1. Connection Method Does Not Define Connection Behavior
The presence of bolts or welds does not by itself determine whether a connection behaves as simple, semi-rigid, or fully restrained.
Connection behavior depends on factors including:
- plate and angle geometry;
- bolt layout and hole configuration;
- weld type, size, length, and orientation;
- stiffness of the connected members;
- prying action and local deformation;
- panel-zone and supporting-member behavior;
- fabrication tolerances;
- loading direction and repetition;
- the force and deformation assumptions used in the structural model.
A welded connection is therefore not automatically rigid, and a bolted connection is not automatically pinned. Both methods can be used in shear, axial, moment-resisting, bracing, splice, and base-connection details when properly designed.
Under an AISC-based design, ANSI/AISC 360 provides the generally applicable structural-steel requirements. Bolting, welding, seismic, fatigue, and other specialized requirements may introduce additional rules.
2. How Bolted Connections Transfer Forces
Bolted connections transfer forces through fasteners and the connected material. Depending on the detail, resistance may involve:
- bolt shear;
- bolt tension;
- combined shear and tension;
- bearing at bolt holes;
- tear-out;
- net-section rupture;
- block shear;
- prying action;
- slip resistance between faying surfaces.
These mechanisms are not interchangeable, and several may need to be checked within one connection.
2.1 Snug-Tightened Joints
In a snug-tightened joint, the connected plies are brought into firm contact in accordance with the applicable installation requirements. The joint is not assigned the same minimum pretension requirements as a pretensioned or slip-critical joint.
Where permitted by the governing specification, snug-tightened joints can provide an efficient solution for connections whose required behavior does not depend on specified bolt pretension or slip resistance.
2.2 Pretensioned Joints
Pretensioned joints use bolts installed to achieve at least the specified minimum pretension.
Pretension may be required because of the connection type, loading, performance objective, or governing standard. However, a pretensioned joint is not automatically a slip-critical joint.
2.3 Slip-Critical Joints
Slip-critical joints use pretensioned high-strength bolts together with faying surfaces that satisfy the specified surface condition so that the joint develops the required slip resistance.
They may be required where slip would adversely affect structural performance or where the governing standard specifically requires slip-critical behavior. Their design and execution depend on factors including:
- bolt pretension;
- number of slip planes;
- faying-surface condition;
- coating qualification where applicable;
- hole type;
- applied tension that reduces clamping force;
- the specified slip limit state.
Slip-critical joints should not be described simply as stronger bolted connections. Their defining purpose is to control slip under the specified design condition.
The 2025 RCSC Specification for Structural Joints Using High-Strength Bolts covers high-strength bolted-joint design, components, installation, and inspection within its scope.
Terminology note: Informal references to a “bearing-type connection” usually describe a joint whose design relies on bolt shear and bearing-related resistance rather than specified slip resistance. In the current RCSC framework, the formal joint categories are snug-tightened, pretensioned, and slip-critical.
3. How Welded Connections Transfer Forces
Welded connections transfer forces through deposited weld metal and the connected base material. Their behavior depends on the weld configuration, loading direction, material, geometry, fabrication quality, and surrounding components.
3.1 Fillet Welds
Fillet welds transfer forces through their effective throat and are widely used for:
- shear tabs and connection plates;
- stiffeners;
- gusset plates;
- built-up members;
- base-plate attachments;
- secondary steelwork;
- other lap, tee, and corner-joint arrangements.
The required weld size and length depend on the applied forces, load direction, weld group, connected material, and governing standard.
3.2 Groove Welds
Groove welds are used where forces are transferred through prepared or naturally formed grooves between connected components.
A partial joint penetration (PJP) groove weld does not extend through the complete thickness of the joint. A complete joint penetration (CJP) groove weld provides complete penetration and fusion through the thickness of the joint as defined by the applicable welding standard.
Engineering note: A CJP designation describes weld penetration. It does not establish that the complete connection develops the full strength of the connected member. Base-metal strength, connection geometry, access, stress distribution, adjacent components, and applicable limit states still require verification.
For projects adopting an AWS framework, AWS D1.1/D1.1M establishes requirements for structural-steel welding within its scope, including procedure and welder qualification, fabrication, inspection, and acceptance.
The edition named in the governing code, contract, and project specifications controls. A newly published edition does not automatically replace the edition adopted for an existing project.
5. Project Conditions That Should Drive Selection
5.1 Required Structural Behavior
The engineer should first determine:
- which forces the connection must transfer;
- whether rotation or slip must be controlled;
- whether fatigue, vibration, impact, or load reversal matters;
- whether the connection forms part of a seismic force-resisting system;
- whether member strength or a particular deformation capacity must be developed.
Only after these requirements are understood should the fastening method be selected.
