By Mubashir · Senior Structural Engineer · May 2026
Why Connection Type Is a First-Order Engineering Decision
Structural connections are where the clean lines of a frame analysis model meet fabrication and construction reality. A beam in an analysis model is a line element with end releases and boundary conditions — but in the steel shop and on the erection site, it is a physical assembly of plates, welds, bolts, and tolerances that must be fabricated, transported, lifted, and connected without exposing workers to unnecessary hazard. The wrong connection type for a given situation can increase fabrication cost by 30%, slow the erection schedule, introduce structural failure modes that the analysis did not capture, or complicate future inspection and maintenance.
At Sixteens, connection design is treated as a first-class engineering task — never delegated to standard detail tables without verifying the actual force demands. This article explains the engineering logic behind the bolted vs welded decision and the factors that govern it under AISC 360.
Bolted Connections: Types, Strengths, and Limitations
Bolted connections use high-strength fasteners to clamp connected elements together and transfer force through bearing on the bolt shank (bearing-type connections) or through friction between the faying surfaces (slip-critical connections). AISC 360 Chapter J governs both types.
Bolt grades in the AISC system are primarily ASTM A325 (now F1852 / A325M) and ASTM A490 (now F2280). A325 bolts have a nominal tensile strength of 120 ksi; A490 bolts reach 150 ksi. The equivalent metric grades widely used on international projects are Grade 8.8 (comparable to A325) and Grade 10.9 (comparable to A490). Bolt grade selection affects shear and bearing capacity directly, and higher-grade bolts are more susceptible to hydrogen embrittlement if improperly handled.
Bearing-type connections transfer shear through bolt bearing on the connected plies. Thread exclusion from the shear plane (threads excluded, X suffix) gives higher shear capacity than threads included (N suffix). Bearing-type connections are simpler, cheaper to install, and adequate for the vast majority of statically loaded connections. They allow slip under load — bolt holes are typically 1/16" larger than the bolt diameter — which means the connected elements shift slightly as load is applied until bearing contact is established.
Slip-critical connections are required where slip under service load is structurally unacceptable: connections in moment frames where slip would relax the rotational stiffness, connections subject to load reversal or fatigue, and connections in seismic lateral systems where cumulative slip under cyclic loading would degrade energy dissipation. Slip-critical connections require Class A or Class B faying surfaces (mill scale cleaned or blast-cleaned), specified pretension of the bolt (usually by turn-of-nut or direct tension indicator methods), and explicit pretension inspection.
The primary advantages of bolted connections are practical: they are adjustable during erection (within the tolerances of slotted or oversized holes), they are visually inspectable without specialised equipment, and individual bolts can be replaced if a defect is found. Their primary structural limitation is that bolt holes reduce the net cross-section of the connected element, which must be verified for net section fracture under the critical tensile loads.
Welded Connections: Types, Strengths, and Limitations
Welded connections fuse the base metal of connected elements through the deposition of filler metal under controlled heat input. AISC 360 Chapter J and AWS D1.1 (Structural Welding Code) govern the design and fabrication of structural welds.
Fillet welds are the most common weld type in structural steel. They are deposited in the corner formed by two perpendicular faying surfaces and transfer force through the minimum throat (the 45-degree dimension from the root to the theoretical face). Fillet welds can be applied to T-joints, lap joints, and corner joints without requiring groove preparation of the base metal, making them economical and fast. AISC 360 specifies minimum and maximum fillet weld sizes based on the thickness of the thinner connected part.
Complete-joint-penetration (CJP) groove welds develop the full tensile strength of the thinner connected part across the joint. They require groove preparation (bevelling one or both members to create a V or double-V groove), qualified welding procedures, and often back-gouging and back-welding to ensure full penetration. CJP welds are stronger but more expensive than fillets, and are reserved for connections where the full cross-sectional strength must be transferred — typically beam-to-column moment connections and column splices.
Partial-joint-penetration (PJP) groove welds penetrate only part of the joint thickness. Their effective throat is less than the plate thickness, so they cannot develop full tensile strength. PJP welds are not permitted as demand-critical welds in seismic applications, where fracture through the weld must be excluded as a failure mode.
