) is reduced to 2.63 sq. ft. (0.25 sq. m). Maximum spacing remains the same. Continuous single wire joint reinforcement is also required in SDC D and higher, and it must be mechanically attached to the anchor in SDC E and higher. The maimum air space dimension is set to ensure performance of the anchor. Corrugated anchors have the most restrictive air space dimension because they are weakest in compression. All veneer anchored must be protected against corrosion. Since the veneer wythe is typically exposed to weather, the anchors must be hot-dipped galvanized, epoxy-coated, or made of stainless steel. The weight of zinc applied varies with the type of anchor and material from which it is made. The standards referenced in the Specification for Masonry Structures, ACI 530.1/ASCE 6/TMS 602, require at least 1.50 oz per square foot (0.031 kg/square meter. A mill-galvanized coating is not sufficient.
ANCHOR SELECTION
The contract documents will most likely specify the anchor to be used. However, the mason contractor may be able to offer a substitution. Some considerations are:
* ability to accommodate dimensional tolerances
* coordination between trades if one part of the anchor is installed by a different contractor
* for veneer over tilt-up concrete with anchors shot onto the face of the concrete in lieu of dovetail slots, the pullout resistance of the connection must be sufficient
CONSTRUCTION CONSIDERATIONS
Proper performance of the veneer anchor is achieved through the use of the proper anchor and its proper placement. The mason contractor has an important role to play in this area. Use of the proper anchor has several factors to it. These include the selection of the correct anchor pieces, the location of the anchor in the mortar joint, and the configuration of the anchor as placed. Of course the anchors must be installed. Investigations of veneer performance following collapses often show that the anchors are not even present. Failure of the mason to install veneer anchors is inexcusable. Dimensional tolerances are a part of any wall construction. Thus it is incumbent on the mason contractor to supply anchors of the proper length to engage the veneer wythe as required for performance. The Code contains requirements for placement as illustrated in Figure 1. Proper embedment means that the anchor can develop the strength to transfer loads from the veneer to the backing. Installing anchors that are too short to have minimum embedment will result in anchors with insufficient pullout due to wind suction. Anchors without sufficient cover to the outside face will have insufficient compressive strength to resist wind pressure, and are subject to corrosion. See Figure 2 on the next page. The answer to this condition is to provide anchors of the correct length to fit into the anchor placement area. Field modification of wire anchors is especially troubling. Anchors that are "bent to fit" may not have proper strength to transfer loads. Do not widen the angle at the point of the wire. See Figure 3 on the next page. When the wires project from the back of the brickwork at modified angles, the tension and compression capacities of the anchor are reduced. Placement requirements also apply to the connection of the anchors. As required by the Code, the eye and pintle of adjustable two piece anchors cannot be offset by more that 1½ in. (31.8 mm). Also, the pintle legs must be of W2.8 (MW18). This is a larger diameter than permitted in joint reinforcement. Similarly, corrugated anchors must have the nail or screw located within ½ in. (12.7 mm) of the 90-degree bend in the anchor. These requirements are to ensure proper load transfer. Anchors must be properly embed-