structural CONNECTIONS Proven Technologies for Fastening to Steel in Corrosive Environments By Christopher Gill
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orrosion of metals used in construction is an age-old problem, the fastening without one with annual costs exceeding $300 billion today in the United accessing the steel backStates alone. In addition to economic losses, corrosion of structural side, and minimal and non-structural elements creates a significant safety risk, particu- disturbance of the base larly with critical connections. Fortunately, technologies have been material coating. developed to help address many of the causes of corrosion of these One example features connections with effective products and processes. a fastener that is driven The type and extent of corrosion are often a function of the envi- into a small pre-drilled ronment. Wet environments in the presence of conductive fluid (i.e., hole in the steel. Forming humidity) can attack the surface of the connection elements (base the hole is accomplished Figure 1. Corrosion of connections is steel, connected material, and accessories such as nuts and washers), by a special stepped drill a common concern. thus causing surface or pitting corrosion. A steel base material could bit that does not penbe carefully coated with a protective coating, only to have that coating etrate through the base steel; simultaneously, the bit removes a small destroyed when a connection is made. For example, effective welding circle of coating or surface rust to prepare for the fastening. The to the steel may require removing the coating, or the welding process blunt-tipped stainless steel fastener is driven into the hole using itself may destroy the coating. Connection methods like through- a battery-actuated tool. The act of driving the fastener point into bolting can cause another form of corrosion called galvanic corrosion the hole displaces some of the base steel, which is then melted by when dissimilar metals are in contact with each other in the presence the friction heat, creating a micro weld. The result is a threaded of a conductive fluid. It should be noted that welding and drilling stud protrusion from the steel, carrying a working load up to 800 can damage the coating both on the connection side of the steel and pounds in tension and up to 850 pounds in shear. Below the threaded the opposite side of the steel (Figure 1). portion, tight against the steel surface, is an Where load transfer requirements are integrated stainless steel washer with a neohigh, welding of the steel members may prene washer bonded to it, which seals and be the only feasible solution. Quality conprotects the uncoated steel surface and prevents trols should be specified to ensure that water infiltration. Even though the stud matecoatings and welds are properly repaired rial and the base steel may be dissimilar, the and re-coated. In many applications, nonprevention of water infiltration helps prevent welded solutions may be appropriate. For galvanic corrosion (Figure 2). example, when framing supplemental steel Also available is a system meeting similar to primary steel, bolting may be indicated. requirements, which can be installed withHowever, in a corrosive environment, the out a battery-actuated drive tool. A hole potential for galvanic corrosion or crevice is drilled with a similar stepped drill bit, corrosion may exist. Additionally, unless which provides the precise drilling depth the hole locations are planned and pre- Figure 2. A welded stud with field repair to coating and removes a small circle of the existing punched in the shop, the process can be damage (left). A properly designed fastening system coating. The blunt tip of this fastener features laborious and expensive. Similarly, the prevents damage (right). high-strength threads. When the fastener is attachment of modular systems (“strut” systems) can be accomplished driven (screwed) into the pre-drilled hole, it cuts fine threads in by through-bolting or clamping. However, the clamping devices may the base steel. Like the system described above, the threaded stud not always be effective, depending on the steel shape that is being protrusion includes a sealing washer, which protects the steel and clamped to, like tube members, for example. prevents water infiltration. Working loads for this fastener can As a structural engineer, you may be called on to design or evalu- be up to 600 pounds. Both systems described above provide a ate critical non-structural connections, such as hangers for utilities ¼-inch or 3⁄8-inch threaded protrusion that can be used to attach and equipment or fastening bar grating to steel on a mezzanine. a baseplate or strut profile or hang a pipe or equipment using a Traditionally, these connections are accomplished with welding, coupler and a threaded rod. through-bolting, or clamping. However, as outlined above, these As long as connections need to be accomplished on construction projects, solutions can invite corrosion. Additionally, they may not be feasible corrosion will continue to cause concern. Manufacturers will in the field if the base steel is too thick to drill or the backside cannot continue to advance the state-of-the-art by developing safer be accessed to complete the connection. Fortunately, technologies have and more practical technologies to address these concerns.■ recently been developed which address corrosion concerns and proChristopher Gill is the Manager of Code and Approvals for Direct vide a practical means for installation. Key features of these products Fastening at Hilti, Inc in Plano, TX. (christopher.gill@hilti.com) include highly corrosion-resistant materials, the ability to complete A U G U S T 2 0 21