The primary advantages of welded connections are structural: they transfer all components of force (axial, shear, moment, torsion) with no slip and no hole weakening. A well-executed CJP weld at a moment connection is structurally equivalent to a continuous member. The limitations are practical: welding requires trained, qualified welders, produces residual stresses and heat-affected zones (HAZ) that affect local material properties, and cannot be easily modified or inspected after completion without destructive testing or specialised non-destructive examination (NDE).
Weld Inspection: UT, MT, and PT
Weld quality inspection is a non-negotiable element of any welded steel structure. AWS D1.1 specifies the acceptance criteria for visual inspection, and additional NDE methods are required for higher-criticality welds. Ultrasonic testing (UT) uses high-frequency sound waves to detect internal defects — porosity, inclusions, cracks, and incomplete fusion — in groove welds. Magnetic particle testing (MT) reveals surface and near-surface cracks in ferromagnetic materials and is used for fillet and groove welds alike. Penetrant testing (PT) detects surface-breaking defects using fluorescent or visible dye. For seismic demand-critical welds under AISC 341, UT inspection is mandatory for all CJP groove welds.
Seismic Implications: AISC 341 Requirements
The choice between bolted and welded connections becomes structurally significant in seismic design categories C, D, E, and F, where AISC 341 governs. The lessons from the 1994 Northridge earthquake — where numerous welded beam-to-column moment connections fractured in an apparently brittle manner despite code compliance — fundamentally changed seismic connection design requirements and remain the foundation of modern AISC 341 provisions.
Demand-critical welds are welds whose failure could cause partial or total collapse of the structure. CJP groove welds in beam-column moment connections are the canonical example. AISC 341 Section A3.4b requires that filler metals for demand-critical welds achieve minimum CVN toughness of 20 ft-lb at −20°F and 40 ft-lb at 70°F. These requirements exclude standard E70 electrodes that do not meet CVN criteria; the project welding specification must explicitly call out compliant electrode classifications (such as E71T-8, or E7018-compatible electrodes with documented CVN test results).
Bolted connections in seismic systems can slip under the cyclic loading of an earthquake, which redistributes forces in the connected elements and degrades the energy dissipation behaviour of the frame. This is why slip-critical connections are required in seismic lateral systems — to maintain the intended force path under repeated load cycles. Bearing-type connections are not permitted in the lateral force-resisting system in high seismic design categories.
Pre-qualified connections under AISC 358 (Prequalified Connections for Special and Intermediate Steel Moment Frames) have been laboratory-tested and analytically verified to meet the rotation capacity requirements of Special Moment Frames (SMF) and Intermediate Moment Frames (IMF). The Reduced Beam Section (RBS, "dogbone") connection is the most widely used pre-qualified moment connection: the beam flanges are trimmed in the protected zone to reduce the section modulus and force the plastic hinge to form away from the column face, protecting the weld from the full strain demand. Using a pre-qualified connection eliminates the need for project-specific connection testing, which would otherwise be required for SMF connections.
Practical Decision Factors Beyond Structural Capacity
Once structural adequacy is established for both alternatives, the practical decision factors often tip the choice. Erection sequence is a major consideration: bolted connections can be assembled quickly at height without hot work, and the erector can adjust plumb and alignment before final bolt tightening. Welded field connections require the ironworker to position members precisely, tack-weld to hold position, and then return after the welder completes the joint — a slower and more hazardous sequence at height.
Fabrication environment matters too. Welds made in a shop under controlled conditions — controlled temperature, no wind, optimum access, qualified procedures, and immediate inspection — are more reliable than field welds made under less controlled conditions. The general engineering preference is to complete as much welding as possible in the shop and use field bolting for the erection connections. This is the basis of the most common connection specification in steel construction: shop-welded / field-bolted (SWFB), where beam end plates or shear tabs are shop-welded to the beam, and the field connection is made by bolting the end plate or shear tab to the column flange with high-strength bolts.
Fatigue and cyclic loading environments favour welded or slip-critical bolted connections over standard bearing-type bolted connections. Under repeated loading, bearing-type bolt holes can elongate as the bolt shifts within the hole — a process called hole elongation — which gradually loosens the connection and shifts load to adjacent bolts. For structures subject to machinery vibration, crane runway loading, or cyclic wind (as in some of the entertainment structures in our project portfolio, such as the Dammam entertainment tower and Buwaid amusement park supports), slip-critical bolts or welded connections are the appropriate choice.
