The Deceiving Tube Joint

November 26, 2007 at 10:58 pm 4 comments

Introduction

Structural joint is one of the most important things in steel construction. Unfortunately, it is often overlooked by engineers. Many engineers nowadays master the sophisticated engineering softwares but lack the feeling and intuition of an engineer.

The most important skill in structural design is the ability to visualize the stress flow in structures and how the structures behave under certain loads. Many engineers lose this art of engineering and rely heavily on finite element analysis, make themselves more an operator rather than a critical engineer.

The Tube Joints

Figure 1 below is a picture I took in my project 2 years ago. Take a good look. Can you tell me what is wrong in the picture? Does it ring the emergency alarm in your head?

img_0054-wrong1.jpg
Fig. 1. Tube joint.

The tube joint in picture above is part of an A-frame structure shown in Figure 2 below. Another tube joint from the same structure is shown in Figure 3 below. Have you figured out what is wrong with both joints?

img_4063-a-frame.jpg
Fig. 2. A-frame structure.

img_0058-wrong2.jpg
Fig. 3. Another tube joint from the same frame.

The Fatal Mistake

The best way to tell the mistake in the joints is by telling what can happen to them. Let us visualize how the load flow through the joint and how the structural member will respond to it. Figure 4 below shows section view of the primary tube. The secondary tube, which serves as structural stiffener, is connected to the primary tube through plate and bolts.

When the A-frame is loaded, the secondary tube may be under compression and, consequently, pierces the primary tube as illustrated in lower part of Figure 4. The rest is disaster. Primary tube will buckle locally and lose a serious amount of axial and bending strength. Then the A-frame will collapse like vegetable.

pipe-joint-buckling.jpg
Fig. 4. Schematic of the badly designed tube joint.

The Right Joint

The right way to join tubes is to virtually intercept the tubes and fully weld it. Figure 5 shows the comparison between the wrong and right tube joints. Example of well-designed tube joint is shown in Figure 6 below. The picture was taken from a shipyard in Tanjung Priok. It is part of an offshore platform under construction. When the frame is under load, the primary tube keeps it shape and thus retains its original axial and bending strength.

pipe-joint-comparison.jpg
Fig. 5. The wrong and right tube joints.

img_2617-platform-leg.jpg
Fig. 6. The right way to join structural tubes.

Why Things Go Wrong

How a terrible mistake like that can happen? Because the designer does not have engineering intuition. He is a 21st century engineer who works mainly with computer aided design and analysis tool. In the finite element model composed of trusses element, the A-frame looks OK. We only see line elements, which represent tubes, connect to each other. Theoretical safety factor is sufficient. Everything looks perfect. But we know it is not.

Be careful with engineering softwares. They are actually toys for operators, not a crucial skill of the engineers. Engineers observe thing and visualize its behaviour in his head. When things get complex, we need computer aided design and analysis tools. But computer is only a tool to verify our intuition. So, pay attention to details and always do visualization in mind to strengthen our engineering intuition. I wish you strong and reliable works always.

** End of Article **

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Entry filed under: Design & Analysis, Designer's Pitfall.

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4 Comments Add your own

  • 1. David Weisenthal  |  July 16, 2009 at 1:26 am

    Great insight-I am a mechanical engineer and I agree with the comment on over-reliance on the software. Thanks for this info on tube design, you have opened many eyes.

    Reply
  • 2. annia  |  September 21, 2010 at 9:56 am

    i am tryin to design an A-frame, where can i get some good sources of information??

    Reply
  • 3. cromagnum  |  January 20, 2012 at 3:26 am

    A thru-plate that goes through a slot cut on each side of the pipe, then welded at both sides might overcome the design issue if it was only from one side of the tube.
    Just a thought, still needs good engineering.

    Reply
  • 4. Russ  |  March 29, 2012 at 9:46 am

    Okay, I’m willing to be educated, but isn’t this structure statically determinant? As such, all members are loaded in tension or compression w/no bending loads. If statically determinant the horizontal and diagonal members will take out each other’s horizontal component, so it doesn’t matter if the vertical tube is horizontally “soft” at the joint – it will see only tension or compression. thanks.

    Reply

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This blog is intended to accommodate sharing of thoughts, ideas, and experience in heavy equipment design and construction. You are free to copy, print, and distribute material in this blog provided that you refer back to its source and you do not use it for commercial purpose. Feel free to drop comment. Have a nice day, mate. //

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