Structural Steel Vs. Formed/Welded Steel Plates: A Comparison Of Strength And Design
The following is a design comparison of structural steel sections versus formed and welded steel plate sections including a comparison of strength in bending. It is important to note that these custom sections are not absolute; They are simply examples for replacing structural steel members.
Structural steel is being replaced by formed and fabricated steel plates more than ever.
Examples of this can be seen on the frames of modern vehicles. Instead of using structural rectangle tubing, two plate channels are lapped and welded.
Structural steel is often times more expensive and wasteful than plate because it is cut to a particular length. If you need a 6' beam, but your vendor only provides an 8' beam, you end up wasting 2'. Steel plates allow you to create your own beams with the exact length, strength, and shape you need. There are a pros and cons to both types of sections, however. The following analysis assumes that all beams are 12' in length and are composed of A36 steel.
I-beam:
The I-beam on the left is a W 12 X 30 and the beam on the right is a welded 3/8" plate beam. The weld size is assumed to be 1/4" welded down both sides of the web.
Comparison:
Section Modulus, Structural, I-beam = 38.6 in^3
Weight, Structural, I-beam = 357 lb
Section Modulus, Welded, I-beam = 36.92 in^3
Weight, Welded, I-beam = 380 lb
The structural beam is 4.40% stronger along its bending axis and 6.24% lighter. However, it is important to recall the benefits of using plate mentioned earlier. Also, structural I-beams only come in certain sizes, and some large beams such as bridge girders have to be created using plate.
Channel:
The channel on the left is a C 6 X 13 and the channel on the right is a formed 1/2" plate beam. The radius of the bend is 1/2".
Comparison:
Section Modulus, Structural, Channel = 5.79 in^3
Weight, Structural, Channel = 155 lb
Section Modulus, Formed, Channel = 6.81 in^3
Weight, Formed, Channel = 176 lb
The structural channel is 16.2% weaker along its bending axis but is 12.7% lighter. Structural channel has a longer flat edge for welding other members to.
Angle:
The angle on the left is a L 6 X 4 X 3/8 and the angle on the right is a formed 3/8" plate beam. The radius of the bend is 3/8".
Comparison:
Section Modulus, Structural, Angle = 4.34 in^3
Weight, Structural, Angle = 146 lb
Section Modulus, Formed, Angle = 6.0 in^3
Weight, Formed, Angle = 143 lb
The structural angle is 32.1% weaker along its bending axis and 2% heavier. The winner here is the formed piece.
Tubing
The square tube on the left is a 6 X 6 X 3/8 and the tube on the right is a welded 3/8" plate tube. The weld is 3/8".
Comparison:
Section Modulus, Structural, Tube = 13.9 in^3
Weight, Structural, Tube = 328 lb
Section Modulus, Formed, Tube = 13.93 in^3
Weight, Formed, Tube = 329 lb
The structural tube is equal in strength to the welded tube and equal in weight. There are other ways to create square tubing from plate, but this is my personal favorite because a full strength fillet weld can be used.
While the up-front cost for plate is cheaper, the weld and shop time required to make formed and welded pieces sometimes equals or exceeds the cost for structural members. Plate is still beneficial because it reduces the number of materials required for a weldment. It also reduces the amount of structural steel kept in stock. However, sometimes it’s just plain easier to use structural steel, especially in the case of small I-beams, which tend to warp when welded if made out of plate.
There are distinct pros and cons to both structural steel and plate so it’s up to the engineer to decide which is the better option when designing a weldment.weldment.