What Are the Different Types of Bolts Used in Steel Structures? Everything You Need to Know

When it comes to steel structures, bolts do more than just hold things together. They define the very strength, safety, and lifespan of the entire framework. From skyscrapers and bridges to pre-engineered buildings and fabricated steel structures, bolts are essential components that ensure secure connections, structural integrity, and load-bearing performance.

In this article, you’ll discover everything you need to know about the types of bolts used in steel construction. We’ll break down what bolts are, how they’re used, the most common types, and how to choose the right one based on your specific needs.

This comprehensive guide also covers best practices, component parts, tool usage, and real-world examples to help engineers, project managers, and procurement professionals make smarter decisions. Read on to strengthen your knowledge, bolt by bolt.

What Are Bolts and How Are They Used in Steel Structures?

Bolts are mechanical fasteners used to securely connect two or more components, typically steel members, in structural construction projects. They consist of a threaded shaft and a head, paired with nuts and washers to ensure tight fastening.

In steel structures, bolts play a vital role in joining beams, columns, and trusses to form load-bearing frames. They help transfer loads between members and resist forces such as tension, shear, and torsion. Because they allow disassembly, bolts offer more flexibility than welding, especially in modular or pre-engineered steel building structures. Their use spans across industries, from bridges and commercial buildings to machinery and fabricated steel assemblies, offering both strength and adaptability.

According to the American Institute of Steel Construction (AISC), bolted connections in structural steel can be either bearing-type or slip-critical, depending on the application. Bearing-type bolts depend on bolt shear and member bearing, while slip-critical bolts rely on friction between connected surfaces, which makes bolt type selection critical.

How Do Bolts Compare to Other Fasteners in Steel Construction?

Bolts differ significantly from other fasteners like screws, rivets, or welds in terms of strength, installation ease, and long-term reliability. In steel structures, bolts are usually preferred due to their superior tensile strength, reusability, and capacity to accommodate dynamic loads.

Welding, while strong, is permanent and requires skilled labor and strict environmental control. Screws are suitable for lighter loads and less critical components. Rivets, though once popular, are largely outdated due to limited adjustability.

On the other hand, bolts are versatile. For instance, in high-vibration environments like bridges or industrial plants, heavy-duty bolts with matching nuts and washers ensure tight, secure fastening. Structural bolts like ASTM A325 or A490 types are specifically designed for large load requirements, often paired with pre-drilled holes for precise alignment and torque-based installation.

Their repeatable performance in real-world scenarios makes them the fastener of choice for modern steel structures, especially in fabricated and pre-engineered steel building structures where flexibility and load management are key.

Also Read: What Are Pre-Fabricated Steel Structures and Why Are They Used?

What Are the Main Types of Bolts Used in Steel Structures?

Bolts used in steel construction vary in design, head style, material composition, and intended application. Choosing the right type ensures safety, cost-efficiency, and long-term performance in a structure.

