Table of Contents
Bearing strength is a fundamental concept in structural steel connection design that directly influences the safety, performance, and longevity of steel structures. When steel members are joined using bolted connections, the interaction between the bolt and the surrounding plate material creates localized compressive stresses known as bearing stresses. Understanding how these stresses develop, how they affect connection behavior, and how to properly design for bearing strength is essential for structural engineers working with steel construction.
This comprehensive guide explores the critical role of bearing strength in steel connection design, examining the fundamental principles, calculation methods, failure modes, design considerations, and best practices that ensure robust and reliable structural connections.
What Is Bearing Strength in Steel Connections?
Bearing strength is particularly critical for connections, joints, and load transfer points where concentrated forces are applied, determining the load-carrying capacity of bolted, riveted, or pinned connections in steel structures. Unlike tensile or compressive strength that considers the entire cross-section of a member, bearing strength focuses specifically on localized contact areas, making it a unique and essential property in connection design.
The stresses developed when two elastic bodies are forced together are termed bearing stresses, and they are localized on the surface of the material and may be very high due to the small areas in contact. In bolted connections, this occurs when the bolt shank bears against the inside surface of the bolt hole in the connected plates.
Bearing Stress Distribution
Average bearing stress fp = P/(db t), where P is the force applied to the fastener. However, this simplified calculation assumes uniform stress distribution. In reality, the intensity of the bearing stress between the rivet and the hole is not constant but varies from zero at the edges to a maximum value directly in back of the rivet. Despite this complexity, the common practice of assuming the bearing stress to be uniformly distributed over the projected area of the rivet hole provides acceptable results for design purposes.
The bearing stress state can be complicated by the presence of nearby bolt or edge, and the bolt spacing and edge distance will have an effect on the bearing strength. This interaction between geometric parameters and bearing capacity is a critical consideration in connection detailing.
Material Behavior Under Bearing Loads
At the microstructural level, bearing strength manifests through localized plastic deformation and compaction of material beneath the loading surface, where dislocations in the crystal structure begin to move and multiply, creating slip planes and eventually leading to plastic flow. This deformation behavior is progressive and depends heavily on the material’s ductility and strength characteristics.
Bearing strength typically increases with tensile strength but at a decreasing rate, and ultra-high-strength steels may not show proportional increases in bearing strength due to reduced ductility. This relationship highlights the importance of considering both strength and ductility when selecting materials for bolted connections.
Types of Bolted Connections and Bearing Behavior
Understanding the different types of bolted connections and how bearing forces develop in each configuration is essential for proper design. The connection type significantly influences the bearing stress distribution and failure mechanisms.
Bearing-Type Connections
In bearing type connections, load is initially transferred by friction until the occurrence of slip, after which the bolt shank and the side of the bolt hole come into direct contact, bearing stresses are developed at the contact surfaces, and as the applied load increases, contact stresses increase until bearing failure finally occurs. This is the most common type of connection in structural steel construction.
Two separate strength criteria must be satisfied: (1) the shear strength of the bolt itself; and (2) the compressive capacity of the elements being joined, as the bolts “bear” on the inside surface of the bolt holes. Both criteria must be checked to ensure adequate connection performance.
Slip-Critical Connections
In the less common cases where no slip is desired — for example, in structures subjected to repeated stress reversals — so-called slip-critical connections are designed on the basis of the clamping force that the bolts place on the steel elements being joined, so that friction between the surfaces clamped together resists the tendency of the bolts to slip within the bolt holes. Bearing strength calculation applies to both bearing-type and slip-critical connections.
They are mostly required in the presence of fatigue with reversal of the loading, oversized holes, slotted holes (except when the load is normal to the slot), and when slipping at the faying surface would be detrimental to the structure’s performance.
Single Shear vs. Double Shear Connections
When the bolt is in double shear, two cross-sections are effective in resisting the load, and the bolt in double shear will have the twice the shear strength of a bolt in single shear. The shear configuration also affects bearing behavior, as the load distribution and plate thickness considerations differ between single and double shear arrangements.
For bolts in single shear, the governing thickness is the thickness of the thinner element being joined, while for bolts in double shear, the relevant thickness is either that of the middle piece, or the combined thicknesses of the two outer (side) pieces, whichever is less (assuming that all elements being joined are made from the same material).
