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Analysis of the cylinder system of grab unloader
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AS 4100:2020 Fatigue in SDC Verifier – How Section 11 Is Implemented
Read Full ArticleHow SODIMEC verified a 198 t ladle lifting beam with SDC for Ansys
Read Full StoryAnalysis of the cylinder system of grab unloader
All ProjectsAS 4100:2020 Fatigue in SDC Verifier – How Section 11 Is Implemented
Read Full Article
Yevheniia Sidakova
Last updated 13 May, 2026
Bolts play a critical role in load-bearing connections for countless mechanical and structural assemblies, from simple machines to towering buildings. But behind every reliable bolted connection lies careful consideration of the forces they’ll face. Whether a bolt is holding together components in an engine or supporting beams in a bridge, understanding the mechanics behind bolt strength calculations is key.
This article explores the fundamentals of bolt strength, specifically focusing on tensile strength, shear strength, and proof load — concepts that guide engineers in designing connections that perform reliably, even under demanding conditions.
Read our previous posts about bolts:
Bolt strength refers to the capacity of a bolt to withstand forces without breaking or deforming. There are different types of loading conditions that bolts may experience, including tension (pulling force) and shear (sliding force). To ensure compliance with industry standards regarding bolt strength, bolt checks are often performed using Eurocode 3 (EN1993) for European steel structures and AISC 360-10 in the USA.
These standards provide specific criteria for assessing bolt strength, making it crucial to follow them when designing bolted connections that can safely endure operational loads. Understanding the strength parameters in these conditions is crucial for designing bolted connections that can endure operational loads safely.
Bolt strength refers to the capacity of a bolt to withstand forces without breaking or deforming. There are different types of loading conditions that bolts may experience, including tension (pulling force) and shear (sliding force). To ensure compliance with industry standards regarding bolt strength, bolt checks are often performed using Eurocode 3 (EN1993) for European steel structures and AISC 360-10 in the USA.
These standards provide specific criteria for assessing bolt strength, making it crucial to follow them when designing bolted connections that can safely endure operational loads. Understanding the strength parameters in these conditions is crucial for designing bolted connections that can endure operational loads safely.
Tensile strength refers to the maximum amount of tensile (pulling) force a bolt can handle before it fractures. It is one of the most fundamental aspects of bolt strength and is directly related to the material properties and diameter of the bolt and should correspond to standards like AISC 360-10. Tensile strength is usually expressed in megapascals (MPa) or pounds per square inch (PSI).
To calculate the tensile strength of a bolt, the following equation is used:
$$
P = S_t \times A_s
$$
Where:
Note: The tensile stress area accounts for the effect of threading and is different from the nominal cross-sectional area. It can be calculated using formulas from standards like the Machinery’s Handbook.
Shear strength refers to the maximum stress that a fastener can withstand before fracture when applied at a right angle to its axis.
This type of force is common in bolted joints subject to lateral (sideways) loads. A bolt typically has lower shear strength compared to its tensile strength, and this parameter is crucial for applications where bolts are subjected to shear loads, such as in structural frames or machinery joints.
Shear strength can be calculated using the following equation:
$$
\tau = \frac{F_s}{A \times n}
$$
Where:
Practical Insight: In a double-shear configuration, bolts often experience increased shear strength. When designing bolted joints, ensure the load path and bolt arrangement are evaluated to determine whether a single or double shear setup is applicable.
Image: bolt tensile strength vs. bolt shear strength
The proof load is the maximum force that a bolt can withstand without experiencing permanent deformation. It serves as a safety threshold, ensuring that the bolt will not stretch or deform under expected loading conditions. The proof load is often specified as a percentage of the bolt’s yield strength, typically ranging from 85% to 95%.
The equation for proof load is:
$$
F_p = A_t \times S_p
$$
Where:
Important: Exceeding the proof load can lead to bolt elongation, which is especially critical to avoid in fatigue-sensitive applications, where the bolt is subjected to cyclic loading.