Bolt Type	Head Style	Key Features	Typical Applications	Material Options
Hex Bolts	Hexagonal Head	High tensile strength, easy torque control with wrench or socket	Steel beams, bridges, heavy machinery, structural frames	Carbon steel, stainless steel, galvanized
Anchor Bolts	J, L, or Straight	Embed in concrete, used for steel-to-foundation connection	Columns, base plates, pre-engineered steel buildings	Galvanized steel, stainless steel
Carriage Bolts	Rounded Head + Neck	Anti-rotation neck, tamper-resistant, aesthetic appearance	Wood-to-steel joints, furniture, non-critical structural parts	Zinc-plated steel, stainless steel
Lag Bolts (Screws)	Hex Head	Coarse thread, no nut needed, ideal for wood-to-metal load-bearing connections	Structural bracing, timber frames, steel-wood junctions	Stainless steel, hot-dip galvanized
Flange Bolts	Hex Head + Flange	Integrated washer, distributes load evenly, reduces loosening	Machinery, pipe flanges, dynamic assemblies	Carbon steel, alloy steel
Torsion Shear Bolts	Dome or Hex Head	Built-in torque control, tail shears off when fully tightened	Bridges, seismic structures, wind turbines	High-strength alloy steel
Eye Bolts	Eye Loop Head	For lifting and suspending loads, WLL must be respected	Hoisting, rigging, industrial steel structures	Forged steel, stainless steel
Machine Bolts	Square/Hex Head	Uniform threading, used with matching nuts for high clamping force	Machinery, heavy-duty steel frames, bridge joints	Carbon steel, zinc-plated steel
Socket Head Bolts	Cylindrical Head	Compact head, fits tight spaces, driven with Allen wrench	Machinery assembly, structural joints with clearance issues	Alloy steel, stainless steel
A325/A490 Bolts	Hex Head	ASTM-standard high-strength bolts for structural steel	Critical joints in fabricated and pre-engineered steel structures	Medium carbon steel, quenched & tempered

Below, we explore the most widely used bolts in steel frameworks, their defining characteristics, and where they’re typically applied.

What Are Hex Bolts and Where Are They Used?

Hex bolts, also known as hex head bolts, have a six-sided head and are one of the most common bolt types in steel construction. They’re typically used in machinery, structural frames, and bridge construction.

Known for their high tensile strength, hex bolts offer robust performance in load-bearing and shear-critical applications. They are compatible with wrenches and socket sets, allowing easy torque control. These bolts are available in various grades, including stainless steel, carbon steel, and galvanized coatings for corrosion resistance.

Their standardization under ASTM specifications (like A307 for general applications or A325 for structural purposes) makes them reliable in everything from steel beams to anchor plates.

What Are Anchor Bolts and How Do They Secure Steel to Concrete?

Anchor bolts are used to attach structural steel elements to concrete foundations. They transfer loads from the steel frame into the concrete base, ensuring stability and alignment.

There are several types of anchor bolts: J-bolts (with a hook shape for grip), expansion bolts (which expand inside holes for a tight fit), and chemical anchors (which use adhesives for bonding). These are essential in pre-engineered steel buildings, where precise foundation anchoring affects overall integrity.

The American Concrete Institute (ACI) recommends anchor bolts for tension-critical areas where seismic or wind loads are present. Incorrect placement or insufficient embedment can result in catastrophic failures, making installation accuracy vital.

What Are Carriage Bolts and When Are They Used in Structural Components?

Carriage bolts feature a round, domed head and a square neck just below it. This design prevents the bolt from spinning during tightening, making them ideal for wood-to-steel or non-critical steel joints.

While not suited for high-tension steel applications, they’re commonly used in lighter structures, such as steel-framed furniture or wood-attached steel railings. Their aesthetic and tamper-resistant design also makes them popular in exposed architectural steelwork.

What Are Lag Bolts and What Load Requirements Do They Serve?

Lag bolts, also called lag screws, are heavy-duty fasteners used to connect wood to steel or secure structural components under high loads. They have a hex head and a coarse thread, allowing them to bite into wood or soft metal without nuts.

They’re widely used in mounting structural brackets, timber joists, or anchoring pre-engineered steel structures to wooden frameworks. The U.S. Department of Agriculture classifies lag bolts as suitable for load-bearing wood-to-steel joints, provided proper diameter and thread depth are chosen.

Lag bolts are torque-sensitive and should be installed using a socket or impact wrench for full strength development.

What Are Flange Bolts and Why Are They Used in Machinery and Pipes?

Flange bolts have an integrated washer-like flange under the head, distributing pressure evenly during fastening. This design eliminates the need for separate washers and prevents loosening due to vibration.

Flange bolts are heavily used in mechanical assemblies, piping systems, and vibration-prone areas of steel structures. Their design simplifies the fastening process while offering a reliable, uniform clamping force, critical in dynamic environments like manufacturing plants or HVAC steel frames.

