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I have been talking to some structural engineers trying to find one to design foundations for my contemplated bespoke building based on hi-cube 40-ft shipping containers. The initial concept is just two containers on concrete slab with garage space between them:

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Ultimately, I would like to build extra levels on top of the containers. But at this point I do not want to design that. For now I just want what is on the picture, albeit with foundations strong enough to carry the extra load of say 2 additional levels.

I have been trying to explain the above to structural engineers, specifically mentioning that any extra levels will not be directly connected to the concrete foundation but to the containers only (which would be reinforced if needed). Essentially, the containers will be the foundations for the extra levels.

But so far I have been told that they need the design of the extra levels.

Can someone please explain: why is it not sufficient to just assume extra load on the containers to calculate the concrete foundations? Why would the exact design of the extra levels make any difference (as far as the containers' concrete foundation is concerned) beyond its weight?

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  • $\begingroup$ It's real simple. They need it for their files. If something goes amiss, they can pull out the file and say "this is what I was designing for". This is a legal duty of professionals of all stripes. You could try offering a letter of indemnity and a ,um, less complete design. $\endgroup$
    – Phil Sweet
    Feb 3, 2021 at 2:24
  • $\begingroup$ @PhilSweet Sounds like a potentially acceptable answer. $\endgroup$
    – Greendrake
    Feb 3, 2021 at 2:38

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Foundation's task is not just to support gravity loads, it should resist lateral loads, wind and earthquake, as well.

Wind loads are dependent on the geometry of the building and seismic loads are dependent on the mass distribution throughout the buliding.

For example of you will show access to the roof of your third floor container for its use as a deck, you have to add 2.5 times live load more than floor loads for that roof (100lbs/square foot versus 40 for a floor).

If this deck is over just half of the roof of the container, it will introduce torque forces due to unsymmetrical loading at the time of the earthquake.

Therefore detailed plans of upper floors are needed for proper structural designs in general and foundation design in particular.

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  • $\begingroup$ also, depending on your local building codes, they may state floor loading values. A bedroom, a bathroom, library, home gym, may all have different floor loading requirements according to code. In addition, the framing can dictate how these load will be transmitted to the foundation and if one part needs more reinforcing than an other of its all essentially equally loaded. $\endgroup$
    – Forward Ed
    Feb 4, 2021 at 18:44

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