Structural Engineering Analysis: Intermodal Shipping Container

[nculhane]

Hello,

For a structural analysis term project I'm currently working on, my team has come across a problem. Here's the setup:

Intermodal shipping containers (dimensions 8ft height, 8ft width, 40 ft length) can (according to company sources) be filled to a max load that grossly weighs 66 kips. Fully loaded containers are often stacked 8 high (we specifically asked the company to affirm this). We're trying to find the force applied on the bottom container of a stack of 8, and where this force is distributed.

Sources have confirmed the majority of the load ends up in the vertical columns (corner posts) of the container, and these columns are very strong. We have calcs that confirm this (they will not buckle and have a factor of safety of 1.25 under the entirety of the max load). However, we're also trying to determine the bending experienced on the top side rail beam of the bottom container (the 40ft long beam of top of the container's frame). This is our problem.

The beam is a hollow square beam 40ft long, with side length 2.4 in & thickness 0.1 in. Looking at max yield stress/strain curves of the beam's material, a maximum force applied on the beam that does not permanently deform the beam would be about 0.03 kips (found using stress=force/area, and taking the "stress" from the highest value on the elastic stress/strain curve (59 kips)), and we're assuming the container's design doesn't allow it to deform under the weight of 7 containers. Basically, we found that after calcs, this means that 99.94% of the forces applied on the top of the bottom container go into the columns, and only 0.16% of the force goes into the 40ft beam. This seems pretty farfetched; could anyone provide insight into what my team has done wrong?

 

[SteamKing]

The side beams of a container are not designed to withstand the loads from the vertical stack; they are supposed to hold the ends together when the container is lifted on or off the vessel. After all, there is supposed to be a gap between the top of the lower container and the bottom of the upper container, so that no load can be transmitted except by the corner posts. See the figure below:

This document discusses more about the strength of containers:

http://www.residentialshippingcontainerprimer.com/Container Compressive Load-Bearing Capacity

This might be a better copy of the report above:

http://thisoldwoodshed.com/fermi_stress_test.pdf

 

[nculhane]

This is exactly what my team needed; thank you for your informative and helpful reply.

 

[AlephZero]

After all, there is supposed to be a gap between the top of the lower container and the bottom of the upper container, so that no load can be transmitted except by the corner posts. See the figure below:

... and when being transported, the containers should be positively locked into position so the loads can only go through the corner posts. The picture in SteamKings's post does NOT show that. They are resting on the corners posts, but not accurately aligned. http://en.wikipedia.org/wiki/Twistlock

Even so, about 10,000 containers end up in the sea every year, and the empty ones can float for quite a long time as a hazard to shipping.

 

[SteamKing]

... and when being transported, the containers should be positively locked into position so the loads can only go through the corner posts. The picture in SteamKings's post does NOT show that. They are resting on the corners posts, but not accurately aligned. http://en.wikipedia.org/wiki/Twistlock

Even so, about 10,000 containers end up in the sea every year, and the empty ones can float for quite a long time as a hazard to shipping.

The picture was selected to show at least two containers resting on top of one another and to show that a gap is formed between the sides when the posts are (mostly) aligned. This picture, along with the attached papers, was intended to show that the load path of a vertical stack of containers is supposed to go thru the corner posts rather than thru the sides.

 

[AlephZero]

I don't think we are disagreeing about anything. I was just adding the comment that by positively locating the containers and locking them together, the load path is forced to go through the corner posts, not just supposed to go through them.

Of course if you are just making a temporary stack of empty containers (and your local weather station isn't forecasting tornadoes or hurricanes!) people often don't bother to lock them.