Failure Modes in Bearing Connections
Bolted connections can fail in several distinct modes, each with different characteristics and implications for design. Recognizing these failure modes and understanding the conditions that lead to each type is crucial for creating safe and efficient connections.
Bearing Failure of Plates
When an ordinary bolt is subjected to shear forces, the slip takes place and bolt comes in contact with the plates, and the plate may get crushed, if the plate material is weaker than the bolt material. This type of failure involves excessive deformation and eventual crushing of the plate material around the bolt hole.
The compressive stresses that occur between the bolt and plate are because of excessive deformations on the bolt and plate, and to compute the bearing stress between the bolt and the hole edge, the assumption of uniform distribution of stresses on this surface is defined.
Bearing Failure of Bolts
The bolt is crushed around half circumference, the plate may be strong in bearing and the heaviest stressed plate may press the bolt shank, though bearing failure of bolts generally does not occur in practice except when plates are made of high strength steel and the bolts are of very low grade steel. This failure mode is relatively rare in modern construction with properly specified materials.
Tearout and Shear-Out Failure
The bearing problem can be complicated by the presence of a nearby bolt or the proximity of an edge in the direction of the load, the bolt spacing and end-distance will influence the bearing strength, and a possible failure mode resulting from excessive bearing is shear tear-out at the end of connected member. This failure mode occurs when insufficient edge distance or end distance allows the material between the bolt hole and the plate edge to tear out.
The specimens exhibited three failure modes: net cross-sectional; tearout; and splitting failures, where end distance and edge distance were two controlled parameters that determined the failure modes. Research has shown that geometric parameters play a dominant role in determining which failure mode will govern.
Net Section Fracture
Two main failure modes were observed in the bolted connection tests, namely the bearing and net section tension failures. Net section fracture occurs when the reduced cross-sectional area at the bolt holes cannot sustain the applied tensile force, leading to fracture across the net section.
Due to a reduction in the net area (i.e., due to bolt holes) of the plate along the bolt line, the tensile strength of the plate will be lesser than the actual value at this section, and because of this, the plate might fail under tension, therefore, it becomes important to calculate the least net area among different bolt lines to find the least tensile strength of the plate and check it for safety for the applied load.
Block Shear Failure
Member can fail due to tension fracture or block shear, and bearing failure of connected/connecting part due to bearing from bolt holes. Block shear is a combined failure mode involving shear along one plane and tension along a perpendicular plane, resulting in a block of material being torn from the connection.
The characteristic block shear strength Rn is calculated based on the yield and rupture limit states along the shear surface or surfaces and the rupture limit states along the tensile surface. This failure mode requires careful consideration in connections with multiple bolt lines and limited edge distances.
Design Standards and Bearing Strength Calculations
Various international design standards provide specific provisions for calculating bearing strength in steel connections. Understanding these code requirements and their underlying principles is essential for compliant and safe design.
AISC Specifications for Bearing Strength
Design Bearing Strength (LRFD J3.10) The strength of connection in bearing is taken at the bolt holes per AISC LRFD section J3.10, and the design bearing strength at the bolt hole is φRn. The AISC specification provides detailed equations for different hole types and loading conditions.
The design bearing strength is for service load when deformation is a design consideration (the hole edge deformation is limited to a maximum of ¼”), and the bolt is also in a connection with standard, oversized, and short-slotted holes independent of the direction of loading, or a long-slotted hole with the slot parallel to the direction of the bearing force: Rn = 1.2LctFu ≤ 2.4dtFu.
The available strength for a bolt in bearing, Rn/Ω = 1.5dbtFu, where the safety factor for bearing, Ω = 2.0. These equations account for the clear distance between bolt holes or to the edge of the connected part.
Bolt Material Specifications
ASTM designations A325, F1852, and A-490, the new ASTM specification F1852 refers to the fasteners frequently referred to as tension control, TC, twist-off, or torque-and-snap fasteners, and High-Strength bolts range in diameter from ½ to 1 ½ in. Different bolt grades have different strength properties that must be considered in bearing calculations.
