written by: John Moehring
• edited by: Lamar Stonecypher
• updated: 1/28/2010
Joining individual and assembled elements together is the bread and butter of structural engineering. A crucial part in many of these joined elements are fasteners: bolts, screws, rivets, welds, clamps, nails, and even adhesives can play a vital role.
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Ignorance Is Bliss
Keeping load bearing assemblies together is the role of structural fasteners. Using the correct geometry, implementation, and composition of these critical elements can spell the difference between blissful ignorance of their success and razor sharp focus on their failure. Choosing the best one would seem to be a simple matter of selecting the geometrically correct fastener made from the strongest materials for a given cost. This is not necessarily ideal. For example, a higher grade of steel bolt may exhibit better tensile strength, but that increase in strength may also make it more prone to brittle fracture. In applications where a certain amount of elasticity is required the lower grade of steel would be not only more desirable, but also possibly life saving.
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Not Always Discrete
There are two broad categories of fasteners used to join two structural elements together:
Welds and adhesives fall into this category. Welding is the process of melting the substrates along a joint or seam, usually with a sacrificial medium melted into the joint as well. Metallic and thermoplastic welds are utilized, but by far the most common type of welding is performed on metals. In some cases metallic welds are stronger and less prone to failure than the substrates, which helps explain some of its usefulness in structural construction.
Adhesives bond pieces together without substantially changing the substrates. Gluing, brazing, and soldering are examples of this technique. Careful matching of adhesive and substrate compositions must be performed in order to achieve structural integrity.
Bolts, screws, rivets, clamps, and nails are typical examples of discrete fasteners. Screws and nails rely on some form of friction between the fastener component and substrate to maintain structural integrity, while rivets and clamps provide mechanical blocking to keep pieces together. Bolts and screws incorporating nuts and/or washers use a combination of friction and mechanical blocking to function correctly.
Within this broad category are many, many specification and application requirements. For example, hex bolts and hex cap screws may appear to be similar but in fact have very different manufacturing and application specs. Hex bolts are used where the mechanical properties of the fastener are critical, and hex cap screws are used where tight machining tolerance of the fastener itself is also important. Blind rivets require less installation effort but do not have the structural integrity of solid rivets. Wood screws have better tensile holding strength than nails but tend to experience brittle failure in shear. And so on.
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The Design Challenge
Given the incredible array of fastening choices facing the structural engineer, it is not always immediately obvious which to use. Do the joined elements need to be disassembled for future inspection or maintenance? Can the substrates withstand the heat stress of welding? Will bolt holes concentrate fatigue stresses? Is an epoxy adhesive adequate for potting foundation bolts? Are six rivets per foot adequate to keep the panel in place during 60 mph winds? Are A325 bolts strong enough? Does the local building code specify nailed sheathing? Specification awareness, experience, and training are vital in these critical design decisions. Consequently, different fastener types may be selected for the same application simply due to the preference of the engineer and other project considerations such as cost, safety factors, availability, and aesthetics.
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The last critical step in the fastening process is the actual assembly. The most detailed and accurate analysis can be worthless if follow up is not conducted to ensure adherence to specifications and to help ascertain whether the correct design decisions were made. Testing, maintenance, inspection, and forensic reviews not only help ensure structural integrity but can also serve to refine application and specification criteria. And continuous improvement in structural fasteners is really something to be fixated on.