A practical physics question: We receive carloads of 4’x8’ sheets of oriented strand board (OSB) from Canada. The units are 34” tall, stacked 3 units high, 9 units wide for 27 units on each side of a center beam car. The width between bulkheads must be 73 to 74 feet as there are only a couple inches between the ends of the units. As is to be expected, there is usually a little bit of shifting forward and backward due to train operation. Because it’s packed in tight, there’s not a lot of room for the units to move. However, they do something weird. Each unit is held together with three narrow nylon bands. Picture a deck of cards with a rubber band at each end and one in the middle. The forward/ backward shifting breaks some of these bands and that’s where it gets weird. The bands that snap are generally just on the end units of the bottom row. Usually it’s just the end bands closest to the bulkheads that snap. It seems to me that the bottom row would be the ones least likely to shift, as that row has the most weight holding it down. Why just the end units and why just the end bands? Wouldn’t the entire bottom row be subject to the exact same forces?
Thanks to Chris / CopCarSS for my avatar.
I would opine that the bottom row is being pushed by the units on top of it.
The bulkheads may be the only place the load has to shift, even if it is in single digit inches.
Odds are the load moves as one mass. Add to the horizontal movement a possible downward vector (diagonally across the load) and you've got another stressor on the bottom end units.
Just my guess. I'm not a physicist.
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Are the bands that break on the bottom row being broken by the movement of the row on top of them as the bottom row remains stationary and the next row up snags and pulls on the bands around the bottom row?
Murphy Siding stacked 3 units high, 9 units wide for 27 units on each side of a center beam car. The width between bulkheads must be 73 to 74 feet as there are only a couple inches between the ends of the units.
Do you ever notice the middle and top unit in a "stack" to be shifted to where it is in contact with the bulkhead?
Euclid Are the bands that break on the bottom row being broken by the movement of the row on top of them as the bottom row remains stationary and the next row up snags and pulls on the bands around the bottom row?
Convicted One Murphy Siding stacked 3 units high, 9 units wide for 27 units on each side of a center beam car. The width between bulkheads must be 73 to 74 feet as there are only a couple inches between the ends of the units. Do you ever notice the middle and top unit in a "stack" to be shifted to where it is in contact with the bulkhead?
Having been in a work environment where I could actually witness some loads shifting, I can offer a few ideas. All things being equal, an impact would want to shift the entire contents of the car. The ones with less weight on them, at the top, will shift more easily, but will shift as an entire bundle, because the strength of the bands exceeds that of the movement.At the bottom of the car, there is a force holding the bundles back, namely the floor of the car. I suspect that the bands in contact with the floor (be it a solid floor or the kind with ribs to lift the loads up a bit) are the ones that are breaking.I presume that there is a small gap between bundles stacked vertically, enough to allow a forklift to get underneath them. Next time, look to see if you could find where those lower bands are breaking...I suspect it is beneath the load, caused by coming in contact with the floor.For the record, the most common inside length of the center-beam cars is 73 feet, simply because it allows loads to take up most of the length, whether the individual bundles are eight feet or twelve feet long. Beats the heck out of the old 60-footers!
Carl
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Somehow stacks of long flat objects tend to rotate in the flat plane about the center. And it seems the lowest layer stays still and the layer above is what moves first (taking with it the layers above). I have seen this in many different items, from beams to sheets of plywood/OSB/etc.
If you have ever carried an armload of a few 2x4's you will have experienced the action. The bottom one remains aimed where you are holding the stack, but the board above it will rotate sideways and fall off the ends on opposite sides. (FRUSTRATING! Grrrrr!)
With bands holding the sheet goods together, the rotation will stretch the end bands, applying more force to them, which will sometimes break them.
I am sure there is some physics explanation, but I have no idea what it would be.
Semper Vaporo
Pkgs.
Well, I'm no expert, but I'd guess you are getting some inverted pendulum action in the stack, the top moves freely while the bottom is anchored (by friction) to the floor, and there is just enough sliding between the sheets to shear the bands.
Back in the day, the railroad might have run some tests to figure out what was going on, and how to prevent it in the future.
Good luck with that today...
Convicted OneWell, I'm no expert, but I'd guess you are getting some inverted pendulum action in the stack, the top moves freely while the bottom is anchored (by friction) to the floor, and there is just enough sliding between the sheets to shear the bands.
