February 21, 2012
A Good Question Considering Shipping Container Structures
This question came in to me by e-mail considering shipping container structures. It is from a graduate student that is doing research on the structural design and analysis of a shipping container building, and he is working on doing it by FEM (Finite Element Method). Finite Element Method design is a very good way to model complex structures and lends itself well to computer analysis. I’d like to share my answer with you all:
——– Original Message ——–
Subject: Dry Cargo Shipping Container Structural Integrity
From: “Shane xxxxxxxxxx
Date: Tue, February 21, 2012 2:13 am
Dear Mr. Runkle,
I want to begin to thank you for taking the time for reading this email. I will try to keep short and clear.
I am a structural engineering graduate student at xxxxxxxxxx and I am involve in a multi-disciplinary team designing the next generation of innovative student housing at xxxxxx campus. The multi-disciplinary team has decided to design a high-performance sustainable green structure that will have a dome-mound of dirt as a green roof and the main structural system will be composed of repurposed shipping containers.
The reason why I am emailing you is that I was researching online about any structural analysis or finite-element analysis that has been done for dry cargo shipping container and came across your name at this website: (http://ronestudio.wordpress.com/2012/02/10/basic-container-design-structural-considerations/)
I wanted to kindly inquire if you know of any published information about the structural integrity of cutting walls and windows to dry cargo-shipping containers.
I have found several technical specification requirements from ISO that accounts for both static and dynamic loading but all are under the assumption that the load path is from the cargo floor>C-beam floor joist>Longitudinal C-beam>Corner fitting>Foundation (or the next lower shipping container column).
So far, roof-loading scenario of soil mound is 600lb/sq. ft. and ISO requires roof of shipping containers to handle 300lb/sq. ft. We are looking to reduce load by using alternative fill material but are still concern about combining 20′ shipping containers and cutting entire wall out. I have found one case study of a cabin that you advise by replacing entire wall with two 6″x3″x1/4″x19′ steel box beams welded the entire length of the containers. I believe this cabin did not have loading scenario as much as we have.
With that said, I kindly ask if you could share any information you have regarding structural integrity of a shipping container used as housing. I will be performing a finite-element model of our final design and I wanted to see what information is out there before I begin. Again, thank you for your time and I look forward to hear back from you!
Shane xxxxxxx Graduate Student
There is no published information about container strength when they are cut up, and because there is a huge number of configurations, a prescriptive type of guidance would not work. You have to break the container down into its constituent parts and model it based on the sections of the parts. I use a 3d FEM program (Bentley RAM Elements) and I have put all the parts in as sections. For the sides, I use thin steel shells. You have the following parts in a container:
1. Rear posts – these are roughly steel angles. It’s fairly simple to calculate the moment of inertia of these, or draw them in AutoCAD and let it figure the modulus with the MASSPROP command.
2. Front posts – these are fairly complex members made from two different size channel sections. The moment of inertia can be found by drawing these in AutoCAD and using the MASSPROP command.
3. Bottom Rails – these are C sections, again fairly easy to calculate section properties.
4. Top Rails – these are steel tube sections
5. Side corrugations – these can be modeled as cold formed hat sections.
6. Top corrugations – again, can be modeled as cold formed hat sections.
To get specific information on containers in the form of drawings, check out this site: http://www.isbu-info.org/
RAM Elements will automatically find all the section properties of the sections once you program them in. The side corrugations are pretty easy, you can use existing AISI hat sections. The other members take a bit of work using the LEO language that comes with the program. Also, I’ve found that it is necessary to stiffen members as you make cuts in the container. That takes again more work with the LEO language in RAM Elements to model the sections, or conversely you could calculate your section properties by hand (not recommended, too much chance of error), or a program such as AutoCAD.
You are right, the ISO testing is not applicable in any way. It assumes the containers are not cut into, and is only applicable for their use as cargo carriers. There is no way it can be used for building, although I did see it used by an engineer in his calculations. In case of a failure, it would make for an interesting lawsuit against the structural engineer, he would lose big time. I hope this helps. A copy of this e-mail is going up on my blog, with your personal information removed, since your question is very good I’d like to share it with all.
George W. Runkle III, P.E., PEng, MIEAust
Runkle Consulting, Inc.
930 New Hope Road, Suite #11-145
Lawrenceville, GA 30045