Wind Loads and Structural Behaviour of Shipping Container Foundation?

[Redact]

Hi there, I need some assistance :

I need to design a foundation system for a standard 20’ shipping container. I am thinking of just putting in 4 pad foundations for each ‘foot’ of the container with ground mounted twist locks anchored into each foundation.

I don’t have much experience with wind analysis and structural behaviour of systems, so I would appreciate any help you can give.

The attached document shows the calcs for the wind load, I am using the Canadia approach because that is the only one I had reference material for.

I am assuming a wind speed of 150 mph ( 67 m/s) as the container is in a hurricane prone country.
The lateral loads that I am getting are quite high, which is why I would appreciate any comments on whether I am doing the wind analysis correctly.

I also have a question for how the 4 separate pad foundations will behave. I am trying to understand how they will work together. I am assuming there would be pressure on the windward wall and the leeward wall, causing the container to be pushed in one direction as a worst case.

As a result, this is causing a high sliding force. Is my assumption of sliding wrong? Does the frame of the container change the structural behaviour causing sliding to not be an issue?

Same with overturning. As the top of the container has a frame, it is essentially propped. Is overturning no longer an issue? What is the structural behaviour of the group of pad footings?

Any help would be appreciated.

 

[canwesteng]

Canadian code uses an hourly average wind speed instead of 3 second gust that ASCE uses. I suspect that you are using a gust speed, or at least if you are dealing with hurricanes the ASCE approach will be more appropriate. Otherwise you need to draw some FBDs to figure out sliding and overturning. Nothing is magical about a sea can.

 

[Harbringer]

And once you cut all your openings in the container walls how do you transfer overturning of wall segments into the pads? Can the container even span between footings for gravity without the sidewalls? Or are you not adding any openings?

Does the frame of the container change the structural behavior causing sliding to not be an issue?

Designing for sliding is real...

As the top of the container has a frame, it is essentially propped. Is overturning no longer an issue?

I don't know what you mean by "it is essentially propped" but designing for overturning is real...

What is the lateral system for this building? Are you installing frames or are you using the corrugated panels as shear walls?
No way for us to evaluate the footings without additional information about the lateral system.

You need a mat or continuous footing most likely. I would probably just do a slab with a turn down edge, that's typically how we do steel or concrete equipment shelters (similar scale). It also sounds like you need a structural engineer.

 

[Redact]

@Canwesteng Thank's the ASCE approach made a big difference to the final wind pressure.

@Harbringer The company will include a door and small window, which will be closed in high winds. It is an existing shipping container, these standard shipping containers are framed with structural members, which provide the structure with adequate stiffness to assist with preventing local buckling when being lifted etc. Although I note your point; in hindsight if the bottom structural framing member corrodes it can cause an issue down the road if it is suspended. I'll look to include side walls just in case.

I guess what I am trying to say regarding the propped comments is that I would like to understand the global behaviour of the system. It is evident that local overturning failure of individual footings most likely wouldn't happen. As the system is effectively connected with the structural framing, I would have thought that the pad foundations would act as a group with the container.

The structural behaviour is what I would like to learn.

The corrugated panels would transfer the wind load to the framing elements, down to the footings.

 

[dhengr]

Redact:
As long as you don’t mangle any of the four sides (two sides and top and bot.) with openings or the ends by removing them, the container should act pretty much as a ridged rectangular box. Just as it does in its original life. Openings in any of these surfaces starts to screw up each of these surfaces in acting as shear diaphragms or tension fields. It will roll over or slide due to sufficient lateral loading, the more so, when it is empty and has the least righting moment or weight. You should not depend on any friction for reaction against lateral loads. I would put a grade beam the size of the container, all around as the foundation, with some enlargements at the corners for the concentrated corner loads. This is partly to keep critters and blown debris out from under the container. Site the container and its elevation high enough so the surrounding grade can be graded to drain away from the container.

 

[Harbringer]

Although I note your point; in hindsight if the bottom structural framing member corrodes it can cause an issue down the road if it is suspended.

No my point was the floor framing may not span the full length of the building for D+L loads once openings have been added. If it is a company that is providing this building they may be providing additional lifting points each side of openings. Do they specify that in their drawings?

It is an existing shipping container, these standard shipping containers are framed with structural members, which provide the structure with adequate stiffness to assist with preventing local buckling when being lifted etc.

It is no longer a standard shipping container once you cut holes in it and designate it a building. What is the intended use of this building?

The drawing you've attached I doubt reflects the reality of the lateral system (unless this box has added moment frames). The green highlighted lines all around you've designated "structural frame" is a meaningless term.

You can't understand the "structural behavior" until you understand the load path. In your design the longitudinal walls will transfer 1/2 the transverse wind load to the roof diaphragm. The other 1/2 of the wind load will be transferred into the floor diaphragm which you are designing to span between footings. How does the load in the roof diaphragm get to the ground? One end wall will be a double door so you don't have a load path there so its a three sided diaphragm. You will have one line of resistance in the transverse direction. The footings on those lines will see more shear. However, your longitudinal walls will also have more shear due to the three sided diaphragm action.

What is causing the moment you've drawing at the footings? How are you making a moment connection at the base of a box? What is your calculated uplift due to wind for each pad? Also I still don't know what you mean that it has a propped frame. Are you assuming some kind of cantilever column?

Some details of the actual design with opening locations would be helpful.

 

[Redact]

Thanks for the comments Harbringer, they are helpful so far and I do appreciate them. I am trying to learn from them, as stated above I don’t have experience with wind effects and the accompanying load paths and I am trying to gain a better understanding. The building is a stock container office, I am not designing the building itself. I was tasked with designing a foundation for it to rest on. The company hasn’t provided shop drawings, they have only provided the overall dimensions and some photos. I have shown an image of the what the building structure looks like though. The intended use is as a small office. The openings are stiffened around the door and windows (see image), does that still mean I can’t consider that entire wall as contributing to the load path?

Your comments about the load path of the building are helpful, as I think I may have approached this the wrong way. I had initially converted the wind pressure into a force by multiplying it by the windward wall area and applying this force centrally. The moment was created from that wind force acting from the mid point of the wall to the bottom of the foundation (assuming the wall transfers the load to the stiffer columns down to the foundation). I essentially halved the force and applied the accompanying shear/sliding and moments to the footings on the windward wall side. The leeward wall had a similar pressure so I did the same on the leeward side footings.

For the connection at the base of the box, I was planning to use a ground-mounted twist lock. I’ve shown conceptually what I was thinking in the attachment. I haven’t calculated the uplift yet, I wanted to check to see if I was on the right path first. I’ll calculate the moment couple to get the uplift

 

[XR250]

I usually use reinforced masonry piers or sonotubes. Clip angles are then anchor bolted to the piers and welded to the structure. I generally use more than just 4 (on a 40 footer) to help with the sliding.
These containers are pretty heavy (6k for a 40 footer IIRC). Overturning has not been a big problem for me in my area (115 mph ultimate)