Bolted and Welded Connections

Introduction and Context

Connections are the force-transfer mechanisms that make a steel structure behave as an integrated system. Members may be adequate individually, yet the structure can fail if a connection cannot transfer tension, compression, shear, moment, or their combinations. Connection design therefore requires more than selecting bolts or weld sizes: the designer must trace the complete load path from the supported member through the connection components and into the supporting member.

This chapter presents the principles used to design and detail bearing-type and slip-critical bolted joints, welded joints, splices, gusset plates, base plates, and simple, moment, and bracing connections. The terminology and resistance equations are consistent in concept with commonly used structural steel standards such as AISC 360, Eurocode 3, and comparable national codes. Exact resistance factors, bolt tables, weld coefficients, detailing limits, and seismic requirements must always be taken from the governing design standard.

Learning Objectives

By the end of this chapter, the reader should be able to:

1. Explain how forces are transferred through bolted and welded connections.
2. Distinguish bearing-type from slip-critical bolted joints.
3. Check bolts, connected material, welds, gusset plates, and base plates for relevant limit states.
4. Design simple shear, moment, bracing, and splice connections using a realistic force path.
5. Detail connections for constructability, inspection, durability, and ductile behavior.
6. Identify common design errors and select appropriate connection technology.

1. Connection Design as Force-Path Design

A connection should be idealized only after its intended structural behavior has been identified. A simple beam connection is generally detailed to transfer shear while permitting rotation. A moment connection transfers bending moment and shear and must develop a couple through tension and compression forces. A brace connection transfers axial force through a gusset plate, bolts or welds, and the supporting member.

The general design sequence is:

1. Establish factored or service design actions: axial force N, shear V, moment M, and torsion where applicable.
2. Define the intended connection behavior.
3. Draw the load path.
4. Select bolts, welds, plates, angles, stiffeners, or other components.
5. Check every plausible limit state.
6. Check deformation, slip, fatigue, erection, corrosion protection, and inspection requirements.
7. Produce clear fabrication and erection details.

A connection is only as strong as its weakest component. For example, increasing bolt diameter does not solve a failure caused by net-section rupture of a thin connected plate. Similarly, a large weld may be ineffective if the supporting flange yields or the connection introduces unacceptable restraint.

Principal Limit States

Important limit states include:

• bolt shear and bolt tension;
• bearing and tear-out at bolt holes;
• slip in slip-critical joints;
• tensile yielding and net-section fracture;
• block shear of connected plates;
• weld-metal or base-metal failure;
• local yielding, crippling, buckling, or punching of supporting elements;
• prying action in tension connections;
• bolt group eccentricity;
• plate bending and yielding;
• fatigue under repeated or fluctuating loading.

The design resistance is normally compared with the required strength. In a limit-states format:

where R_d is the design resistance and E_d is the design effect of actions. In a resistance-factor format, the equivalent check is commonly written as ϕR_n ≥ U, where R_n is nominal resistance, ϕ is a resistance factor, and U is the required factored strength. The applicable code determines the factors and combinations.

2. Bolted Connections

2.1 Bolt Types and Joint Behavior

Structural bolts are manufactured in strength grades or property classes. The nominal tensile strength of the bolt is commonly denoted by F_u,b, and the tensile stress area by A_b, although the precise notation varies by standard.

A bearing-type joint is designed so that applied force is transferred after contact develops between the bolt shank and the hole surface. Some slip may occur under service loading before the joint reaches its bearing condition. A slip-critical joint is pretensioned and designed so that friction between faying surfaces prevents slip at a specified service or factored load level.

The choice depends on function, not preference. Slip resistance is important where movement would damage finishes, impair machinery alignment, reverse stress, enlarge holes, or create unacceptable vibration. Bearing-type joints are often economical and satisfactory for ordinary building framing when limited service slip is acceptable.

2.2 Bolt Shear and Tension

For a group of bolts sharing direct shear equally, the average shear demand per bolt is:

where V is the total shear, n is the number of bolts, and V_b is the assumed shear demand per bolt. This equal-sharing assumption is appropriate only when the load is concentric and the bolts have similar stiffness and geometry.

