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Connectors, Anchors and Fasteners
Anchored Veneer: How Good is Your Fastener?
Brick veneer/cavity wall construction is a popular building practice. The aesthetic value, quality image, and the constructability of brick veneers lends to its diverse application. The advantages associated with the use of brick veneer are numerous, and warrant a dedicated discussion. However, the air barrier integrity of the veneer system has become a significant entity in the design and construction of brick veneered structures. An area of interest amongst design professionals and building owners is the awareness of energy savings and LEED recognition associated with the masonry. The design team and contractor are expected to construct the brick veneer maintaining focus on the essentials of air barrier integrity and moisture control. An improper brick tie and fastener combination can lead to unexpected compromises in the veneer stiffness and potential moisture and air leakage issues.
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?
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
Anchors to concrete or masonry can be torque activated, hammer set expansion types, threaded screws, power actuated pins, or hammer driven nails. Depending upon the tie system selected, the length or type of fastener must comply with the thickness of the base plate to be attached, plus the minimum embedment required for the fastener to achieve the loads induced. Since this fastener must partner with a plate or tie assembly, the functional and material compatibility with the tie is important.
Torque applied anchors are an ideal solution for the plate attachment. Besides providing a measurable clamping force, the installed anchor can be inspected. Applying a 4 foot-pound (48-50 inch-pound) torque can produce a preload greater then 900 pounds. This is a typical installed torque for 1/4-inch torque type anchors. Since the preload exceeds the allowable load and allows for creep and relaxation of the installed torque (usually 35 to 50 percent), the choice would be acceptable (check with anchor manufacturer regarding the appropriate installed torque and the resulting induced clamping force). Stud-type expansion anchors can take four to six turns of the nut to achieve the installation torque. The stud projection from the plate surface could be 1/4- to 3/8-inch above the surface of the nut. The total anchor projection distance of 1/2- to 3/4-inch can interfere with the placement of insulation or the vertical travel of the adjustable tie in the base plate. Therefore, a torque activated anchor having a hex head finish is best.
The use of anchors other then torque-controlled expansion has limited appeal. A nail drive system does not draw the plate tight to the concrete or masonry surface. If not properly inserted and activated the anchor can expand prematurely, and the plate connection would be loose to the surface. This will have a negative impact on the free play of the tie and result in excessive deflection. Except for a visual identification, a physical inspection is not possible that would indicate the tie system is secured to the building. This would be true of any nail or hammer driven anchor. The toggle is torque applied, but the hole size required for a 1/4-inch bolt is 3/4-inch in a hollow application such as CMU. A large drilled hole such as this will play havoc with air and water permeance.
Keys to fastener selection for masonry or concrete
Fastener selection for metal stud
Screw drill tip choice
Screw size – diameter
Screw size – length
Based on these criteria, a #10 screw with a #2pt attaching to a 16-gauge tie base plate against 5/8-inch sheathing and fastening to 16-gauge stud, will require a MINIMUM screw length of:
Using a #12 or 1/4-inch screw for the same conditions, the minimum length should be:
Conservatively, a 2-inch minimum length screw is appropriate for all the combinations. Piercing leg or single pole type tie bases require longer screws predicated on the insulation thickness and the type of base plate.
Screw head type
The Steel Stud Manufacturer’s Association publishes a conservative allowable fastener performance in 16 gauge stud as 137 pounds and 156 pounds tension per screw for the #10 and #12, respectively (without reference to drill point effects). The individual manufacturer’s pull-out data for the selected screw sizes is a more realistic approach for capacity consideration. For example, allowable tension loads varied from 200 pounds to 800 pounds from seven different manufacturers. Selecting the screw to use is based on the performance expectations and the quantity per tie required by the tie manufacture. Single fastener designed ties are cost efficient, and quick research regarding the screw performance will provide the appropriate strength for the connection and less breaches in the air barrier. Based on the live load determined above, the resulting force of 56 pounds tension and compression would produce a safety factor of 2.5:1 and 2.8:1 for the screw sizes listed by the SSMA. If a seismic condition exists, the live load can be 70 pounds or greater and the resulting safety factor can be 2:1 or 2.2:1.
Screw material and finish
Another cost effective selection is to use a self-drilling/self-tapping 410 stainless-steel screw. They are hardened for strength and durability, while still possessing essential alloys for corrosion protection. These characteristics will allow the screw to self-drill a precise pilot hole for the self-threading action of the screw, provide corrosion protection, and offer material strength properties that create an effective long-term fix to the steel stud. Incorporating a sealant washer will keep moisture from influencing the effects of differing material when applied to non-stainless ties.
Keys to screw selection for a metal stud
|Last Updated on Wednesday, 19 May 2010 11:29|