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A: The large majority of industrial structural structures—such as building structures, floors, platforms, conveyor bridges, transfer towers, support towers, equipment supports, and so on—are an assembly of smaller components, and, as such, the components must be attached in some fashion.
The most common ways of accomplishing this are the use of bolted connections and welded connections. More specifically, bolted connections are typically made using high-strength bolts and welded connections are typically made using E70XX electrodes. Both bolts and welds come in a variety of types, sizes, and material. The number and size of bolts and the type, size, and length of weld are selected based mainly on the strength required for a particular connection. The single best source for information on bolted joints and the basis for the content in the AISC steel design manuals is the "Specification for Structural Joints Using High-Strength Bolts" by the Research Council on Structural Connections. The main guide for welding generally referred to by engineers is the "American Welding Society (AWS) D.1.1."
The obvious difference between the bolted connection and the welded connection is the method of accomplishing the connection between two components. Bolting is a mechanical-type connection where two or more pieces are held together by a combination of bolts whereas a welded connection is accomplished via a fusion of the metal between two parts. In each case, the physical transfer of forces from one piece to the next is accomplished through the connection material. It is therefore important that the number and size of the bolts is adequate for the load transfer, or in the case of a welded connection, that the weld size, type, and length of the weld are adequate for the transfer.
Bolted connections in industrial structures primarily use ASTM A325 bolts. This is a high-strength bolt used in shear, bearing, or tensile applications. Beam-to-beam connections, beam-to-column connections, bracing connections, plate connections, and so on, are all examples of connections made with high-strength bolts. "Secondary" connections—such as stair treads, girts, purlins, and handrail—are typically made using lower-strength machine bolts (ASTM A307) as opposed to high-strength bolts. The use of high-strength bolts also requires the use of a hardened washer (F436) and a heavy hex nut (ASTM A563).
The most common bolt size used is 3/4-in. diameter but there are times when 5/8-in., 7/8-in., 1-in., or 1-1/4-in. are more appropriate. The number of bolts required for a given connection is directly related to the size selected—the larger the bolt, the fewer are required. By the same reasoning, the smaller the bolts are, the larger the connection will be to accommodate more bolts.
Shop connections (connections assembled in the fabrication facility) can easily be made either way. The quality of the connection is certain as shop connections are done in the most ideal environment/conditions—inside a building with the appropriate equipment for locating and positioning the steel pieces in their proper locations; turning larger, heavier pieces/assemblies into an ideal position for proper welding, and so on. In addition, various automated welding equipment is available in most shops, computer-controlled for precise welding, along with equipment for punching holes and bending and cutting steel.
Field connections, like shop connections, can also be bolted or welded. The difference here is that when given a choice, the engineer will most likely prefer the bolted connection. One reason for the bolting preference is that the steel used in producing the bolts and the connection material meet very stringent ASTM specifications. The connection strength is certain. In the case of a field-welded connection, the weak link in the system is that the strength depends on the quality of the weld that the welder provides. Any number of things can be wrong with the weld alone if the welder is not properly trained or does not properly apply the weld bead. "Bad" welds can result for an assortment of reasons in the field—carelessness on the part of the welder or mis-interpretation of a weld symbol shown on the drawing. Access to the connection for welding may be awkward for the welder. Or the entrance for a welding rod may be extremely tight thus affecting the quality of the weld. In the event the weld is not a "good" weld for any reason, then the strength of the welded connection is compromised. It is for this reason that the engineer will call out on the drawings for, as a minimum, a visual inspection of certain selected welds. This inspection is performed by a certified weld inspector trained to detect imperfections in welds. Any imperfections would be corrected immediately.
A second reason for preferring field bolting to field welding is that the erection time is generally less when using bolts. As the structural steel has been detailed and fabricated prior to field erection/assembly, the pieces will readily go together and fit well. When welding, the components must be positioned and clamped prior to welding. Then a proper weld must be made.
It should be noted that there are many instances where only one or the other type of connection can be used and no choice of type is available, such as a ladder rung is welded to a side rail, not bolted. Or, if a member has to be removable for access purposes or for equipment removal/replacement, then a bolted connection is required, or at least, desired.
In summary, bolted connections and welded connections hold the world of structural engineering together. Without bolts and welds, things would literally come apart at the seams—if in fact they were ever together to begin with. To finally answer the question "should I use a bolted connection or a welded connection?" the answer for the most part is "it’s up to you!"
Answered by William E. Casciola, P.E.
William is Lead Structural Engineer and joined WL Port-Land Systems in 2006. He has been involved in the engineering industry including analysis and design of structural steel and concrete structures/foundations, as well as project management experience since 1973.