The Tricky Ribs

November 24, 2007 at 3:12 am 12 comments

Introduction

The devil is in the details. This time we are dealing with a very simple and trivial aspect of steel structure: ribs. Figure 1 below shows wide-flange beams which are reinforced with ribs. This is part of my latest design called LIMOV STM-50T, a sling test machine with capacity of 50 ton pull. If you need any heavy-duty test machine, or anything serious, just ring us here at LIMOV, OK? It is the only engineering company that matters. Seriously. 😉

img_1835-rib.jpg
Fig. 1. Wide-flange beam reinforced with ribs.

Rib is used to hold the adjacent structural members in its position. It keeps web and flange of beam from buckling. What are web and flange? See Figure 2. When rib is used inside a closed-box structure, it is called diaphragm. Figure 3 shows a series of diaphragms inside a closed-box structure. It is one of my early designs and a bit silly from my present point of view. But this will be discussed some other time. This time we only focus on rib and why it matters.

wideflangebeam.jpg
Fig. 2. Cross section of a wide-flange beam.

img_0290-diaphragm.jpg
Fig. 3. Series of diaphragms inside a closed-box girder.

Welding of Ribs

When we weld the ribs onto the wide-flange beam, we will expect some distortion. Usually the beam will be a bit twisted due to thermal expansion-contraction during welding. From experience, rib takes very little stresses compared to web and flange because it is only subjected to secondary load. So the first simple rule is: (1) don’t use thick plate for ribs. The thicker the plate, the thicker the weld required, and the more twisted the beam.

Dimension of Ribs

Here is a very trivial issue regarding ribs. It may seem very unimportant but I assure you that most structures I see have this pitfall: most engineers design the rib as wide as the flange. Figure 4 shows a bit more complex structural arrangement reinforced with ribs. This is also one of my very early designs. Look at the spots indicated by the red arrows. The edge of the ribs exactly meet the edge of the flange. Why is this bad?

bogie-rib.jpg
Fig. 4. Frame with rib as wide as the flange.

Stay Away from the Edge

In a plain steel plate, the weakest point is on its edge. Crack or defect on the edge of a plate has higher stress concentration than crack of a same size in the middle of the plate. When we weld the ribs onto the flange, we distort the material properties around the weld joint and make it a bit weaker. If we make the weakest point weaker, it can be the place where failure initiates.

Even worse, if the rib is of the same width as flange, welder will find it difficult to make a continuous pass on the edge. See Figure 4 above and imagine yourself as the welder as he reaches the spots indicated by the red arrows. The weld will be discontinued and most probably has defects.

Is it bad enough? No. It can still get even worse. Imagine the welding is finished and somebody else comes and take a look at the swell on the edge where the rib meets the flange. This guy probably says to himself: “Oh, what an ugly weld bead. But no problem, I can straighten this thing up.” Then he comes back with a grinder and grind off the weld bead on the edge of the flange, turns it back into a straight-edge and nice-looking flange again. Now we have little weld material left on the edge. Figure 5 is the picture I taken on board of an oil barge belongs to a famous international oilfield services company. Ribs are on the edge and somebody ground off the weld bead.

img_0120-groundweld.jpg
Fig. 5. Ground off weld bead on the edge of flange-rib arrangement.

So the second rule is: (2) Stay away from the edge. Figure 6 shows more detailed view of ribs in LIMOV STM-50T frame. This is the right thing. The width of the ribs are a bit narrower than the flange. The edge of the flange stays smooth and welder can easily make a continuous J-turn pass when he reaches the edge of the ribs. You know what the welder of this frame told me when I inspected the fabrication? He said: “Sir, I thought you put wrong dimension in shop drawing of this rib. It is a bit too short. But it is also easier to weld too.”

img_1837-goodrib.jpg
Fig. 6. Details of ribs with edge narrower than the flange.

Finally, no engineering teachers or colleges will teach you this trivial matter. Most designers with no field experience do not care about it. Drafters will prefer drawing a same-width rib-flange because it is easier and looks nicer in their drawing sheets. But let us do what is right and begin with small details. Observe things around you and think critically. I wish you strong and reliable works in the future. And remember that we at LIMOV care for every details. 😉

** End of Article **

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

The Cheesy Casting The Deceiving Tube Joint

12 Comments Add your own

  • 1. rendra  |  November 30, 2007 at 8:35 am

    Nice writing bro..,

    Bro, When the rib use for support structure which only bear static load, is it also important thing to give attention about weld rib on the edge? the picture of the structure : http://i16.photobucket.com/albums/b13/rendrafirmansyah/DSC03154.jpg and http://i16.photobucket.com/albums/b13/rendrafirmansyah/DSC03156.jpg.

    Btw, if you have experience with FRP pipe, its pifall or other aspects about it that must concern to, please write in your blog bro..

