When designing a building, choosing the right structural components is crucial for efficiency, cost savings, and long-term durability. Steel joists and joist girders are two widely used structural elements that provide lightweight, high-strength support for floors and roofs. But how do they work together, and when should you use one over the other?

In this guide, we’ll break down what steel joists and joist girders are, their key differences, how they are used together, and how to choose the best option for your project. Whether you’re working on a warehouse, stadium, shopping mall, or industrial facility, this article will help you understand how to optimize your building’s structure for maximum strength and efficiency.

Let’s get started! 

Steel Joists: Key Element for Load Distribution in Roofs and Floors

What is a Steel Joist?

A steel joist, also known as an Open Web Steel Joist (OWSJ), is a lightweight steel truss made up of a top chord, bottom chord, and web members. Its primary function is to support roofs or floors and transfer loads to the building’s main structure, such as beams and columns.

The most distinctive feature of a steel joist is its open-web design. Compared to solid steel beams, it is lighter yet stronger, allowing it to span longer distances while reducing material usage and construction costs.

In a steel structure, joists serve as the secondary load-bearing members, working in conjunction with joist girders and steel beams to create an efficient support system. Here’s how they function:

✅ Joists span across joist girders or beams, distributing loads evenly to reduce structural stress.
✅ They provide horizontal stability, ensuring the roof or floor remains securely in place.
✅ Compared to solid steel beams, open web steel joists reduce overall material usage, making structures lighter and more cost-effective.
✅ They allow for longer spans, reducing the need for interior columns and maximizing usable space.
✅ Used in various structures, including warehouses, shopping malls, industrial facilities, and stadiums.

💡 Simple Analogy: Think of a steel joist as a bridge between primary structural members. It efficiently transfers loads to the main framework while keeping the structure lightweight yet strong.

Key Features of Steel Joists:

✅ High Strength & Stability – Despite being lightweight, steel joists can support significant loads.
✅ Lightweight Design – The open-web structure reduces material usage and decreases overall building weight.
✅ Span Capability – Able to span longer distances, reducing the need for interior columns and maximizing open space.
✅ Efficient Installation – Prefabricated for quick and easy on-site assembly.
✅ Versatile Applications – Suitable for both roof and floor structures in various building types.
✅ Cost-Effective – Lower material and labor costs due to reduced weight and faster installation.
✅ Durability – Steel is resistant to pests, fire, and weather, ensuring long-term durability.
✅ Design Flexibility – Customizable in size and shape for specific design needs.
✅ Sustainability – Steel is highly recyclable, supporting eco-friendly construction.

Steel Joist Components

Steel joists are engineered with a truss-like open web design, allowing them to be lightweight yet strong. Each component plays a crucial role in load distribution and structural integrity.

1️⃣ Top Chord

• The uppermost structural member of the joist.
• It carries compression forces and helps distribute roof or floor loads.
• Typically connects to bridging members or roofing materials.

2️⃣ Bottom Chord

• The lowermost structural element in a joist.
• Primarily resists tensile forces, helping maintain structural stability.
• Often used as an attachment point for ceiling components or utilities.

3️⃣ Web Members

• Diagonal and vertical steel members that connect the top and bottom chords.
• Responsible for shear force distribution, transferring loads efficiently.
• Can be triangular, V-shaped, or zigzagged, depending on design.

4️⃣ End Bearing

• The supporting ends of the joist that rest on joist girders, beams, or walls.
• Transfers the load from the joist to the main structure.
• Usually bolted or welded for secure attachment.

5️⃣ Bridging [Not visible in the provided image]

• Small horizontal or diagonal members connecting multiple joists.
• Prevents lateral movement and improves overall stability.
• Two types: horizontal bridging (runs parallel) and diagonal bridging (forms an X-shape).
• Typically installed mid-span or at regular intervals to maintain alignment and load distribution.

6️⃣ Bearing Seat [Not visible in the provided image]

• The area where the joist makes contact with the supporting girder or beam.
• Designed to distribute the joist’s load across a wider surface, reducing stress concentration.

