Bolting in Steel Construction

 Bolting, as a fundamental component of steel construction, holds immense significance in ensuring the stability, safety, and resilience of structures. The American Institute of Steel Construction (AISC) provides comprehensive guidelines and standards for the proper use of bolts in steel building projects. Understanding these standards is crucial for engineers, architects, and construction professionals to implement effective bolting practices in their projects.

The Importance of AISC Standards in Bolting

AISC, through its specifications and manuals, outlines precise requirements for the design, installation, and inspection of bolted connections. These standards cover various aspects, including bolt types, materials, installation procedures, and quality assurance measures, ensuring the integrity and performance of the connections in steel structures.

Types of Bolts Specified by AISC

The AISC standards recognize several types of bolts commonly used in steel construction:

1. Structural Bolts (A325 and A490): These high-strength bolts are designed for use in structural connections and have specific mechanical properties and dimensional requirements outlined in the standards.

2. Bearing-Type Bolts (A307): These bolts are suitable for non-critical and light structural connections and have lower strength compared to structural bolts.

Field bolting and Shop bolting

Shop Bolts:

Definition: Shop bolts, also known as pre-fabricated bolts, are bolts that are pre-assembled or fabricated in a controlled environment such as a factory or shop floor. These bolts arrive on-site ready for installation without requiring further fabrication or modification.

Field Bolts:

Definition: Field bolts, also known as site-installed bolts, are bolts installed on-site during the construction phase. These bolts are adjusted and installed according to specific requirements during the assembly of the steel structure.

Common bolt-tightening methods

Torque Control:

Torque Wrench: Using a torque wrench is one of the most common methods for tightening bolts. It involves applying a specific amount of rotational force (torque) to the bolt head, which is measured in units such as foot-pounds or Newton-meters. Torque values are specified based on the bolt size, type, and material to achieve the desired tension.

Turn-of-Nut Method:

This method involves tightening the bolt snugly using a standard wrench or spud wrench and then rotating it a specified additional fraction of a turn. This additional rotation is based on the bolt diameter and length and is intended to achieve the required tension without over-tightening.

Direct Tension Indicators (DTIs) or Load-Indicating Washers:

DTIs or load-indicating washers are specialized washers placed under the bolt head or nut. As the bolt is tightened, these washers deform or indicate a change in shape, providing a visual indication of the achieved tension, helping to ensure proper tightening.

Common bolt-joint types

Snug Tight Joints:

Snug tightness refers to tightening bolts enough to bring the connected parts into firm contact without preloading the bolts. It aims to eliminate gaps between the surfaces being joined.

Pre-Tensioned Joints:

Pre-tensioning involves tightening bolts to a specific tension or preload using torque or other methods, inducing an axial force in the bolt before the structure is loaded.

Slip-Critical Joints:

Slip-critical joints rely on the friction between connected surfaces to resist shear forces without relying on the grip of the bolts. They require high bolt pre-tension to prevent movement or slippage between the connected surfaces.