I agree with your inverted pendulum but with the action at right angles to what you indicate. More likely the force involved is shock, either in train handling or humping. The pendulum stack selectively compresses the clearance between the sheets and perhaps even the edge regions as the top 'levers' over, with much more force as you get nearer the bottom of the stack; this is the rotation that pops the bands vertically on the unstressed side.
If CO and I am right, relatively simple blocking or dunning that holds all the piles vertical and doesn't allow 'longitudinal' motion either at the top or significant intermediate points should eliminate the phenomenon.
What I want to know, and perhaps Murphy Siding can answer this best. Why is it not considered a problem shipping this stuff open under the elements?
I see these movements being shipped through rainstorms, and just envision warpage, delamination, and disintegration.
Overmodbut with the action at right angles to what you indicate.
Could be. I was pondering that there might be some form of harmonic in the platform, sort of a "diving board" type resonance. causing the stacks to rock endwise. I'm sure the center spline is intended to mitigate this movement, but I don't really know if this completely eliminates this type of movement.
The physics of a thousand miles or so of vibration of movement coupled with couplings during switching as well as the various impacts of slack running in and out while the car is in transit.
Riding a centerbeam flat car is not like riding in a Rolls-Royce. They don't have air-ride suspensions and while some may have cushioned underframes - that cushioning does come to the end of its travel from time to time.
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When some of our members get done with their debate on Entropy in the S&P forum, I'm sure they'll come over here and give the correct answer, which nobody will understand... :D
Convicted One What I want to know, and perhaps Murphy Siding can answer this best. Why is it not considered a problem shipping this stuff open under the elements? I see these movements being shipped through rainstorms, and just envision warpage, delamination, and disintegration.
Yeah, I've seen the wrapped shipments too, and thought it looked like the right way to go. But I've seen shipments that were not wrapped and wondered what the deal was. So those are headed to discounters most likely? Makes sense.
Seems like the hardwood veneers on modern plywoods are not adhered to the substrate nearly as well as they were 40 years ago, either.
Murphy Siding Convicted One What I want to know, and perhaps Murphy Siding can answer this best. Why is it not considered a problem shipping this stuff open under the elements? I see these movements being shipped through rainstorms, and just envision warpage, delamination, and disintegration. The units are wrapped. The ones you see with no wrap are usually sitting in the yard at a Lowes or Home Depot. They save about a nickel a sheet and assume the consumer doesn't think about warpage, delamination, and disintegration. Even drywall is shipped like that.
The units are wrapped. The ones you see with no wrap are usually sitting in the yard at a Lowes or Home Depot. They save about a nickel a sheet and assume the consumer doesn't think about warpage, delamination, and disintegration. Even drywall is shipped like that.
Most all such shipments are wrapped - that being said that wrapping may not make it intact all the way to the consignee. I suspect the wrappings may be a victim of the issues Murphy identified in the original post.
I look at this like a structural geology problem. With things stacked up, the greatest strees is on the bottom of the stack. In this case, a stack of OSB sheets, rather than layers of rocks. If all the stack of wood comes into abrupt contact with the bulkhead, maybe thru slack run-in or rough coupling, stress will be concentrated in the botton front of the stack. All that force may be enough to deform the bundel of OSB enough to expand it, breaking the bottom band closest to the bulkhead.
Convicted One Yeah, I've seen the wrapped shipments too, and thought it looked like the right way to go. But I've seen shipments that were not wrapped and wondered what the deal was. So those are headed to discounters most likely? Makes sense. Seems like the hardwood veneers on modern plywoods are not adhered to the substrate nearly as well as they were 40 years ago, either.
Where is your product coming from? All the OSB and plywood mills around here use boxcars. I've only seen them use centrebeams in times of extreme car shortages.
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MidlandMike I look at this like a structural geology problem. With things stacked up, the greatest strees is on the bottom of the stack. In this case, a stack of OSB sheets, rather than layers of rocks. If all the stack of wood comes into abrupt contact with the bulkhead, maybe thru slack run-in or rough coupling, stress will be concentrated in the botton front of the stack. All that force may be enough to deform the bundle of OSB enough to expand it, breaking the bottom band closest to the bulkhead.
I look at this like a structural geology problem. With things stacked up, the greatest strees is on the bottom of the stack. In this case, a stack of OSB sheets, rather than layers of rocks. If all the stack of wood comes into abrupt contact with the bulkhead, maybe thru slack run-in or rough coupling, stress will be concentrated in the botton front of the stack. All that force may be enough to deform the bundle of OSB enough to expand it, breaking the bottom band closest to the bulkhead.
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