5.2 Shop and Site Conditions
A common strategy is to complete repeatable welding in the fabrication shop and use bolts for site assembly. Shop conditions can provide better access, positioning, equipment, environmental control, and production consistency.
Field bolting can support rapid erection, but only when the fabricated geometry fits. Bolt-hole alignment, member sweep, connection-plate position, tolerances, and erection sequence remain important.
For deeper treatment of these interfaces, see steel fabrication tolerances in buildings.
Field welding may be appropriate when the detail and site controls support it. However, it can require:
- suitable access and working platforms;
- wind or weather protection;
- controlled joint preparation and fit-up;
- preheat and interpass-temperature control where specified;
- qualified welding procedures and personnel;
- planned inspection access;
- coordination with erection stability.
The decision should be based on actual site conditions rather than a general rule that field welding is unacceptable.
5.3 Transportation and Erection Sequence
Connection design affects shipping size, lifting points, temporary stability, assembly order, and crane access.
Large shop-welded assemblies may reduce field connections but increase transportation and lifting constraints. Smaller bolted assemblies may be easier to ship but require more site work and more connection components.
This is one reason connection design should be coordinated with the building’s prefabrication and assembly strategy.
5.4 Inspection and Quality Planning
Bolted and welded connections both require planned quality control. Neither should be accepted solely because it looks complete.
Bolted-joint inspection may include verification of:
- bolt and nut identification;
- compatible assemblies and washers;
- storage and lubrication condition;
- hole type and geometry;
- connected-ply condition;
- pre-installation verification where required;
- installation method;
- pretension where specified;
- faying-surface requirements for slip-critical joints.
Weld inspection may include verification of:
- approved welding procedure specifications;
- welder or welding-operator qualifications;
- joint preparation and fit-up;
- filler metal;
- preheat and interpass temperature;
- weld size, length, profile, and workmanship;
- visual inspection;
- specified nondestructive examination;
- repair procedures and records.
Not every weld requires the same nondestructive examination, and not every bolted joint requires pretension verification. The governing documents must identify what applies.
5.5 Corrosion Protection
Connection selection must be coordinated with the coating or galvanizing system.
Relevant questions include:
- Will faying surfaces receive paint or another coating?
- Does a slip-critical joint require a qualified coating condition?
- Can the connection be accessed for coating application and inspection?
- Will welding burn or damage an existing coating?
- How will welded areas be cleaned and repaired?
- Can water and contaminants accumulate in crevices?
- Will future maintenance have access to bolts, welds, and adjacent surfaces?
A connection that is structurally adequate but difficult to protect may create avoidable maintenance problems.
5.6 Future Modification and Disassembly
Bolted connections may make future removal or component replacement more practical, but this benefit is conditional.
Before dismantling or reassembling a bolted connection, the project team must determine:
- whether the structure can be unloaded and stabilized safely;
- whether the bolts are permitted to be reused;
- whether holes or connected material have been damaged or elongated;
- whether replacement bolts and assemblies meet the project requirements;
- whether the revised arrangement needs engineering approval.
A demountable appearance should not be treated as proof that a structural connection can be altered without analysis.
Welded connections usually require cutting, removal, preparation, and an engineered repair or replacement detail. Heat input and access can also affect the feasibility of modification.
5.7 Seismic and Fatigue Requirements
Neither bolting nor welding is inherently the preferred solution for all seismic or fatigue applications.
Performance depends on the complete system, including:
- connection configuration;
- member proportions;
- bolt and weld details;
- material properties;
- stress range;
- cyclic demand;
- expected deformation;
- fabrication quality;
- qualification and inspection requirements.
A connection used in a seismic force-resisting system may require additional design, qualification, protected-zone, fabrication, and inspection provisions under AISC 341 or the governing regional standard.
These requirements are examined separately in Seismic Connection Detailing in Steel Structures.
6. Hybrid Connections: Shop Welding and Field Bolting
Many steel-building connections combine bolts and welds.
Examples include:
- an end plate shop-welded to a beam and field-bolted to a column;
- a base plate shop-welded to a column and connected to the foundation with anchor rods;
- a gusset plate shop-welded to one member and field-bolted to another;
- a welded built-up member with bolted site splices.
This approach can combine controlled shop welding with efficient field assembly. It can also reduce the amount of site welding and simplify transportation into manageable assemblies.
However, bolts and welds should not be assumed to share the same force arbitrarily. The engineer must define the load path and determine which component transfers each force.
For example, welded flanges may transfer flexural forces while a bolted web connection transfers shear. This is different from assuming that a bolt group and weld group automatically share the same force in proportion to their nominal capacities.
The governing design framework must be applied consistently. Provisions from AISC, Eurocode, GB, or another system should not be combined within one connection without a coordinated and technically justified design basis.
