How is a Girder Different From a Beam?

Posted by Ken Brakefield

Topics: Structural Steel Fabrication

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You may hear the words “girder” and “beam” used interchangeably, but important differences distinguish the two from one another. Understanding how a girder is different from a beam is a necessary skill for any builder, engineer, or construction worker.

Girders are the main horizontal supports of a structure, and support smaller beams. All girders are beams, but not all beams are girders. Here’s a look at the fundamental differences between these two elements.

Size

The main difference between a girder and a beam is the size of the component. In general, workers in the construction industry refer to large beams as girders. There is no strict width, length, or weight cut offs that decide when a beam is actually a girder. Instead, builders look mainly at how the component is used. If it is the chief horizontal support in a structure, it is a girder, not a beam. If it is one of the smaller structural supports, it is a beam. For example, the structural support of a bridge is typically a girder, while the smaller supports of a residential home are beams. Most horizontal supports in large structures are girders due to their immense sizes.

Functionality

There is no difference between how a beam and girder behave. Both have the same goal – to resist forces by bending. A girder is simply a type of support beam. It is the main horizontal support of a structure or the large beam that supports smaller beams. Like beams, girders typically have I-shaped cross sections composed of two load-bearing flanges and a web for stabilization. Girders may also take on a box or Z shape, as well as other forms. The industry typically uses girders to build bridges, as well as trusses for buildings and other structures.  

Load-Bearing Capabilities

Girders carry dynamic loads and rolling loads. This is what makes them preferable for bridge construction, where load amount is not constant. Dynamic loads are those that exert varying amounts of force upon a structure. Dynamic loads are contrast with static loads, which exert the same amount of force at all times. It takes a special kind of beam to withstand dynamic loads with consistent, unfailing strength. Girder beams have the ideal structure and capabilities for withstanding heavy dynamic loads.

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Jobs

A girder is a primary beam. Its main job is to transfer loads to the columns upon which it rests. A beam is a secondary beam. Its main job is to transfer its loads to girders, which then transfer the load to the columns. Beams bend to accommodate shear stresses, while girders are more stiff to support the small beams. When differentiating between a beam and a girder, determine the component’s main job. If it’s to transfer load to a larger beam, it’s just a beam. If it’s to transfer load directly to the columns it sits upon, it’s a girder.

Fabrication

During the design stage of a girder, fabricators must consider requirements such as the erection of the girders, stability, deck placement sequence, plate sizing, flange sizing, and welded connections. Fabricating a beam requires similar considerations but not in the same capacity. For example, girder fabrication consists of load requirements from smaller beams the girder will support. Beam fabrication does not.

Custom Fabrication for Beams and Girders

During a building project, you often need both girders and beams to complete the truss. Custom beam fabrication is an efficient and cost-effective way to ensure you get the exact horizontal support structure you need. We can help you understand the difference between a beam and a girder, and give you professional advice about which you might need for your project. We can fabricate your beams and create them custom according to your individual needs and specifications. Contact us today for a free quote on your next project.

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Topics: Structural Steel Fabrication