The nominal bolt shear stress can be taken as 68 ksi for Group A (A325) bolts and 84 ksi for Group B (A490) bolts: when divided by the safety factor for bolt shear, Ω = 2.00, the allowable stresses become 34 ksi for Group A and 42 ksi for Group B bolts.
International Design Codes
The calculator currently supports three major design codes: IS 800:2007 (Indian Standard), AISC 360-16 (American), and Eurocode 3 (EN 1993-1-8), and the calculations automatically adjust based on the selected code’s requirements and safety factors. While the fundamental principles remain consistent across codes, specific calculation methods and safety factors may vary.
Although the mechanical behaviour of stainless steel and carbon steel differs significantly, design provisions for stainless steel connections in current standards are essentially based on the rules for carbon steel, and for bolted connections, the design resistances in EN 1993-1-4 and the SCI/Euro Inox Design Manual for Structural Stainless Steel are based on those in EN 1993-1-8 and EN 1993-1-3 with only some minor modifications.
Critical Factors Affecting Bearing Strength
Multiple factors influence the bearing strength of steel connections, and understanding their individual and combined effects is essential for optimizing connection design.
Material Properties
The material properties of both the connected plates and the bolts significantly affect bearing strength. The ultimate tensile strength (Fu) of the plate material is a primary parameter in bearing strength calculations. For connections made from different types of steel, bearing capacity should be computed for each element, based on its own thickness and material properties, with the smaller capacity governing the connection design for bearing.
Engineers typically limit design bearing stresses to 50-70% of the material’s ultimate bearing strength to account for uncertainties in loading, material variability, and environmental factors, and this approach ensures sufficient margin against localized deformation. This conservative approach provides adequate safety margins while allowing efficient use of materials.
Safety factors for bearing strength vary by application: 2.0-2.5 for general structural applications, 3.0-4.0 for critical connections subject to dynamic loading, and 1.5-2.0 for temporary structures with well-defined loads.
Edge Distance and End Distance
Edge distance and end distance are among the most critical geometric parameters affecting bearing strength. Experiments and numerical analysis were carried out to investigate bearing-type bolted connections with end distances and edge distances ranging from one time the bolt hole diameter to two times the bolt hole diameter, and the results showed that end distance and edge distance were two controlled parameters that determined the failure modes.
Insufficient edge or end distance can lead to premature tearout failure before the full bearing capacity is developed. Design codes specify minimum distances to prevent this failure mode, but optimal distances often exceed these minimums to ensure ductile behavior and full bearing capacity development.
Bolt Spacing
The spacing between bolts affects the stress distribution and potential failure paths in connections. Both shear and bearing failure of the plate can be avoided by providing sufficient centre to centre distances between the bolts as mentioned in section 10 of IS 800: 2007. Adequate spacing prevents interaction between adjacent bolt holes and ensures that each bolt can develop its full bearing capacity.
Minimum spacing requirements typically range from 2.67 to 3 times the bolt diameter, with preferred spacing of 3 times the diameter providing optimal load distribution and fabrication tolerance.
Plate Thickness
Plate thickness directly affects bearing capacity, as bearing strength is proportional to the projected bearing area (bolt diameter times plate thickness). Thicker plates provide greater bearing area and generally higher bearing capacity, though other failure modes may govern in very thick plates.
A modification to the bearing coefficient provisions for thin G550 and G300 sheet steels is necessary to account for the reduced bearing resistance of the connected materials, and this reduction in bearing resistance is related more to the steel thickness than to the steel grade (G550 vs. G300). Thin sheet steel connections require special consideration due to out-of-plane deformation effects.
Bolt Hole Type and Size
These holes are classified as: i) Standard holes – 1/16 in. larger than the nominal bolt diameter ii) Oversized holes – not allowed in bearing-type connections iii) Short-slotted holes – allowed in both slip-critical and bearing-type. The hole type affects the bearing area and the potential for deformation before full bearing contact is established.
Standard holes are typically 1/16 inch larger than the nominal bolt diameter for bolts up to 1 inch diameter, providing adequate fabrication tolerance while minimizing the reduction in net section area. Oversized and slotted holes have specific applications and require special design considerations.
Design Procedure for Bearing Strength
A systematic approach to designing for bearing strength ensures that all relevant limit states are considered and that the connection performs as intended throughout its service life.