The nominal shear resistance of one bolt may be expressed in the general form:

where k_s is a code-specified coefficient depending on the number of shear planes and whether the shear plane passes through the threaded portion, F_u,b is bolt ultimate strength, and A_b is the relevant bolt area. The applicable standard must be used because coefficients and resistance factors differ.

For a bolt in direct tension, a general nominal resistance expression is:

where A_t is the tensile stress area. In a combined shear-and-tension check, the governing interaction equation must be taken from the design standard; shear and tension should not be checked independently when both are significant.

2.3 Bearing, Tear-Out, and Hole Effects

Bearing resistance is controlled by bolt diameter, plate thickness, material strength, edge distance, pitch, and the direction of force. A bolt close to a free edge may cause tear-out before the bolt itself reaches its shear resistance. End distance is particularly important when the force acts toward the edge.

Designers must distinguish between nominal hole diameter and bolt diameter. Standard, oversized, short-slotted, and long-slotted holes have different detailing and slip requirements. Oversized and slotted holes can simplify erection or permit adjustment, but they generally reduce bearing or slip resistance and require code-approved washers and detailing.

The net area of a tension element is reduced by holes. A typical net-area calculation is:

where A_n is net area, A_g is gross area, d_h is hole diameter, t is plate thickness, s is stagger along the force direction, and g is transverse gauge spacing. The stagger term applies when holes follow a zigzag path. The exact effective-net-area provisions depend on the governing code and connection geometry.

2.4 Slip-Critical Joints

The nominal slip resistance of a pretensioned bolt is conceptually related to the clamping force and the friction coefficient of the faying surfaces:

where μ is the faying-surface slip coefficient, h_s is a hole or surface factor specified by the code, T_p is specified bolt pretension, and n_s is the number of slip planes. A resistance factor or safety factor is applied according to the design method.

Slip-critical design requires controlled surface preparation, correct pretensioning, suitable washers, inspection, and protection against contamination or paint systems that are not approved for the required slip coefficient. Pretension alone does not guarantee slip resistance if the faying surfaces are oily, damaged, or incorrectly coated.

2.5 Eccentric Bolt Groups

When a shear force acts at an eccentricity from the bolt-group centroid, the connection is subjected to direct shear and a moment:

where e is the eccentricity. The direct shear is distributed among the bolts, while the moment produces additional forces related to each bolt’s distance from the instantaneous center of rotation. For preliminary elastic analysis, the moment-induced force in bolt i is proportional to its radial distance r_i:

where the summation includes all bolts and assumes elastic bolt-group behavior about the selected centroidal axis. For large rotations or highly ductile bearing-type joints, instantaneous-center methods may provide a more realistic ultimate-strength model. The selected method must be consistent with the code and connection behavior.

3. Welded Connections

Welding joins steel by fusion, pressure, or both. Structural building work commonly uses fillet welds, complete-joint-penetration groove welds, partial-joint-penetration groove welds, plug welds, and slot welds.

3.1 Fillet Welds

The effective throat of an equal-leg fillet weld is:

where w is the nominal leg size and t_e is effective throat, assuming a standard 45° fillet profile. The effective weld area is:

where L_e is effective weld length. A general nominal weld resistance is:

where F_w is the code-specified nominal strength of the weld metal or the applicable directional strength expression. The design standard determines whether weld-metal and base-metal resistances are checked separately and how resistance factors are applied.

The effective length must satisfy minimum limits and should exclude ineffective end regions where required by code. A weld that is too short can have poor force distribution and may not develop its nominal strength. Maximum fillet weld size may also be limited relative to the thickness of the connected material to avoid overheating or damaging a thin edge.

3.2 Groove Welds and Weld Groups

Groove welds are selected where force must be transferred through the full thickness or where access and geometry make fillet welds inefficient. Complete-joint-penetration welds can develop high force, but they require qualified procedures, appropriate joint preparation, inspection, and control of distortion.

A weld group subjected to a concentric force can often be proportioned by distributing force over its effective area. An eccentric weld group must resist direct force plus moment. The designer should consider both weld-metal stresses and stresses in the connected plate, including local bending and block shear.

