Fastening Facts

Professional sign installers encounter virtually every conceivable type of building surface and structure in their daily work. Because signage is frequently an afterthought for project planners — and many signs are installed on existing buildings — it’s usually up to the erector to determine the best method of installation. During my contracting days, we installed such a large variety of signs that I developed a good, working knowledge of fasteners and their capabilities. When we finished bolting down a sign, someone always felt compelled to remark, "That’s one sign that will never move." The shortened form of this expression is the ever-popular, "It’ll hold." But without an engineering degree or years of experience, how do you really know if the sign you’re installing will hold? Turn of the screw If you’re installing a single-faced sign on the facade of your customer’s store, you’re probably concerned about which type of fasteners to use. If the sign is installed indoors at a mall, its total weight ("dead weight") is the only concern, and choosing the proper fasteners is a simple matter of dividing this weight by the number of bolts or screws used to attach the sign. This yields a per-bolt load that can be compared to the load ratings of various common fasteners. For outdoor signs, however, wind-loading forces must also be considered. Depending on the sign’s mounting height and its orientation relative to prevailing winds, building corners or parapet edges, wind pressure can exert a strong outward pull on the sign. This is known as "leeward wind presssure," which, under high-wind conditions, multiplies the load stresses on the mounting bolts. Because a fundamental rule of safe engineering is to design structures for the worst possible case, your fasteners must be rated to withstand the maximum anticipated wind-loading for the particular size and weight of the sign. Sign fasteners are exposed primarily to two types of forces: Lateral load or "shear load" refers to the amount of vertical force required to pull a screw out of lumber in a direction perpendicular to the screw’s axis. Similiar force applied to a bolt connecting two steel plates attempts to shear off the bolt. "Withdrawal load" or "tension load" is the axial force that attempts to extract the fastener from its anchorage. For example, the fasteners for an indoor, ceiling-mounted sign are exposed almost exclusively to withdrawal or tension loading. The fasteners for wall-mounted, indoor signs are primarily exposed to lateral or shear loads exerted by the sign’s dead weight. Outdoor sign fasteners, however, are subject to both of these forces working in concert. When you attach a sign to a standard lumber-framed wall with ordinary wood screws, you normally can’t determine the grade of studs that you’re penetrating. We previously touched on the importance of anchoring wall signs to these internal studs for safe installations. A wood screw’s holding strength depends on four main factors: The screw’s cross-sectional area (nominal diameter) The quality of lumber penetrated The orientation of the framing member’s grain in relation to the vertical load The depth of the screw’s penetration The lumber grade has a significant effect on a specific fastener’s rating. The National Design Specification (NDS) for wood construction accounts for this and other "adjustment factors" in establishing load ratings for common sizes of lag bolts. Most single-faced, outdoor wall signs manufactured today are not particularly heavy for their size, but wind pressures can be substantial. This explains why manufacturers typically equip wall signs with internal or external mounting hardware designed to accept 3/8- to 5/8-in. bolt diameters. Probably the most common fasteners used to attach single-faced, cabinet-type electric signs to framed or masonry walls are 1/2-in. lag bolts. Although these bolts exceed the structural requirements for most wall-sign installations, the varying factors of geographical location, mounting position and building construction must be considered for mass-produced signs. That’s why quantity manufacturers dictate the fastener size by the particular bolt-hole size they incorporate in the sign’s mounting hardware or wall irons. It’s far better to have too much fastening stength than not enough. Although many non-electrical wall signs (plywood, Alcan Composities USA Inc.’s Sintra® ™, etc.) don’t exceed the loading specifications of ordinary wood screws, lag bolts are frequently used for these installations. Lag bolts are available in longer lengths (to install thicker signboards), and they are much easier to install than wood screws. If you’ve ever attempted to drive in a large wood screw with an ordinary screwdriver, you can appreciate the convenience of using a socket wrench to tighten a lag bolt. To avoid splitting the lumber studs in framed walls, however, it’s important to drill a smaller-diameter pilot hole before inserting a lag bolt. Alcan Composites

Nuts and bolts

Hex-head steel bolts (with machine threads) are commonly used to attach signs to poles that incorporate welded mounting plates, and also to connect various sign-structure components. Bolt heads carry rating marks that indicate their tensile strength according to standards developed by the Society of Automotive Engineers (S.A.E.). Tensile strength is the maximum tension load (in pounds per square inch of cross-sectional area) that a bolt can resist before the metal fails and it pulls apart. Any specific diameter of S.A.E. Grade 8 bolt has approximately twice the tensile strength of the same-diameter Grade 2 (hardware-grade) bolt. To determine the bolt’s grade, just add 2 to the number of radial lines on its head.

Special, hardened steel bolts are required to attach large, pole-mounted signs and structural components. In most cases, these are A325 bolts, which are available in three types: medium-carbon steel (Type 1), low-carbon martensite steel (Type 2) and weather-resistant, galvanized steel (Type 3). These bolts must be used with conforming, hardened steel nuts and washers. Hardened nuts are identified by circumferential marks stamped on at least one face, or by the marks "2," "2H," "D" or "DH."

According to Mike McGuire, Fastening magazine’s editor, "85% of all fastener failures occur because of improper installation or application and poor design. Only 15% of the failures are metallurgical." This means that sign companies can virtually eliminate attachment failures by carefully specifying and installing various fasteners. When using any of the new generation of industrial fasteners that are anchored with special cements or adhesives, always observe the manufacturer’s specifications carefully. If you’re working from an engineer’s drawing, it’s essential to use the precise type and grade of fasteners indicated on the plan. When in doubt concerning fastener size, it pays to consult an engineer experienced with sign structures. As we’ve seen, all nuts and bolts are not created equal, and you certainly don’t want any weak links in your sign installations.

 

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