I recently saw a train that was almost completly double stack containers going through Mulkiteo, WA and many of the cars had longer containers stacked on top of shorter ones. Is this standard practice with double stacked freights? (I don’t see many double stacks around here so I don’t know). If it is, does anyone know why they do that?
Thanks for answering my odd question.
emmar
Depending on which design the car(s) you are seeing are, the bottom container has to be a standard size such as a 40 foot, two 20 foot, etc. All longer containers have posts at the 40 foot spacing so they can be stacked on top of the 40’s, or any other length, since all have the strong posts at the same spacing. Take a closer look at containers on the cars, or even on trucks, you’ll see the posts at this spacing, even on the longer containers.
Some of the newer cars are designed to handle the longer containers in the bottom well, but with the posts at the standard 40 foot spacing, they can have any length, down to 40 foot, stacked on top of them.
I also had the understanding that it was due the proper distribution of weight. AFAIK it’s the floors and ends that are the strongest part of a container. If you climb up on the roof of one the roof feels to move under your feet.
Anyway since the roof isnt especially stong it must be ensured that the container that is placed on top is as long, or longer, than the one on the bottom. By doing this it means no weight is forced on the roof of the bottom one because the end points of that container will be directly supporting the floor of the one on top. The downward force of the top container is then channeled into it’s floor, then down the ends and corners of the bottom container and then onto the chassis of the railroad car.
I’m fairly certain these are the reasons rather than specifically the anchoring devices built into the railroad car although that probably does play a part. Even at yard and ports and the like container that I see stacked up never seem to have a shorter one on top regardless of whether containers are on a train, truck, ship, or on the ground.
Hope you can make sense of what im trying to say here! [:)]
There is a problem with putting 20 foot containers on top. There is no reinforcemnt in the middle of the longer containers and the 20 footers would just cave it in.
The longer containers may be allowed to stick out over the end of the car while the 40 footer may fit in the well.
There is no reason that 40’ ft containers can not be placed on top of 45, 48’ or 53’ containers. The stacking points are all at 40’ excepl 20’ containers. Only certain well cars will accept two 20’ containers on the bottom and are labeled as such. I do not beleive I have ever seen 2 20’s on the bottom with 2 20’s on top. You will never see either one or two 20’s on the top of any other container as there is no locking or structural support between the 40’ posts.There are no restrictions that I am aware of concerning the container on the top must be larger than the one on the bottom except for the 20’ containers. [2c] As always ENJOY
I learn things here too thanx guys. I’m not all that familiar with the container sizes here I think some sizes may be domestic US? Anyway what I said above was really my observation of 20 and 40 or 45 foot containers since they are most easy to tell apart from a distance. I guess it would make sense that some longer containers have more structural supports. So my reply above probably only applies to 20 footers on top.
Can a 40 foot container hold twice the weight of a 20 foot or only twice the volume? Apparently some containers such as the chemical tank variety are only built in 20 foot lengths due to the density of different liquids.
A key driving factor is the car size. There are more 40’ and 48’ well cars available right now than 53’ cars. So, if you have a 40’ car, you can put 2 - 20’s or 1 - 40 in the bottom position, then you can put a 40, 45, 48 or 53 on top… because, as mentioned above, the stacking frame is at the 40’ point for all of these box sizes.
Similarly, if you have a 48’ car, you can put put 2 - 20’s, 1 - 40, 1 - 45, or 1 - 48 in the bottom position, then you can put an equal or larger 40, 45, 48 or 53 on top.
But - the 48 can’t go in the bottom well of a 40’ car and a 53’ can’t go in the bottom of a 48’ car. That means generally, the smaller box ends up on the bottom. (Unless you have 2 of the same size.)
No, it can’t take twice the weight. 20’s are designed to maximize containership utilization.
When a ship is loaded the ideal is to load it “down and full”. That means it is carrying all the weight it can carry and all the cubic capacity is used fully. That’s hard to do. 20’s handle high desity freight - if you put that cargo in a 40’ or larger box, you’d waste the space inside that wasn’t filled. (You can only load so much weight in an intermodal box because of highway weight limits.)
So, in order to avoid wasting cubic capcity on a ship, they use 20’s. You don’t see domestic 20’s - they’re stictly for cargo that moves on containerships for part of the journey.
As an aside, it’s always been interesting to me that international container lengths still are standardized in “feet”. As far as I know, the US is the only developed economy that still uses “feet”. (I would like to thank Ronald Reagan for that - I don’t want to be 1.93 meters tall, or try to figure out what the “real” temperature is when it’s 8 Celcius) World commerce conforms to US measurements. Cool!
You were da man Ron, you were da man!
My brother asked me a little while ago if they ever put shorter containers on top of longer ones. Never having seen one, I answered probably not, but then under a week later I saw a CN doublestack with shorter containers on top. Looked like 45’ on top of 48’ containers.
In theory there’s no reason you couldn’t put a shorter one (minimum 40 foot) container on top of a longer one, the sturdy support posts on the containers are at the 40 foot spacing on the longer ones. The main limiting factor would be the limits of the container well in the car in question.
Just yesterday I saw 48’ and 53’ containers stacked over 48’ containers and all the support posts were at the 40’ points. This is easier to notice when you’re sitting on the upper level of a Metra gallery coach and the stack train is on the next track.
Way back in history… (Well, the 1990s anyways!) BN had some 28’ containers, and had some kind of special saddle that would enable them to be stacked on top of 40’ (or 45’ ?) containers. Looked kinda weird, from what I have seen. UPS also has a ton of 28’ containers (and I’ve been in enough of 'em!), though I don’t know how they are handled for intermodal transport.
So this is one example (though somewhat incomplete) that shorter-than-40’ containers are occasionally stacked on top of 40-footers…
That is because the stacking saddle transfers the load(28’) to the 40’ stacking points. There are many sizes of specialty containers made for specific needs that are not fully compatable with standard sizes. These 28’ boxes also may reguire special lifting adapters to go from truck to train etc. thus restricting them to certain yards. [2c] As always ENJOY
If one reads any of the Maritime media and their discussions of Container Ship capacity it is normally stated in a value of ‘TEU’, ie. 2000 TEU etc.
TEU = Twenty foot Equivalent Unit.
While the majority of present day containers my be 40 foot, for capacity issues they still state capacity in TEU.
The strength of all containers is concentrated in their corner post. Twenty foot containers end to end - end up having their ‘far’ corners at 40 feet…the same distance as the 40 foot containter. Containers longer than 40 foot, have their ‘mounting’ corner posts built at the 40 foot marks so that they will fit atop 40 foot or twin 20 foot containers.