Bolt proof strength represents the stress level at which a bolt begins to experience irreversible deformation. In other words, it defines the boundary between elastic and plastic behavior under load.
What is proof strength of bolts?
It is the maximum stress a bolt can withstand while still returning to its original shape after unloading. Once this limit is exceeded, permanent deformation occurs.
In design practice, proof strength is used to ensure that operating loads remain safely below the plastic deformation threshold. This makes it a key parameter in high tensile bolt strength applications where preload control is critical.
Yield strength is the stress at which a bolt begins to deform plastically. Beyond this point, the bolt will not return to its original shape even after the load is removed.
What is yield strength of a bolt?
It defines the transition from elastic to plastic behavior and is one of the most important design limits in structural engineering.
Bolt yield strength formula is typically expressed as:
$$
F_y = S_y \times A_s
$$
Where:
Fy = yield load
Sy = material yield strength
As = tensile stress area
High tensile bolt strength grades (such as 10.9 or 12.9) are specifically designed to provide higher yield limits for demanding applications.
Bolt thread strength refers to the ability of the threaded portion of a bolt to resist stripping or shearing under load. This is often a critical failure mode, especially in softer mating materials or insufficient thread engagement.
Bolt thread strength calculation considers the shear area of engaged threads and the material strength of both bolt and nut. In many cases, thread stripping governs failure before the bolt reaches its tensile limit.
How to calculate bolt thread strength:
Engineers evaluate the shear area of engaged threads and apply allowable shear stress values:
$$
F_{\text{thread}} = A_{s,\text{thread}} \times \tau_{\text{allowable}}
$$
Where:
Fthread = thread stripping capacity
As_thread = effective shear area of threads
τ_allowable = allowable shear stress
Moreover, identifying bolt grades is essential for ensuring the correct application based on strength and hardness. Here’s a brief overview:
| Grade | Tensile Strength | Yield Strength | Proof Load |
| Grade 1 | 60,000 psi (415 MPa) | 36,000 psi (250 MPa) | 33,000 psi (230 MPa) |
| Grade 2 | 74,000 psi (510 MPa) | 57,000 psi (395 MPa) | 55,000 psi (380 MPa) |
| Grade 5 | 120,000 psi (830 MPa) | 92,000 psi (635 MPa) | 85,000 psi (590 MPa) |
| Grade 8 | 150,000 psi (1,035 MPa) | 130,000 psi (895 MPa) | 120,000 psi (825 MPa) |
| Grade 8.2 | 150,000 psi (1,035 MPa) | 120,000 psi (825 MPa) | 120,000 psi (825 MPa) |
| Grade 9 | 180,000 psi (1,240 MPa) | 140,000 psi (965 MPa) | 140,000 psi (965 MPa) |
| Class 8.8 | 830 MPa (120,000 psi) | 640 MPa (93,000 psi) | 600 MPa (87,000 psi) |
| Class 10.9 | 1,040 MPa (151,000 psi) | 940 MPa (136,000 psi) | 830 MPa (120,000 psi) |
| Class 12.9 | 1,220 MPa (176,000 psi) | 1,220 MPa (176,000 psi) | 970 MPa (140,000 psi) |
Each grade reflects the bolt’s ability to handle stress under different conditions. Selecting the appropriate grade ensures safety and reliability in engineering applications.
When performing bolt strength calculations, you should factor in not only the individual strength parameters but also the specific conditions in which the bolt will be used. Here are a few key considerations, mentioned in the previous article:
In engineering applications, accurately calculating bolt strength is critical for ensuring the safety and reliability of bolted connections. By understanding key parameters like tensile strength, shear strength, and proof load, engineers can make informed decisions when selecting and designing bolts for specific applications. Each of these parameters plays a distinct role in how a bolt behaves under different loading conditions, and together they ensure that bolts perform optimally within their intended design limits.
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