Step 1: Determine Design Forces
The first step in connection design is determining the factored design forces that the connection must resist. These forces should account for all applicable load combinations and load factors specified by the governing design code. The distribution of forces among multiple bolts must be determined based on the connection geometry and loading configuration.
Step 2: Select Bolt Size and Grade
Bolt selection involves choosing an appropriate diameter and material grade that can resist the applied shear and tension forces. The bolt diameter affects both shear capacity and bearing area, while the bolt grade determines the material strength properties. Common bolt grades include A325 and A490 for structural applications, with A490 bolts providing higher strength for demanding applications.
Step 3: Calculate Bolt Shear Capacity
The shear capacity of bolts must be calculated considering whether threads are included or excluded from the shear plane. Shear Strength of Bolts: Ω = 2.0 Bolt Diameter = 0.768 in Nominal Shear Strength, Fnv = 54 ksi Nominal Shear Strength (per bolt), Rnv = 0.6 Fnv Ab = 0.6 (54 ksi) 0.463 in^2 = 25.0 kips (Rnv / Ω) =12.5 kips. The number of shear planes (single or double shear) must also be considered.
Step 4: Calculate Bearing Capacity
Bearing Strength of Standard Bolt Holes: Ω = 2.0 (Ignoring bolt hole deformation at service load level) edge distance, le = 0.84 in (clear) distance to adjacent hole, lc = 2.19 in, and since edge distance is less than the adjacent distance to the next bolt hole, edge distance will control. The bearing capacity calculation must consider the clear distance to the edge or to adjacent holes, using the smaller value.
Bolt value is the least of Design Bearing Strength (Vdpb) and Design Shear Strength (Vdsb) of the bolt. The governing capacity is the minimum of the shear and bearing capacities.
Step 5: Check Plate Limit States
Multiple plate limit states must be verified, including:
- Shear yielding: The gross area along the shear plane must be adequate to prevent yielding
- Shear rupture: The net area (accounting for bolt holes) must resist fracture
- Tension yielding: The gross area perpendicular to the load must not yield
- Tension rupture: The net area must resist fracture in tension
- Block shear: The combined shear and tension failure path must be checked
The strength of a bolted connection is the least of bolt value (design strength of the bolt) and the design tensile strength of the plate.
Step 6: Verify Geometric Requirements
All geometric requirements specified by the design code must be verified, including minimum and maximum edge distances, end distances, and bolt spacing. These requirements ensure proper force distribution, prevent premature failure modes, and provide adequate fabrication tolerances.
The design bearing strengths are given for different edge distances (1.25 in. and 2 in.), different Fu (58 and 65 ksi), and different bolt diameters (5/8 – 1-1/2 in.) Design tables in steel manuals provide tabulated values for common configurations, streamlining the design process.
Special Considerations in Bearing Design
Certain conditions and applications require additional considerations beyond standard bearing strength calculations.
Deformation Limits
In some applications, deformation at service loads may be a design consideration even if ultimate strength is adequate. Bearing deformation can lead to excessive hole elongation, affecting the serviceability of the structure. Design codes may specify deformation limits, typically around 1/4 inch of hole elongation, which require using more conservative bearing strength equations.
The bearing resistance of bolted connections has been determined in previous studies either on the basis of a strength or a deformation criterion. The choice between strength-based and deformation-based design depends on the specific application and performance requirements.
High-Strength Steel Connections
Bearing-type bolted connections are frequently used for steel structure connections and are typically small when used to connect high-strength steels, however, studies on the failure mode and ultimate bearing capacity of high-strength–steel-bearing-type bolted connections are, to our best knowledge, limited. High-strength steels may exhibit different bearing behavior due to reduced ductility and different strain-hardening characteristics.
Bearing strength often conflicts with ductility requirements, and higher-strength steels typically offer superior bearing resistance but may exhibit reduced plastic deformation capacity before failure, potentially leading to more sudden failure modes.
Thin Sheet Steel Connections
Mild sheet steels (G300) have an increased deformation capacity in comparison to G550 sheet steels due to their elevated ductility, and an increase in the capacity for out-of-plane deformation can lead to tears which originate not near the edge of the piled material in front of the bolt, but closer to the centre of the originally placed bolt hole. Thin sheet connections may experience curling and out-of-plane deformation that affects bearing behavior.
