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Step bolts are appropriately named because they are used as climbing steps on communication and utility poles and towers. They are also used to fasten wood to wood and wood to metal. As a member of the round head bolt family, they have a "large circular head with a low rounded top surface and flat bearing surface, and an integrally formed square neck under the head." Similar to carriage bolts, step bolts may be the preferred choice for decking and soft wood because the lower profile head is less obtrusive, and the larger head diameter reduces the chance of pull-through. Since the head is designed not to turn, they are usually tightened by torquing a nut. Whenever the head is exposed, they can be used with an internally threaded (tapped) hole and tightened by turning the step bolt's square neck.
Threads are standard right-hand and Unified inch coarse series (UNC, Unified National Coarse). Short lengths are fully threaded while longer lengths follow a standard formula: for bolts 6" and shorter, the minimum threaded length is two times the basic bolt diameter plus 1/4", and for lengths greater than 6", two times the diameter plus 1/2".
Typical step bolt sizes are 1/4", 5/16" and 3/8" in diameter while lengths span from about 3/4" to 3". Measure length from under the head to the threaded end of the bolt.
Head diameter averages about 3.25 times the size of the bolt. The width across flats of the square neck is about the size of the bolt. Table 1 lists head diameter and height, square neck width and depth and minimum threaded length.
Grade 2, steel, zinc plated, is a common grade, material and finish.
Zinc, the most popular and least expensive commercial plating, offers moderate corrosion resistance. However, hot dip galvanized is a thick coating of zinc that protects against corrosion in harsh environments. Hot dip galvanized and stainless steel are usually recommended if the bolts (less than 1/2" in diameter) will be used with pressure preservative treated wood such as "ACQ" (Alkaline Copper Quaternary)—check local building codes and contact your lumber supplier for recommendations.
The square neck prevents the bolt from turning as the nut is tightened. When installing a step bolt in soft wood, consider drilling the hole the same size as the bolt's shank to ensure a tight fit for the square neck; use a "soft" hammer or mallet to drive the bolt into position and prevent damage to its finish. In hard wood, a slight counterbore for the neck is sometimes needed to prevent the square neck from splitting the wood and to ensure that the head will pull down flush.
When installing step bolts in wood, use a large diameter washer under the nut to distribute clamping force over a larger area and minimize compression of the wood.
Refer to American Society of Mechanical Engineers Standard ASME B18.5, Round Head Bolts, for specifications relating to step bolts.
Step Bolt Dimensions
Table 1. Step Bolt Dimensions. Nominal size in fractional and decimal
inches, head diameter and height, square width and depth and minimum
threaded length, in inches. (Note: Average size is calculated and rounded
to nearest 64th.)