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Why is an I-beam shaped the way it is? How did an I-beam's eventual final design come to fulfill its primary function?
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Jack Dahlgren, California Architect
Answered Oct 22, 2012 · Upvoted by Isaac Gaetz, Licensed Structural Engineer and Ravi Babu, M.tech Structural Engineering, National Institute of Techno…
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When a beam bends the top of the beam is in compression and the bottom is in tension.

These forces are greatest at the very top and very bottom. So to make the stiffest beam with the least amount of material you would want the material to be only at the top and bottom sides. However you still need to connect them together or they would just be two separate plates and would not be stiff at all. So you put a web in the middle to connect them and make them work together. The resulting shape is the traditional "I-beam" or wide flange beam.

This shape is used when the load is parallel with the flange. As you can see, the shape is not so good with lateral forces unless you turn it sideways. When the load will come from two directions, a square tube is used.
In all these cases the idea is to remove material that is not carrying much load and concentrating the material where the load is highest.
Increasing the depth of the beam increases the bending strength by the depth cubed, so we can gain a lot of stiffness this way. However, we start to get limited by things such as buckling when the sections start getting too thin. When folded sheet metal studs are used, this is done by placing a small bend (lip) at the end to add stiffness.

You can experiment with these structural shapes with some cardboard or popsicle sticks and glue to get a physical understanding of how they work, or look in any structural engineering text book (Statics) for details on how to calculate.
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Matthew Sutton, worked at Structural Engineering
Updated Nov 28, 2016
In the world of structures there are two basic families of members--flexural and axial. Members that resist flexure are known as beams and members that resist axial are split up into categories depending on whether or not they are under compression or tension (cables, columns, rods, etc). The most commonly occurring axial loaded member in structural engineering is a compression member, called a column. Tensile rods and cables are common in bridges but rare elsewhere. So many members in structural design act as either a beam, a column, or a hybrid beam-column.
But back to beams...the I-shape is an ideal shape for beams, that is, for resisting flexure.
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9 ANSWERS

Jack Dahlgren, California Architect
Answered Oct 22, 2012 · Upvoted by Isaac Gaetz, Licensed Structural Engineer and Ravi Babu, M.tech Structural Engineering, National Institute of Techno…
Continue Reading
When a beam bends the top of the beam is in compression and the bottom is in tension.

These forces are greatest at the very top and very bottom. So to make the stiffest beam with the least amount of material you would want the material to be only at the top and bottom sides. However you still need to connect them together or they would just be two separate plates and would not be stiff at all. So you put a web in the middle to connect them and make them work together. The resulting shape is the traditional "I-beam" or wide flange beam.

This shape is used when the load is parallel with the flange. As you can see, the shape is not so good with lateral forces unless you turn it sideways. When the load will come from two directions, a square tube is used.
In all these cases the idea is to remove material that is not carrying much load and concentrating the material where the load is highest.
Increasing the depth of the beam increases the bending strength by the depth cubed, so we can gain a lot of stiffness this way. However, we start to get limited by things such as buckling when the sections start getting too thin. When folded sheet metal studs are used, this is done by placing a small bend (lip) at the end to add stiffness.

You can experiment with these structural shapes with some cardboard or popsicle sticks and glue to get a physical understanding of how they work, or look in any structural engineering text book (Statics) for details on how to calculate.
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If you have a 2 to 12 year old child, you must watch this.
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Matthew Sutton, worked at Structural Engineering
Updated Nov 28, 2016
In the world of structures there are two basic families of members--flexural and axial. Members that resist flexure are known as beams and members that resist axial are split up into categories depending on whether or not they are under compression or tension (cables, columns, rods, etc). The most commonly occurring axial loaded member in structural engineering is a compression member, called a column. Tensile rods and cables are common in bridges but rare elsewhere. So many members in structural design act as either a beam, a column, or a hybrid beam-column.
But back to beams...the I-shape is an ideal shape for beams, that is, for resisting flexure.
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