Building a structure made of steel takes a good bit of work. Design, construction planning and erection are all accounted for from conception to completion of a typical building project to withstand gravity loads, earthquakes, wind and snow. Now imagine that structure, still made out of steel, has to float in the water while carrying 1,500 tons or more. This is the daily business of Brownsville Marine Products, taking care of the assembly of these massive structures and almost literally “throwing” them into the river when completed (safely, of course).
One of the trickier operations in this line of work is the rake assembly. The rake is the front portion of the barge that is typically bowed upward to the tip of the deck. To start the build of this piece, the deck is first welded together; that is, the rake is built upside down to its operating orientation for ease of assembly. The other miscellaneous pieces mentioned are put together after this, but now the deck has to be positioned right side up so it can be fabricated to the main body of the barge. This is where the 50 ton sweep has to be very carefully thought out – it’s not exactly like flipping your favorite breakfast food on a dime.
Angular Momentum-inducing Stands
When the rake is picked up, it’s going to want to rotate for the same reason you swing on a Skycoaster. Your center of mass is not directly underneath the pivot point, and until it is, it will continue to swing back and forth until it has induced enough air resistance and friction at the rotation point to ease to a smooth stop. But we don’t exactly want all of that tonnage to be freely oscillating, causing either a rope to snap or something to be clubbed by it. In order to prevent this type of motion, a careful plan was laid out so the trunnion position would allow for a small angle of rotation while allowing it to come to an optimal rest position so the complete flip can be realized smoothly. Trunnions were first used on cannons to allow it to have freedom to aim at any angle desired. For this application, they are the “hooks” which heavy-duty rope is wrapped around, attaching to a high-capacity overhead crane which lifts and turns the rake.
Figure 5 shows the location of the center of mass (which I can’t elaborate enough what a behemoth that was to calculate!) and trunnions. As it is picked up, the rake rotates until the pivot point is directly below the trunnion. It will begin to swing counterclockwise because the center of mass (COM) is slightly off center of this imaginary vertical line, oscillating until friction subdues its motion completely, and then lowering into its final resting position. This wasn’t the final orientation for assembly to the main body, however. A separate calculation was done to determine the strongest rope necessary to orient the deck of the rake at the proper angle.
Basic Lift and Flip Procedure Designed for Minimal Angular Rotation
This is why the location, or “pick point”, is so important. If it was closer to the deck, the vertical position would have happened much sooner in the rotation, causing a larger angle for the COM to rotate before equilibrium and therefore, a faster and prolonged swing. It would naturally follow, then, to think you should just put the trunnion wherever you want for a nearly zero degree rotation! Not so fast, skipper!
The rake, as you can see in the first photo, isn’t a flat surface on the side from its curvature alone. Additionally, there are “rub bars” on the exterior of the main plates which cause discontinuity in the flatness of the surface. The plate that bridges the trunnion and rake has to be fully welded all the way around due to the immense weight and load put on that connection. The vertical line connecting the COM and trunnion also have to be on the side of the pivot point it is desired to turn to, otherwise it will go the wrong way when set back down! Lastly, there has to be room on the inside of the location of the trunnion to add additional stiffener plates if none exist there already, otherwise the actual plate of the barge might buckle from the high local stresses.
As you can see, a lot of factors go into what turns out to be a very simple operation. But a well-executed plan is what was necessary for this project to find its rest. See the video here!
Allegheny Design Services
Allegheny Design Services is the link between owners, architects and contractors. We help the architect realize their vision while providing simple, buildable solutions to the contractor. We pride ourselves in offering timely solutions to complicated engineering issues, whether structural, mechanical, electrical, or plumbing in nature. ADS utilizes state of the art software including BIM (Building Information Modeling), AutoCAD, AutoCAD MEP, AutoDesk Revit Structure, AutoDesk Revit MEP, Speclink and RAM Structural System.
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