Types of Steel Joists

Steel joists are classified into four main series: K-Series, LH-Series, DLH-Series, and CJ-Series, each designed for different spans, load capacities, and applications. Additionally, within the K-Series, there is a KCS Joist, specifically designed to handle concentrated loads. Below is a breakdown of their differences and typical uses:

Type	Full Name	Span Range	Common Applications	Key Features
K-Series Joists (Standard)	Standard Open Web steel joist	Short Span (8-60 ft.)	Offices, malls, schools, small warehouses	The most commonly used steel joist, lightweight and cost-effective for smaller spans.
KCS Joists (Enhanced)	K-Series Constant Shear Joist	Short Span (10-60 ft.)	Roof equipment support, mechanical rooms, localized floor loads	Designed for concentrated loads such as HVAC units and piping, stronger than standard K-Series.
LH-Series Joists (Long Span)	Long Span Open Web Steel Joist	Long Span (20-96 ft.)	Industrial buildings, large warehouses, shopping centers	Stronger than K-Series, ideal for mid-to-large span structures.
DLH-Series Joists (Deep Long Span)	Deep Long Span Open Web Steel Joist	Long Span (60-240 ft.)	Stadiums, aircraft hangars, exhibition halls	Designed for ultra-long spans, reducing interior columns and creating open spaces.
CJ-Series Joists (Composite)	Composite Open Web Stee-l Joist	Medium to Long Span (20-120 ft.)	Parking garages, office buildings, large floor structures	Works with concrete slabs to increase floor load capacity while minimizing steel usage.

Steel Roof Joist: Types and Applications

Steel joists play a crucial role in roof construction, offering lightweight, high-strength support while allowing for long spans and open interior spaces. Different roof joist designs are used depending on structural needs, drainage, and architectural style.

1️⃣ Common Types of Steel Roof Joists

Steel joists come in various shapes beyond the standard Parallel Chord design. Each type serves a specific function:

Joist Type	Description	Common Applications
Single Slope Joist	One side is higher than the other, designed for roof drainage.	Warehouses, industrial buildings, structures requiring effective water runoff.
Double Slope Joist	Similar to a traditional pitched roof, providing a symmetrical slope.	Factories, workshops, commercial buildings needing additional interior height.
Arch Joist	Curved design, reducing the need for interior columns and enhancing aesthetics.	Stadiums, sports arenas, architectural structures.
Gable Joist	A symmetrical, peaked roof joist for efficient load distribution.	Factories, warehouses, large-scale commercial buildings.
Scissor Joist	Forms an inverted V-shape, creating higher interior ceilings.	Churches, theaters, auditoriums, large event spaces.

🔹 For most buildings, standard parallel chord joists are the most cost-effective option, while special shapes are used for specific architectural or drainage needs.

2️⃣ Why Use Steel Joists for Roofs?

Compared to other roof support systems, steel joists provide several advantages:

Feature	Steel Roof Joist	Solid Steel Beam (H-Beam/I-Beam)	Wood Roof Truss
Weight	Lightweight, reduces structural load	Heavy, increases foundation load	Light, but less durable
Span Capability	Long spans (up to 240 ft.)	Limited span unless using large beams	Moderate spans, often shorter
Installation Speed	Prefabricated for quick assembly	Requires heavy equipment and on-site welding	Requires framing and careful on-site construction
Cost	Cost-effective for large projects	Expensive due to material and labor costs	Cheap but high maintenance
Durability	Resistant to fire, pests, and weather	High strength, but needs coatings for corrosion resistance	Prone to rot, fire, and termites

💡 Key Takeaway: Steel joists offer an optimal balance between strength, weight reduction, and cost efficiency compared to solid beams or wood trusses, making them ideal for industrial, commercial, and long-span structures.

Cost of Open Web Steel Joist

When planning a construction project, understanding the cost factors of open web steel joists (OWSJ) is essential. The price of steel joists varies based on design specifications, material costs, span length, and additional fabrication requirements.

1️⃣ Key Cost Factors of Open Web Steel Joists

Several factors influence the cost of steel joists, including:

✅ Material Type & Steel Prices

• The price of raw steel fluctuates, affecting the overall cost of joists.
• High-strength steel may cost more but reduces weight, leading to savings in other structural components.

