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December 2022 #14281 Page #124
Mechanical Anchors: Mechanical Anchors: Screw vs. Expansion: Screw vs. Expansion: Which Which is Right for Me? is Right for Me? By Simpson Strong-Tie Staff
Not all post-installed mechanical anchors are created equal. There are key differences between screw and expansion anchor types — differences that include how they gain their holding strength, installation requirements, and overall anchor performance. In the following article, field engineers Todd Hamilton, Chris Johnson and Derek Gilbert compare the two anchor types. Post-installed structural mechanical anchors are generally categorized as either expansion anchors (aka wedge anchors) or screw anchors. These types of anchors are used after concrete is cured or CMU is placed and offer quick and easy installation in a wide range of applications. What’s the difference? When should you specify one rather than the other? This article will help you understand the similarities and differences and guide you in selecting the best anchor for your specific project need. By definition, a screw anchor is a threaded mechanical fastener placed in a predrilled hole (see Figure 1). The anchor derives its tensile holding strength from the mechanical interlock of the fastener threads with the grooves cut into the concrete during anchor installation (see Figure 2). Screw anchors are tested and evaluated in accordance with ACI 355.2 and ICC-ES AC193 for concrete (cracked and uncracked) and ICC-ES AC106 for masonry (hollow and Figure 1: Simpson Strong-Tie® Titen HD® and grout filled). In practice, screw anchors are Strong-Bolt® 2 Anchors used across a variety of structural applications and industries to anchor items such as sill plates, threaded rod uplift systems, shearwall bottom plates, wood and cold-formed steel holdowns, column/post baseplates, ledgers, rigid kneewalls, racking systems, railings, mechanical equipment, electrical conduits, cable trays and piping supports. In contrast, a wedge-type expansion anchor consists of a steel stud with nut and washer, threaded on the top end with a formed tapered mandrel on the opposite end around which an expansion clip is positioned (see Figures 1 and 2). The anchor is installed in a predrilled hole and set by tightening the nut to a specified torque, thereby causing the expansion clip to expand through deformation or torque over the
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