3.3 Welding Practice and Quality

Weld design is inseparable from welding procedure. Important variables include weld process, electrode classification, preheat, interpass temperature, hydrogen control, weather protection, access, welding position, and inspection category. Weld symbols on drawings must communicate size, length, location, contour, and whether a weld is intermittent or continuous.

Welding can introduce residual stress and distortion. Balanced sequencing, temporary restraint, appropriate joint design, and heat control reduce these effects. Critical or dynamically loaded welds may require nondestructive examination such as visual, magnetic-particle, ultrasonic, or radiographic testing, as specified by the contract and code.

4. Splices and Gusset Plates

4.1 Member Splices

A splice joins two segments of a structural member. It must transfer the actions present at the splice, not merely the member’s axial force. A beam splice may require shear, moment, axial force, or combinations. A column splice may require compression, tension under uplift or frame action, shear, and moment caused by imperfect alignment or frame behavior.

For a flexural splice, the moment may be represented by a tension-compression couple. If the distance between the centroids of the tension and compression resultants is z, the approximate flange-force couple is:

where M is the design moment. This relation is a force-path model, not a substitute for checking the web, flange plates, bolts, welds, prying, local yielding, and stability.

4.2 Gusset Plates

Gusset plates connect braces, trusses, and other members. They may be loaded in tension, compression, shear, or combined actions. Checks commonly include gross-section yielding, net-section fracture, block shear, bolt bearing, bolt shear, weld resistance, Whitmore-section yielding, and out-of-plane buckling.

The Whitmore section is an idealized effective section spreading from the brace connection toward the gusset edges at a code-defined angle or geometry. It is useful for estimating yielding across the spread section, but it does not eliminate checks for actual plate edges, bolt holes, weld termination, or buckling.

For compression braces, gusset geometry should provide adequate clear length and restraint. In seismic systems, gusset plates may be deliberately detailed to permit stable inelastic rotation, subject to the applicable seismic provisions. A gusset that is strong but excessively rigid may force brittle failure into the brace or weld.

5. Base Plates and Column Bases

A column base transfers axial force, shear, and often moment into concrete through a base plate, anchor rods, grout, and the foundation. The load path is three-dimensional and includes steel yielding, concrete bearing, anchor tension, shear transfer, plate bending, and foundation reinforcement.

For concentric compression, the average concrete bearing pressure is approximately:

where P is compression force and A₁ is the effective bearing area. The pressure distribution must remain within code limits, and the concrete bearing resistance depends on concrete strength and the relationship between loaded and supporting areas.

For moment and axial compression, the pressure distribution may be nonuniform. If the resultant lies within the effective compression region, a compression block can transfer the moment. If uplift occurs, anchor rods and the base plate must develop the tension couple. Base-plate thickness is commonly governed by cantilever bending of plate projections beyond the column flanges or web, but the actual geometry should be analyzed rather than assigned a universal thickness.

Shear may be transferred by friction, anchor rods, shear lugs, or bearing against a prepared surface. Friction should not be assumed without considering the applied compression, surface condition, and code requirements. Anchor rods require checks for steel tension and shear, concrete breakout, pullout, pryout, edge distance, embedment, and interaction.

6. Common Connection Types

6.1 Simple Shear Connections

A simple connection is intended to transfer shear with negligible moment resistance in the frame analysis. Typical forms include single-plate shear tabs, double-angle connections, seated connections, and fin plates. The connection must accommodate beam rotation without unintended restraint. Detailing should provide erection clearance, access for bolts or welding, and adequate ductility.

A single-plate connection should be checked for bolt shear, bolt bearing, plate yielding, plate net-section fracture, block shear, weld strength, and local failure of the beam web and supporting column or girder. The supporting member may require stiffening where concentrated forces are high.

6.2 Moment Connections

A moment connection transfers a couple and shear. Common forms include bolted end plates, flange plates with a bolted web shear connection, welded flange connections, and proprietary systems. The design must establish how tension and compression forces flow between beam and column.

For an end-plate connection, tension-side bolts may experience prying action. The end plate bends between bolt rows and the beam flange, increasing bolt tension above the simple couple estimate. The column flange, column web, continuity plates, panel zone, welds, and beam sections must also be checked. A connection that develops beam plastic strength may require column-side strengthening and a capacity-design approach.

