Joist vs. Beam vs. Girder | All You Need to Know

Joists, beams, and girders, all these play a crucial role in supporting and distributing loads in buildings, bridges, and other structures.  However, while they might sound similar, they serve different purposes based on their size, placement, and the loads they carry. 

While joists handle lighter loads and are used more frequently across the structure, beams carry heavier loads and span longer distances. Girders, being the largest and strongest, provide the essential support for beams, ensuring that all the loads in a building are safely transferred to the ground.

In this article, we’ll break down the differences between joists, beams, and girders, exploring their roles, applications, and key characteristics.

What Is a Joist?

Joists are horizontal structural elements designed to support floors and ceilings. Typically spaced closely together, they carry loads from walls or floors and transfer them to beams or other supporting structures. 

Joists are relatively small compared to beams and girders and are generally made from wood, steel, or engineered materials like LVL (Laminated Veneer Lumber).

Placement and Function: Joists are used mainly in floors and ceilings, running between walls or other supports. They ensure that the load of the surface they support is evenly distributed.

Spacing: They are placed at regular intervals, often 12″, 16″, or 24″ on center, depending on the material and span requirements.

Materials: Joists can be made from wood (common in residential construction), steel, or engineered materials like I-joists, which combine wood and metal for greater strength.

Typical Use: Joists are most commonly found in residential structures, supporting floors or roofs.

What Are the Three Types of Joists?

There are several types of joists used in construction, each designed to meet specific structural requirements based on material, load, and span. Here are the main types of joists:

1. Wooden Joists

Wooden joists are the most common in residential construction, especially for framing floors and ceilings. They are usually made from softwood lumber such as pine or fir.

Solid Lumber Joists: These are single, solid pieces of wood cut to specific dimensions (like 2×10 or 2×12). They are easy to work with and relatively inexpensive but can be prone to warping, twisting, or splitting over time.

Engineered Wood Joists (I-Joists): I-joists are made from laminated wood and designed to resemble the shape of a steel I-beam. They are more dimensionally stable than solid lumber and can span longer distances with less material. These are often used in newer homes for floor and roof systems.

2. Steel Joists

Steel joists are commonly used in commercial and industrial construction where longer spans and greater load capacities are needed. They are lighter and stronger than wood joists, making them ideal for large structures like warehouses or office buildings.

Open-Web Steel Joists: These joists consist of a top and bottom chord connected by a web of diagonal bars, forming an open truss-like structure. The open design allows for mechanical systems like ducts or wiring to pass through easily. They come in two types:

Cold-Formed Steel Joists: Made from thin sheets of steel formed into structural shapes, these joists are often used in light-gauge steel construction. They are more common in residential or small-scale commercial buildings where lightweight steel framing is used.

3. Concrete Joists

Concrete joists are used primarily in larger buildings and high-rise construction where strength and fire resistance are crucial. They are cast on-site or prefabricated and used in combination with concrete floors and ceilings.

Precast Concrete Joists: These are manufactured off-site and then transported to the construction location. They are often combined with hollow-core slabs for floors and ceilings, offering excellent fire resistance and load-bearing capacity.

Cast-in-Place Concrete Joists: These are formed and poured directly on the construction site, allowing for flexibility in design. The joists are integrated into the overall concrete slab, forming a monolithic structure that provides exceptional strength.

There are also composite joists that combine two or more materials, usually steel and concrete, to take advantage of the strengths of each.

What Is a Beam?

Beams are horizontal load-bearing members that carry significant weight and transfer it to girders or columns. They are larger than joists and play a crucial role in supporting entire floors, roofs, or walls.

Load Transfer: Beams receive loads from joists or other structural members and distribute the weight to the girders or columns below.

Size and Material: Beams are larger and stronger than joists, often made from materials like steel, concrete, or wood. Steel beams, like I-beams or H-beams, are prevalent in commercial buildings.

Span Capability: Beams span longer distances compared to joists, often placed at wider intervals and capable of handling greater loads.

