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What is a Steel I-Beam?

Mary McMahon
Updated May 17, 2024
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A steel I-beam is a type of joist or girder made from structural steel. I-beams are used as major support trusses in building, to ensure that a structure will be physically sound. Steel is one of the most common materials used to make I-beams, since it can withstand very heavy loads, although other materials, such as aluminum, are sometimes used. Composite I-beams are also available, with layers of other materials encasing the outside of the steel to disguise it as something else, such as wood.

The shape of a steel I-beam strongly resembles a capital “I” in cross section, which explains the name. It has a strong central core capped with flanges on either side. Various lengths of beam are available to suit construction project needs, and each beam also carries a rating, indicating how large it is and how much weight it is able to bear. When engineers are designing a structure, they determine what the load limits of the I-beams used in the structure should be.

There are advantages and disadvantages to using steel I-beams in construction. They are much less likely to bend or warp than wood, allowing builders to use them to create large open spaces that would not be possible with ordinary wooden beams. A steel beam also does not need to be as large as a wooden one bearing the same load, so the support beams in a structure do not need to be so obtrusive. Steel can fail catastrophically if exposed to heat, however, making it necessary to insulate the beam for safety.

Several fabrication techniques are used to create steel I-beams. Many are rolled or extruded, processed on metalworking machinery that creates standardized beams very rapidly. Others, sometimes called plate girders, are made by riveting or welding together sections of steel plate. However the I-beam is manufactured, it will be extremely heavy, requiring strong workers and specialized equipment to be handled.

When a house is renovated, steel I-beams may be used to replace old structural supports. When removing structural supports, it is crucial to know how much weight they are supporting and how they have been installed. For this reason, it is highly recommended that homeowners use professionally certified individuals for construction work that may involve removing such supports to ensure that they do not compromise the structural integrity of the home.

About Mechanics is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a About Mechanics researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Discussion Comments
By anon323096 — On Mar 03, 2013

What are the advantages of a steel I-beam?

By anon318478 — On Feb 07, 2013

How far apart would my columns be if my span were 49'7" to support an I-beam that size?

By anon308964 — On Dec 13, 2012

I am just a student in the seventh grade learning in science about how make structures strong and stable. We were starting with the topic of beams so I came here to do a little research of the different beam's advantages.

By markcgilmer — On May 20, 2009

to support a 20 ft wide opening for a gable roof with 2x6 trusses 5 in 12 slope with comp shingles

roof. What size I beam on 3 runs(each side and center) to support this much weight?

By jbernier — On May 16, 2008


We are looking to cut a beam from our 1789 post and beam house. It was in the corner of our house but we put a small 7x11 addition on and it's no longer in the corner, but clearly an important beam.

Our stove is going to be right in front of the beam. We want to keep the beam for aesthetic reasons but we know we need to support it. I have been told that an I beam could work where we notch the beam out. I just don't completely understand the logistics of putting the I beam in, and I'm also wondering if there's any possibility that we won't need to reinforce it at all.

Of course, I'd rather be safe than sorry but I just don't know how to reinforce it correctly.

Thanks, in advance!

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

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