✅ Span & Load Capacity

• Longer spans require heavier joists or more reinforcement, increasing costs.
• Higher load-bearing requirements may demand custom joists with thicker chords and web members.

✅ Fabrication & Customization

• Standard K-Series joists are more cost-effective due to mass production.
• Custom shapes (e.g., scissor joists, arched joists) increase fabrication time and costs.

✅ Coating & Corrosion Protection

• Galvanized or coated joists for corrosion resistance (used in humid environments) have a higher cost.
• Fire-resistant coatings or paint add to expenses.

✅ Transportation & Handling

• Long-span joists (DLH-Series, LH-Series) require larger trucks and specialized handling, increasing logistics costs.
• Prefabricated joists reduce onsite labor costs but may need careful delivery planning.

✅ Installation Costs

• Prefabricated joists speed up installation and reduce labor expenses.
• Additional support structures (joist girders, columns) impact total cost.

2️⃣ Cost Comparison: Steel Joists vs. Alternative Support Systems

Structural Component	Relative Cost	Key Cost Considerations
Open Web Steel Joist	Low to Medium 💲💲	Cost-effective for long spans, lightweight, fast installation
Hot-Rolled Steel Beams (H-Beam, I-Beam)	High 💲💲💲💲	More expensive, heavier, but strong in shorter spans
Cold-Formed Steel Joists	Low 💲	Lighter and cheaper but less load capacity
Concrete Beams	Medium to High 💲💲💲	Heavy, requires more labor, but good for high loads

3️⃣ How to Optimize Steel Joist Costs?

To reduce costs without compromising structural integrity, consider these strategies:

🔹 Use Standard Joist Sizes – Custom joists are costly; standard K-Series joists are more economical.
🔹 Optimize Span Length – Avoid excessive spans that require deep long-span (DLH) joists.
🔹 Plan for Prefabrication – Pre-engineered joists reduce onsite labor and waste.
🔹 Choose Efficient Load Distribution – Pair joists with joist girders to minimize unnecessary steel usage.
🔹 Compare Suppliers & Bulk Ordering – Working with multiple vendors and ordering in bulk may lower unit costs.

Joist Girder: Primary Load-Bearing Component

What is a Steel Joist Girder?

A joist girder is a stronger and larger steel truss girder compared to a steel joist. It is primarily used to support multiple steel joists and transfer loads to columns or other primary structural members. Joist girders have a higher load-bearing capacity and are suitable for longer spans in large-scale structures.

Type	Span Range	Common Applications
Joist Girder	20-120 ft.	Industrial buildings, large warehouses, shopping centers, stadiums

Key Features of Joist Girders

✅ High Load Capacity – Supports multiple steel joists and handles heavier loads.
✅ Structural Stability – Can be part of wind and seismic-resistant designs.
✅ Fewer Interior Columns – Ideal for large-span buildings, optimizing open space.
✅ Customizable Design – Tailored to meet specific span, load, and height requirements.
✅ Efficient Material Use – Maximizes strength with minimal material for cost-effective construction.
✅ Durability – Offers long-term durability and resistance to environmental factors when properly treated.
✅ Prefabricated Components – Often prefabricated for quicker on-site assembly.

🔹 Standard Specifications

• Joist girders typically have an end-bearing depth of 7.5 inches.
• They are commonly connected to columns using two 3/4-inch A325 bolts for secure fastening.

Steel Joist vs. Joist Girder: Key Differences

Comparison	Steel Joist (Floor/Roof Support Beam)	Joist Girder (Primary Beam)
Function	Supports roofs or floors and transfers loads to a main beam or joist girder.	Supports multiple steel joists and transfers loads to columns or main structures.
Load Distribution	Designed for uniform loads (e.g., floors, roofs).	Designed for concentrated loads (supports multiple steel joists).
Load Capacity	Suitable for light to medium loads.	Ideal for large-span, heavy-load structures.
Span Range	Short to medium spans (8-240 ft.), depending on K, LH, or DLH Series.	Long spans (30-120 ft.), commonly used in large-span buildings.
Structural Design	Open-web truss, lightweight, reduces material use.	Heavier structure with higher load-bearing capacity, typically connected to columns.
Connection Method	Welded or bolted to a joist girder or main beam.	Typically secured to columns using 3/4-inch A325 bolts.
Installation Process	Installed on top of joist girders or main beams as a secondary structure.	Installed first, then steel joists are placed on top.
Common Applications	Warehouses, malls, office buildings, schools—used for floor or roof support.	Stadiums, aircraft hangars, exhibition centers, warehouses—used to minimize interior columns.
Cost Efficiency	Lower unit cost, ideal for standard structures.	Higher cost but reduces the number of columns, optimizing space usage.
Selection Guide	✅ For spans under 60 ft. → Steel joist is a more cost-effective choice.	✅ For large spans (100+ ft.) → Joist girder helps optimize space and long-term costs.