Connection Force Extraction: The Critical First Step
A connection cannot be designed correctly without accurate demand forces. The most common source of connection design errors is using simplified approximate forces — tributary area estimates, standard reaction tables — rather than extracting actual member end forces from the analysis model. For simple gravity-only framing, approximate forces are often adequate. For moment connections, seismic connections, or connections with significant axial force (as in braced frames and trusses), only the forces from the structural analysis provide a reliable basis for connection design.
At Sixteens, connection force extraction is part of the structural design deliverable, not an afterthought. Member end forces for all governing load combinations are tabulated for each connection type and used directly in the connection calculations. The connection design report cites the governing load combination and force values, and cross-references the analysis model output, so any reviewer can trace the force path from the analysis to the connection check.
Our standard specification for most steel structures: shop-welded / field-bolted. Welds are made under controlled shop conditions for maximum quality; field connections use high-strength bolts for safe, efficient erection. This approach captures the structural advantages of both methods while minimising the practical risks of each.
When to Mix Bolted and Welded in the Same Structure
Mixed connection strategies within a single structure are common and correct engineering practice — not a compromise. A typical steel frame might use: CJP welded moment connections at beam-column joints in the moment frame (where full moment transfer is required and seismic ductility governs), bolted shear-tab connections at gravity framing beams (where only shear transfer is required and field bolting is more efficient), welded base plates to columns (shop operation, full moment or axial transfer to foundation), and bolted anchor rods for field installation of columns to concrete (allowing column levelling and alignment adjustment before grouting).
The key principle is that each connection type is applied where it is structurally appropriate and practically efficient, and each is explicitly designed and documented. Mixing connection types is not inherently a problem; the problem is mixing them without a clear engineering rationale for each choice.
Frequently Asked Questions
When are slip-critical bolts required?
Slip-critical bolts are required in three main situations under AISC 360: connections subject to significant load reversal (where slip in one direction is followed by loading in the other, potentially causing fatigue of the connected plies), connections in seismic lateral force-resisting systems (SDC C through F, per AISC 341), and connections where slip under service load would be structurally unacceptable — typically moment connections designed to be rigid, or connections in precision-fit assemblies where dimensional tolerance is critical. Standard gravity shear connections in non-seismic construction generally do not require slip-critical bolts.
Can you mix bolted and welded connections in the same structure?
Yes, and it is standard practice. The most common approach is shop-welded / field-bolted: end plates, shear tabs, or clip angles are shop-welded to beams or columns under controlled fabrication conditions, and field connections are made by bolting the pre-welded elements together at the erection site. This approach provides the quality of shop welding for the critical weld, and the erection efficiency of bolting for the field assembly. AISC 360 Chapter J does not prohibit sharing load between bolts and welds in the same connection, but does require that if welds and bolts are designed to share load at a joint, the welds must be CJP groove welds (not fillet welds) or the connection must be specifically designed for the differential slip behaviour between the two fastener types.
What is a demand-critical weld?
A demand-critical weld is defined in AISC 341 as a weld whose failure could result in partial or complete collapse of the structure. The canonical examples are the CJP groove welds at beam-to-column moment connections in Special Moment Frames, and column splices in structures where the splice is within the expected plastic hinge zone. Demand-critical welds must use filler metals meeting enhanced CVN toughness requirements (20 ft-lb at −20°F, 40 ft-lb at 70°F), must be performed to AWS D1.8 (Structural Welding Code — Seismic Supplement) procedures, and must receive ultrasonic testing (UT) inspection per AISC 341 inspection requirements.
How does connection type affect the overall structural model?
Connection type affects the structural model through the rotational stiffness assigned to beam-column joints. Pinned connections (shear tabs, single-plate connections) are modelled with a moment release — no moment is transferred. Fully restrained (FR) moment connections are modelled as rigid joints — full moment continuity. Partially restrained (PR) connections, where some moment is transferred but the connection has measurable rotational flexibility, require explicit modelling of the connection's moment-rotation curve, which is typically determined from published curves for specific connection types. Most practical building designs use either fully pinned or fully rigid joints; PR connections are more common in research and in lightweight structures where connection flexibility has a significant effect on frame behaviour.