6.3 Bracing Connections

Brace connections are often governed by high axial force and eccentricity. The gusset should be proportioned so that the brace force reaches the supporting member through a short, clear load path. Bolts or welds should be arranged to minimize unintended eccentricity, and the connection should be checked for both tension and compression directions where load reversals are possible.

In seismic construction, brace connections must satisfy special requirements for overstrength, ductility, net-section rupture, block shear, gusset yielding, and brace buckling. These requirements cannot be replaced by ordinary gravity-load connection rules.

7. Worked Example: Bearing-Type Shear Connection

This illustrative example demonstrates the design logic; it is not a substitute for a complete code design.

A beam web transfers a factored shear of 180 kN to a column through a double-sided bolted connection. Eight identical high-strength bolts are arranged in two lines, with four bolts in each line. Assume the force is concentric and that the selected bolt and plate arrangement has been checked for required edge distances and spacing.

The average shear demand per bolt is:

Suppose the applicable standard gives a design shear resistance of 58 kN per bolt for the selected bolt, thread position, and shear-plane arrangement. The bolt shear check is therefore:

Bolt shear is adequate. However, this result alone does not approve the connection. The designer must also check bearing at every hole, tear-out at the end bolts, net-section fracture of the web or angles, block shear, welds if a plate is welded to the column, local yielding of the column, bolt installation access, and beam rotation. If the connection has eccentricity, equal sharing is no longer sufficient and the bolt-group moment must be analyzed.

8. Applied Case: Braced Industrial Frame

Background

Consider an illustrative single-storey industrial frame with roof bracing intended to resist wind reversal. The brace is a double-angle member connected to a column through a gusset plate. The factored brace force is 420 kN in tension and 300 kN in compression.

Problem

An initial design used a short gusset plate with four bolts in one line. The calculated bolt shear resistance exceeded 420 kN, but the net section of the brace angles and the gusset block-shear path were inadequate. The bolt line was also eccentric to the brace centroid, producing secondary bending.

Analysis and Approach

The revised design began with the complete force path: brace angles → bolts → gusset plate → column flange and web → foundation. The connection was redesigned with a longer gusset, two bolt lines, increased end distance, and a welded attachment sized for the resultant force. The brace angle outstanding legs were checked for net-section fracture and block shear. The gusset was checked for gross yielding, net fracture, block shear, Whitmore-section yielding, and compression buckling. Because wind reverses, both tension and compression cases were considered.

The weld group was arranged as symmetrically as practicable about the brace force line. A construction gap and a clear gusset geometry were provided so that the brace could rotate without premature contact during compression buckling. The column web and flange were checked for local forces, and inspection requirements were placed on the drawings.

Outcome

The revised arrangement had a more direct load path, reduced eccentricity, improved erection tolerance, and adequate resistance in all governing limit states. It also avoided the common error of judging adequacy from bolt shear alone.

Learning Points

1. Connection strength is governed by the complete system, not a single fastener.
2. Reversible loading requires tension and compression checks.
3. Eccentricity can create important secondary forces.
4. Gusset stability and deformation are as important as nominal yielding resistance.
5. Clear drawings and inspection provisions are part of structural safety.

9. Detailing, Constructability, and Professional Practice

Good detailing prevents theoretical strength from being lost in fabrication or erection. Drawings should identify bolt diameter, grade, hole type, pretension requirement, washer arrangement, weld size and length, weld electrode or process where required, surface preparation, inspection, coatings, and tolerances.

Bolted joints require access for tightening tools and inspection. Bolt heads and nuts should not be placed where steelwork blocks installation. Welded joints require access, suitable joint preparation, and a sequence that limits distortion. Drainage and seal welding should be considered where water could collect and cause corrosion.

The designer must coordinate connection details with fire protection, architectural finishes, mechanical services, temporary works, lifting points, and erection sequence. A connection that is impossible to assemble safely is not a satisfactory design.

Durability and Inspection

Corrosion protection may involve paint, galvanizing, weathering steel, or a combined system. Faying surfaces in slip-critical joints require compatible coating systems. Crevices, trapped water, and damaged coatings should be avoided.