How Are Steel Joists and Joist Girders Used Together?

In most industrial and commercial buildings, steel joists and joist girders are used together. Joist girders act as primary beams, carrying major loads, while steel joists serve as secondary beams, supporting floors or roofs. 

However, in buildings that already have a primary load-bearing structure, steel joists can be used alone, directly connecting to steel beams or concrete beams. Joist girders, on the other hand, are rarely used alone, as they typically require steel joists to distribute and support the floor or roof loads.

1.  Standard Combination: Steel Joist + Joist Girder (Most Common Use)

✅ Joist girders (Primary Beams) connect directly to steel or concrete columns and carry the main structural loads.
✅ Steel joists (Secondary Beams) span across Joist girders to support floors or roofs.
✅ Joist girders distribute loads from multiple steel joists and transfer them to the columns.
✅ Steel joists support roofing materials, metal decks, concrete slabs, or other covering structures.

📌 Typical Applications:

• Industrial facilities & warehouses → Fewer interior columns, creating large open spaces.
• Shopping malls & supermarkets → Supports large-span roofing for spacious interiors.
• Stadiums & aircraft hangars → Used for long-span roof structures.

💡 Simple Analogy:
A Joist Girder is like a main trunk, while steel joists are the branches. The main beam (trunk) supports multiple secondary beams (branches), which then carry the roof or floor load.

2. Using Steel Joists Alone (Without Joist Girders)

🔹 In some buildings, steel joists can connect directly to steel beams (H-Beams) or concrete beams without requiring Joist girders.
🔹 This is common in smaller-span structures or buildings with an existing main load-bearing framework.

📌 Examples:

• Multi-story office buildings → Steel joists attach to a reinforced concrete frame and directly support the floor slab.
• Low-rise warehouses → Uses H-shaped steel beams as the primary structure instead of joist girders.

💡 Simple Analogy:
If a building already has strong steel or concrete beams, steel joists can be used without joist girders.

3. Using Joist Girders Alone (Without Steel Joists) – Rare Cases

🔹 In some heavy-duty structures (e.g., bridges, industrial plants), Joist girders may directly support floors or roofs without using steel joists.
🔹 However, this is uncommon, as Joist Girders usually still require steel joists to distribute loads more efficiently.

📌 Key Takeaway:
While possible, joist girders are rarely used alone, as they are more effective when paired with steel joists for load distribution and structural efficiency.

Usage	Are Both Joist Girder & Steel Joist Used?	Typical Applications
Standard Combination (Most Common)	✅ Yes – Joist girders as primary beams, steel Joists as secondary beams.	Industrial buildings, warehouses, malls, stadiums, hangars.
Steel Joists Alone	⚠ Possible – Only when supported by concrete or steel beams.	Office buildings, low-rise warehouses.
Joist Girders Alone	🚫 Rare – Usually still requires steel joists for load distribution.	Heavy-duty industrial plants, bridges.

💡 The best structural system depends on the building type, span, and load requirements!

Conclusion: Why Choose Steel Joists and Joist Girders?

Steel joists and joist girders are lightweight, strong, cost-effective, and easy to install, making them essential components in modern construction. Whether for industrial facilities, commercial buildings, or stadiums, they provide efficient structural solutions that help reduce costs and improve construction efficiency.

If you are looking for high-quality steel joists and joist girders, we are here to help! We offer customized steel structure solutions to make your building safer, stronger, and more efficient.

📩 Contact us today for a quote or consultation! 🚀