The Tricky Ribs
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. 😉
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.
Fig. 2. Cross section of a wide-flange beam.
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?
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.
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.”
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. 😉
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