Inspection should be proportionate to risk. Bolt pretension may be verified through direct-tension indicators, calibrated torque procedures, turn-of-nut methods, or other approved procedures. Weld inspection should follow the project specification and the applicable welding standard. Records should identify nonconformances and their disposition.

Digital Tools and AI

Connection design software can calculate bolt groups, weld groups, plate stresses, and code checks efficiently. However, software output is only as reliable as the geometry, load combinations, material properties, code settings, and boundary assumptions entered by the engineer. AI tools may assist with drawing review, clash detection, document classification, and preliminary detailing, but they should not independently approve a connection. Professional responsibility, verification, traceability, and compliance with the governing code remain with the qualified design team.

10. Challenges and Limitations

Simplified connection models are useful but have limits. Equal bolt sharing ignores stiffness variation and eccentricity. Elastic weld-group methods may not represent large plastic rotations. Nominal bearing resistance does not predict service slip. A base-plate pressure calculation may overlook grout flexibility, anchor interaction, or foundation behavior. A gusset yield-line model may not capture local buckling or three-dimensional restraint.

Uncertainty also arises from material variability, fabrication tolerances, hole misalignment, weld defects, incomplete pretension, corrosion, residual stress, and changing load paths during construction. Designs should therefore include appropriate robustness, ductility, inspection, and conservative judgment, particularly for fatigue, seismic loading, temporary conditions, and unusual geometries.

Best-Practice Design Checklist

• Identify all force components and load reversals.
• Draw the complete force path before sizing components.
• Select bearing-type or slip-critical behavior intentionally.
• Check bolts, holes, plates, welds, and supporting members.
• Check gross yielding, net fracture, block shear, bearing, tear-out, and buckling where relevant.
• Include eccentricity, prying action, and local member effects.
• Design for erection access, tolerances, inspection, and corrosion protection.
• Use the governing national standard and current material specifications.
• Independently review critical connection calculations and details.
• Ensure drawings communicate the design assumptions to fabricators and erectors.

Chapter Summary

Connections transfer structural actions between steel members and foundations. Reliable design begins with the intended behavior and a complete force path. Bearing-type bolted joints transfer force through bolt shear and bearing after contact, whereas slip-critical joints rely on controlled pretension and friction to prevent service slip. Bolted connections require checks for shear, tension, interaction, bearing, tear-out, net-section fracture, block shear, and eccentric bolt-group effects.

Welded connections require attention to effective throat, weld length, weld-metal resistance, base-metal failure, access, procedure qualification, distortion, and inspection. Splices transfer the actual member actions, including moment and shear, rather than only an assumed axial force. Gusset plates require strength and stability checks, while base plates transfer forces through steel, grout, concrete, and anchor systems.

Simple, moment, and bracing connections differ in their intended force-transfer mechanisms. A sound design combines analytical resistance with constructability, durability, inspection, code compliance, and professional judgment. Software and AI can support engineering work, but they do not replace verification or accountability.

Key Terms

Bearing-type joint: A bolted joint in which force is transferred primarily through bolt bearing against the hole.

Slip-critical joint: A pretensioned joint designed to prevent slip by friction between faying surfaces.

Faying surface: The contacting surface between connected parts of a bolted joint.

Pretension: Intentional tensile force installed in a structural bolt.

Bearing: Local contact stress between a bolt and the material surrounding its hole.

Tear-out: Shear failure of material between a bolt hole and a nearby free edge.

Net section: The remaining cross-sectional area after deducting bolt holes.

Block shear: A combined tension-and-shear failure path around a group of bolts or welds.

Effective throat: The design thickness of a fillet weld resisting force.

Weld group: A set of weld segments acting together to transfer force.

Splice: A connection joining two segments of a structural member.

Gusset plate: A plate connecting braces, truss members, or other axial-force members.

Whitmore section: An idealized spread section used to assess gusset yielding near a brace connection.

Base plate: A steel plate distributing column forces to concrete or masonry.

Prying action: Additional tension induced in bolts by bending of a connected plate.

Bolt-group eccentricity: The offset between the applied force line and the bolt-group centroid.

Friction coefficient: A coefficient describing resistance to slip between faying surfaces.