    Thanx.. 🙂

    Reply
  • 2. isadikin  |  November 30, 2007 at 5:50 pm

    Halo, bro Rendra!

    I have seen the first picture (DSC03154.jpg). The second picture cannot be opened. I think this is a badly designed beam. As per our discussion above, it is trapped into the pitfall: The ribs are designed as wide as the flange. Another mistake: Some mounting plates are already bent like hell, most probably during handling to the truck. The designer is ignorant of how the beam will be handled and transported. I don’t see any lifting lugs or padeyes required for lifting the beam during transportation and installation. The mounting plates are designed between 2 ribs which makes welding difficult due to restricted access. In brief: Bad design.

    Regarding your question: The design principal is applicable for both static and dynamic load. So we shall always give good attention to it.

    In broader sense, this is not only about ribs. It is more about observation and imagination in engineering. In the case of “the tricky rib” and “the deceiving tube joint”, you will find that we only discuss the very basic principle of engineering. How load flows inside a structure and how the structure reacts to it. How welder finds a small difference in design could make his work easier and better. We are not talking about numbers, calculations, or other sophisticated theories of advanced materials.

    The simple yet elegant engineering intuitions are lost today. Teachers don’t tell us to imagine how stress flows. They just give us equations and numbers. That is why our failure analysis ability is very low because we think in numbers instead of visualizing how system works and reacts.

    About FRP, unfortunately, up to this moment, I don’t have any experience with FRP pipe. 😦 But I encourage you to approach the problem with insightful observation and visualization. I am sure you shall have no problem in dealing with it.

    If you like design, and want to see the best designed structures in the world, I suggest you to observe an excavator. I personally think that heavy equipments, especially the mining class excavators, have the finest and best structural design. I love to observe detailed structural arrangements of mining excavators and crawler cranes. They are simply beautiful. I’ll try to post something about it later. 🙂

    Reply
    • 3. abbas naqvi  |  July 15, 2011 at 1:33 pm

      boss you look like hell good engineer…. please tell me how to calculate the weld throat in welding ribs of I beam … i m confused !! reply fast !

      Reply
  • 4. rendra  |  November 30, 2007 at 9:47 pm

    Hereby, I rewrite the link of the second picture can’t be opened before 😀 :

    Thank you for the explanation. Keep share with us 🙂
    Btw, excavator was my subject in my “Machine Element Assignment” with Mr Andi about five years ago :D. We’ll waiting for your post about it. Post it!!! 😀

    Reply
  • 5. Neil  |  February 13, 2008 at 1:59 am

    I am setting up a block library in AutoCad. When it comes to the Wide Flange, I am looking for a way to calculate the radius between the web and the thickness. What do you use if the thickness and the web are not equal?

    Reply
  • 6. isadikin  |  February 13, 2008 at 11:59 am

    Hello Neil,
    I suppose by “thickness” you refer to “flange”. The radius you mentioned must be the “r” in Figure 2 above. Wide flanges usually have standard radius which you can look up in tables of sections. You can drop me your e-mail and I will send you tables of standard wide flanges to complete your block library. My e-mail address is in “About me” section.

    Reply
  • 7. Beadwork  |  March 6, 2010 at 5:31 am

    I am new to beading, where do I start? We recommend our ‘Basic Bead stringing’ and ‘Wire Work’ Classes. From there, speak with an associate, tell them what you are interested in making and they can recommend books, and classes key to your interests.

    Reply
  • 8. Beading Loom  |  March 6, 2010 at 7:55 am

    Do you do beading parties? Yes we do. We offer birthday parties for children ages 4 and up. We do bridal showers, wedding showers, adult birthday parties, family get-togethers, office parties, or a demonstration or class for your organization.

    Reply
  • 9. Ivory Chiles  |  February 4, 2011 at 11:47 am

    Well, this is fine, but how about additional choices we have here? Would you mind making one more article regarding them as well? Thank you!

    Reply
  • 10. Sandip Patil  |  May 9, 2011 at 12:58 pm

    I want to know one thing that when we are going to join two beams and two strengthen the joint can we weld the plate without notch to strengthen the joint

    Reply
  • 11. Laurine Bernacchi  |  July 9, 2011 at 2:23 am

    This one is in my bookmarked links, will check on back later.

    Reply
  • 12. abbas naqvi  |  July 15, 2011 at 1:10 pm

    we have an I beam and we want to make a concealed crane ,so we have to weld supporting ribs like you have done in your above project,,,, please help me how i can calculate the weld throat required to bear the load of 20 KN direct…. the I beam typically can bear this load but we are not sure about the impact loads when the crane is stopped suddenly thats why we are reinforcing it with ribs !!! please tell me how to calculate the throat of weld required !

    Reply

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