What Are Torsion Shear Bolts and Why Are They Ideal for Dynamic Loads?

Torsion shear bolts provide consistent clamping force using a built-in torque control mechanism. When the correct torque is applied, the bolt tail shears off, indicating proper tension.

These are especially useful in seismic zones or high-stress steel constructions like bridges, towers, and stadiums. Their uniform preload ensures structural elements remain tightly joined, even under cyclic or dynamic loads. These bolts often comply with ASTM A490 or Japanese JSS II standards.

What Are Eye Bolts and How Are They Used in Lifting and Securing?

Eye bolts have a looped head that allows ropes, cables, or chains to be attached. They are used in temporary or permanent lifting applications and structural suspensions.

In steel structures, eye bolts support equipment hoisting or bracing installations. They are critical in bridge construction or industrial buildings where load-bearing support points are required. The Working Load Limit (WLL) must be followed to ensure safe use.

Also Read: How to Choose the Right Steel Space Frame Supplier for Your Project

What Are the Key Components That Make Up a Bolt Assembly?

A typical bolt assembly includes several components: the bolt (with a threaded end), a matching nut, and a washer. These work together to ensure secure fastening under tensile and shear forces.

The bolt head design (hex, socket, flange) affects the tool used, while the nut must match the thread size and pitch. Washers distribute the load, prevent surface damage, and reduce loosening from vibration.

Material choice, like stainless steel for corrosion resistance or carbon steel for strength, also affects performance. Proof stress and tensile strength should always be matched to structural requirements.

How Do You Select the Right Bolt Type for Steel Structures?

Choosing the right bolt involves more than size; it includes material strength, environmental conditions, load type, and installation methods.

For example, high-strength bolts like A325 or A490 are used in critical connections, while galvanized bolts suit outdoor applications. Anchor bolts must meet embedment and alignment specs, and flange bolts are ideal where vibration is expected.

Understanding torque requirements, bolt grade, and matching nut specifications helps avoid connection failures. ASME and ISO provide standardized dimensions and strength classifications to assist in selection.

What Are the Common Tools Used for Bolt Installation in Steel Construction?

The most used tools for bolt installation include socket wrenches, torque wrenches, impact drivers, and Allen wrenches. Each ensures accurate tightening based on bolt type and load requirement.

Socket sets are essential for hex bolts and lag bolts, while torque wrenches help meet structural tension specs. For torsion shear bolts, a manual wrench or hydraulic tool might be used to achieve the correct shear point.

Always ensure tools match the bolt size and head type to prevent rounding or torque misapplication.

What Are the Best Practices for Ensuring Secure and Durable Bolt Connections?

Bolt connections should be torqued to spec using calibrated tools, checked for alignment, and periodically inspected. Using washers and lock nuts can prevent loosening in vibrating structures.

Anchoring bolts in concrete must follow embedment depth guidelines and curing times. Avoid mixing different metals to prevent galvanic corrosion.

According to research published in the Journal of Constructional Steel Research, poor bolt installation is a leading cause of joint failure in prefabricated structures, highlighting the importance of adhering to manufacturer and industry standards.

How Do Bolts Contribute to the Strength of Fabricated and Pre-Engineered Steel Structures?

Bolts are foundational to the integrity of fabricated steel structures. They allow modular assembly, absorb dynamic loads, and ensure flexibility during transport and construction.

Manufacturers like Purun Steel Structure Manufacturing rely on high-strength bolts to connect frames, roof trusses, and wall panels. In pre-engineered steel building structures for industrial projects, bolt connections are designed to handle wind, seismic, and live loads, ensuring code compliance and long-term durability.

Whether you’re sourcing from a Top Fabricated Steel Structure Supplier in the USA or opting for Affordable Pre-Engineered Steel Building Structures, bolt quality directly impacts safety and lifespan.