Standards cannot be used to accurately predict the failure mode of thin sheet steel bolted connections loaded in shear, and typically, net section fracture is predicted when test results reveal that bearing distress in the sheet steel is the controlling mode of failure. Special provisions or modified bearing coefficients may be necessary for thin sheet applications.
Stainless Steel Connections
The investigation showed the deformation behaviour of stainless steel connections to be somewhat different from that of carbon steel connections, with stainless steel exhibiting pronounced strain hardening, however, the locations of fracture initiation obtained from the numerical models matched those observed during experimental studies of both carbon steel and stainless steel connections.
The nonlinear stress-strain behavior of stainless steel, particularly austenitic grades, results in significant strain hardening that can increase bearing capacity beyond initial yield. However, this also complicates the prediction of deformation and failure modes.
Common Design Mistakes and How to Avoid Them
Understanding common pitfalls in bearing strength design helps engineers avoid costly errors and ensure safe, efficient connections.
Insufficient Edge Distance
One of the most common mistakes is providing inadequate edge distance, which can lead to premature tearout failure. While minimum edge distances are specified in codes, using distances near the minimum may result in reduced bearing capacity and brittle failure modes. Providing edge distances of 1.5 to 2 times the bolt diameter typically ensures adequate bearing capacity and ductile behavior.
Neglecting Material Compatibility
Using higher grade bolts than needed can be wasteful and may cause the plates to fail before the bolts (bearing failure). The connection should be designed so that all components work together efficiently, with similar utilization ratios for different limit states. Mismatched material strengths can lead to inefficient designs or unexpected failure modes.
Excessive Number of Fasteners
The connection components should fail in a ductile, predictable manner, and adding too many fasteners can weaken plates due to reduced net section area, so there’s an optimal number based on load distribution and geometry. More bolts do not always result in a stronger connection, as the reduction in net area may govern the capacity.
Ignoring Load Distribution
Assuming equal load distribution among all bolts may not be accurate in all cases, particularly in long connections or connections with eccentric loading. Elastic analysis or more sophisticated methods may be necessary to determine actual bolt forces and ensure adequate bearing capacity at critical locations.
Advanced Topics in Bearing Strength Analysis
For complex connections or special applications, advanced analysis methods may be necessary to accurately predict bearing behavior and connection performance.
Finite Element Analysis of Bearing Connections
This paper presents an experimental and numerical study to investigate the bearing strength and failure modes of single-bolt connections, and then a FE modelling method considering tensile and shear material properties on the corresponding regions is proposed to explore the bearing behaviour of single-bolt connections. Finite element analysis can capture complex stress distributions, material nonlinearity, and contact behavior that simplified hand calculations cannot address.
The bearing strength exceeding 12% overestimation and incorrect failure mode would be obtained using the conventional FE modelling method for bolted connections under shear dominated stress due to steel shear behavior. Proper modeling techniques, including appropriate material models and failure criteria, are essential for accurate results.
Strain Hardening Effects
Material strain hardening can significantly affect bearing capacity, particularly in ductile materials that undergo substantial plastic deformation before failure. Advanced analysis methods can account for strain hardening by using nonlinear material models that represent the full stress-strain curve, including the strain-hardening region.
In the numerical simulation of the models, non-linear stress-strain material behavior of stainless steel was considered by using expressions which represent the full range of strains up to the ultimate tensile strain. This approach provides more accurate predictions of ultimate capacity and deformation behavior.
Fracture Mechanics Approaches
Fracture criterion is incorporated into FE models to investigate the relationship between the bearing strength and failure mode of the bolted connections. Fracture mechanics-based approaches can predict the initiation and propagation of cracks in bearing connections, providing insights into failure mechanisms and ultimate capacity.
Peak plastic strain in the plate material in front of the bolt reaches the localized fracture strain of the material, fracture occurs and the maximum load is said to have been achieved. Defining appropriate failure criteria based on strain, stress, or fracture mechanics principles is crucial for accurate prediction of bearing capacity.
Practical Design Examples
Working through practical examples helps solidify understanding of bearing strength calculations and design procedures.