Application: Beams are used in a wide range of construction types, from residential homes to large commercial buildings, bridges, and industrial structures.

What Is a Girder?

Girders are the largest and strongest of the three, serving as the primary support in a structural framework. They bear the weight transferred from beams and distribute that load to columns or foundation walls.

Structural Hierarchy: Girders are the backbone of a structure, carrying the loads from multiple beams or trusses. They are typically located at critical points of a structure where substantial weight needs to be supported.

Material and Size: Girders are usually made from steel or reinforced concrete due to the massive loads they must support. They are significantly larger than both beams and joists.

Load Distribution: In large buildings or bridges, girders serve as the primary support, carrying loads from beams and transferring them to columns or piers.

Common Use: Girders are found in large structures like skyscrapers, bridges, and large industrial buildings where significant strength and stability are required.

What Is the Difference Between Girders and Beams and Joists?

Here’s an overview of all the key differences between girders, beams, and joists. 

Aspect	Joist	Beam	Girder
Purpose	Supports floors and ceilings, transfers loads to beams	Transfers loads from joists to girders	Primary load-bearing member, transfers loads to columns or foundations
Size	Small to medium	Larger than joists, smaller than girders	Largest structural member
Material	Wood, steel, engineered wood	Steel, wood, concrete	Steel, reinforced concrete
Spacing	Closely spaced (12″, 16″, or 24″ on center)	Wider spacing compared to joists	Often spaced at critical support points
Application	Residential floors, ceilings	Floors, roofs, walls, bridges	Major structural frameworks like bridges and skyscrapers
Load Capacity	Supports smaller loads, transfers to beams	Supports heavier loads, transfers to girders	Supports the heaviest loads, transfers to columns or foundations
Span	Short to medium spans	Medium to long spans	Longest spans in a structure

Are a Girder and a Beam the Same? 

Beams and girders are both critical load-bearing elements but differ mainly in size and capacity. Beams handle the intermediate loads, receiving weight from joists or smaller structural members and distributing it to the girders. They span medium to long distances and are essential for transferring floor and roof loads.

Girders, however, are the largest and most robust structural elements. They receive loads from beams and carry them to the foundations or columns. 

While both beams and girders are often made of steel or reinforced concrete, girders need to be much stronger and larger due to the substantial weight they support. Girders essentially form the backbone of the entire structural system.

Joist vs Beam

Joists and beams are similar in function but differ in their structural hierarchy. Joists are smaller, lighter, and more numerous, providing localized support for floors and ceilings. They are typically spaced closely together, often at intervals like 12″, 16″, or 24″ on center, depending on the load and span.

Beams, on the other hand, are larger and span longer distances. They carry heavier loads and transfer these to girders or columns. Beams are spaced more widely apart than joists, as they handle the combined loads from multiple joists. Materials also differ: joists are usually made from wood or engineered wood, while beams are often made from steel, wood, or concrete for greater strength.

Joist vs Girder

The joist-girder relationship shows a clear difference in scale and role. Joists are smaller secondary elements that carry localized loads, such as those from floors or ceilings. They transfer these loads to beams, which in turn pass them on to girders.

Girders are much larger and form the main support for the entire structure. They bear the heaviest loads from beams and distribute them to columns or foundations. Joists may be made of wood or steel, but girders are typically constructed from steel or reinforced concrete to handle the immense weight of large buildings or bridges.

Frequently Asked Questions 

What is another name for beam?

A beam might be called a girder, joist, rafter, or purlin, depending on its size and position within the structure.

Is a truss a girder?

Girders are specialized trusses designed to handle additional loads from the structural elements they support. If a single-layer girder isn’t strong enough, designers can create a multi-layer girder to handle the increased load.

Conclusion   

In summary, joists, beams, and girders are all critical components of a structure, each playing a distinct role in transferring and distributing loads. Joists provide localized support for floors and ceilings, beams span larger areas to transfer weight to girders, and girders act as the primary structural support, distributing the weight to the foundation. Together, they work in harmony.