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Why Does A Rolled Steel Joist Have An I-Shaped Cross Section?

One of the greatest symbols of construction in the modern era is the rolled steel joist, also known simply as the i-beam or steel beam.


Patented by Alphonse Halbou in 1849, the RSJ was initially rolled in wrought iron, although as soon as the Bessemer process was invented that made steel cheap, it would become a legendary part of construction and the foundational innovation that allowed the creation of buildings that reached the heavens.


The reason why it was shaped like an “I”, however, is based on wanting the beam to withstand the biggest bending load whilst using the least material.


The principle is that more stress of a given material load comes the further it is away from the neutral axis of a beam, and so as long as you have enough material to avoid the beam shearing entirely, it is better to place material at the top and bottom of the beam.


In practice, this makes RSJs ideal for use in structural steel buildings, such as metal bridges and as the foundations of skyscrapers, wherever a load needs to be supported.


This was used to incredible effect in the world’s first steel-framed skyscraper, the Rand McNally Building in Chicago Illinois, which was completed in 1889 and would eventually be demolished in 1911. Many other larger skyscrapers would not exist without the RSJ.


The principle behind the RSJ has even seen use in wood and timber construction, with the I-joist being found to be both lighter and less likely to warp than a solid wooden beam would.


For more information and advice from steel specialists in Sussex, get in touch today.