Example: Simple Shear Connection
Shear connections between I-shaped sections are some of the most common connections in steel design, and to help understand the required design checks in accordance with AISC 360, this article will use a design example to explain them. A typical shear connection between a beam web and a column or girder involves checking bolt shear, bearing at bolt holes, plate shear yielding and rupture, and block shear.
The design process involves selecting an appropriate number and size of bolts, determining the required plate thickness, and verifying all limit states. Try four rows of bolts, 1/4-in. plate thickness, and 3/16-in. fillet weld size. Iterative design may be necessary to achieve an efficient connection that satisfies all requirements.
Example: Tension Splice Connection
Tension splice connections require careful consideration of net section capacity in addition to bearing and shear. The connection must develop the full capacity of the tension member while maintaining adequate safety margins for all limit states. Double-cover-plate splices provide symmetric load transfer and are commonly used for tension members.
Calculate the bolt value of a 20mm diameter bolt of grade 4.6 to join main plates of 12mm thick under the following cases. Assume Fe410 for all plates. The bolt value calculation considers both shear and bearing, with the minimum governing the design.
Quality Control and Inspection
Proper quality control during fabrication and erection is essential to ensure that connections perform as designed.
Hole Preparation
Bolt holes should be drilled or punched to the specified size with clean, smooth edges. Punched holes may require reaming if the material thickness exceeds certain limits or if the hole quality is inadequate. Damaged or oversized holes can reduce bearing capacity and should be repaired or the connection redesigned.
Bolt Installation
Proper bolt installation is critical for connection performance. High-strength bolts must be tightened to the specified pretension using calibrated wrenches or other approved methods. Undertightened bolts may not develop adequate friction in slip-critical connections, while overtightened bolts may be overstressed or damage the connected material.
Inspection Requirements
Visual inspection should verify that all bolts are properly installed, holes are properly aligned, and there are no signs of damage or distress. For critical connections, more detailed inspection including ultrasonic testing or other non-destructive methods may be specified. Documentation of bolt installation, including pretension verification, should be maintained for quality assurance.
Future Trends and Research Directions
Ongoing research continues to refine our understanding of bearing behavior and improve design methods for steel connections.
Advanced Materials
The development of new high-strength and high-performance steels requires updated bearing strength provisions. Material selection decisions often balance bearing strength against fabricability and cost, and while higher-strength steels offer better bearing resistance, they may present challenges in hole formation and may be more notch-sensitive. Research into the bearing behavior of these materials helps optimize their use in structural applications.
Performance-Based Design
Performance-based design approaches that consider the full range of connection behavior, from initial loading through ultimate capacity and post-peak response, are becoming more common. These methods can lead to more efficient designs that better match the actual performance requirements of the structure.
Digital Tools and Automation
Structural Central’s Steel Connection Tool is structural engineering software that allows you to quickly generate the calculations required to determine the strength of a steel connection, and the calculations evaluate each of the possible limit states for the coped beam, bolts, welds, angles, plates, girder and/or column in order to determine which failure mode will control the load-carrying capacity of the design. Advanced software tools streamline the design process and reduce the potential for calculation errors.
Machine learning and artificial intelligence are being explored for connection design optimization and failure prediction, potentially leading to more efficient and reliable connections in the future.
Conclusion
Bearing strength is a critical consideration in steel connection design that directly affects the safety, performance, and economy of steel structures. A thorough understanding of bearing behavior, failure modes, and design procedures is essential for structural engineers working with bolted connections.
Proper bearing design requires consideration of multiple factors including material properties, geometric parameters, loading conditions, and applicable design standards. By following systematic design procedures, verifying all relevant limit states, and avoiding common mistakes, engineers can create connections that safely and efficiently transfer loads between structural members.
As materials and construction methods continue to evolve, ongoing research and development of design provisions ensure that bearing strength calculations remain accurate and applicable to modern construction practices. The integration of advanced analysis methods and digital tools further enhances our ability to design optimized connections that meet the demanding requirements of contemporary structures.
For additional resources on steel connection design, engineers can refer to the American Institute of Steel Construction (AISC) for comprehensive design guides and specifications, the Steel Construction Institute for technical guidance and research publications, and BG Structural Engineering for practical design examples and tutorials. These resources provide valuable information for both learning and professional practice in steel connection design.