Panel zone: The web region of a beam-column joint resisting shear associated with moment transfer.

Capacity design: A method that selects a ductile yielding mechanism while protecting other components from premature failure.

Review Questions

Short Answer Questions

1. What is the fundamental difference between a bearing-type and a slip-critical bolted joint?
2. Why must a connection be designed using a complete force path?
3. Define effective throat for an equal-leg fillet weld.
4. What is the purpose of a gusset plate in a braced frame?
5. Name four limit states that may govern a bolted connection.

Long Answer Questions

1. Explain the design procedure for an eccentric bolted shear connection, including direct shear and moment effects.
2. Discuss the principal design and detailing considerations for a welded moment connection.
3. Explain how a base plate transfers axial force, moment, and shear to a concrete foundation.
4. Compare simple, moment, and bracing connections in terms of intended behavior and force transfer.
5. Discuss how fabrication, erection, inspection, and corrosion protection influence connection safety.

Analytical / Application Questions

1. A concentric shear force of 240 kN is carried by 8 identical bolts. If the design shear resistance is 42 kN per bolt, determine the average bolt demand and assess bolt shear adequacy. State the additional checks required before approval.
2. A beam splice transfers a factored moment of 360 kN·m. If the approximate distance between the tension and compression resultants is 0.75 m, calculate the couple force and explain why this is not the only required design check.
3. A bracing gusset is subjected to tension in one load case and compression in the reverse load case. Develop a design checklist identifying the bolt, weld, plate, brace, stability, and supporting-member checks required.

Multiple Choice Questions with Answers

Question 1

Which statement best describes a slip-critical bolted joint?

A. It transfers all force through weld metal.

B. It is designed to prevent slip through friction from bolt pretension.

C. It permits unlimited movement before bearing develops.

D. It requires bolts to remain completely loose.

Correct Answer: B. It is designed to prevent slip through friction from bolt pretension.

Question 2

For an equal-leg fillet weld, the effective throat is commonly expressed as:

A. 0.500w

B. 0.577w

C. 0.707w

D. 1.414w

Correct Answer: C. 0.707w

Question 3

A 180 kN concentric shear is carried equally by 8 bolts. What is the average shear demand per bolt?

A. 12.5 kN

B. 22.5 kN

C. 45 kN

D. 1440 kN

Correct Answer: B. 22.5 kN

Question 4

Which limit state involves a combined tension-and-shear path around a bolt group?

A. Block shear

B. Concrete creep

C. Weld undercut only

D. Elastic buckling of the entire building

Correct Answer: A. Block shear

Question 5

What is the principal purpose of a gusset plate in a bracing connection?

A. To provide architectural cladding

B. To connect the brace force to the supporting structure

C. To eliminate all compression forces

D. To replace foundation reinforcement

Correct Answer: B. To connect the brace force to the supporting structure

Question 6

If a shear force V acts with eccentricity e from a bolt-group centroid, the associated moment is:

A. M = V/e

B. M = e/V

C. M = V + e

D. M = Ve

Correct Answer: D. M = Ve

Question 7

Which action commonly increases bolt tension in an end-plate connection?

A. Prying action

B. Concrete shrinkage only

C. Uniform temperature with no restraint

D. Removal of all edge distance

Correct Answer: A. Prying action

Question 8

Which check is especially relevant to a base plate under combined compression and moment?

A. Pressure distribution and anchor tension where uplift occurs

B. Only bolt thread pitch

C. Only weld electrode color

D. Only beam lateral bracing

Correct Answer: A. Pressure distribution and anchor tension where uplift occurs

Question 9

For a beam splice, the approximate force in a tension-compression couple is calculated as:

A. F = Mz

B. F = M/z

C. F = z/M

D. F = M + z

Correct Answer: B. F = M/z

Question 10

Why is bolt shear resistance alone insufficient to approve a bolted connection?

A. Bolts never transfer shear.

B. Other limit states such as bearing, net fracture, block shear, and supporting-member failure may govern.

C. Shear resistance is unrelated to bolt strength.

D. Only welds can be checked.

Correct Answer: B. Other limit states such as bearing, net fracture, block shear, and supporting-member failure may govern.