What is the best foundation for a shipping container home?

A foundation is often the first significant site work undertaken for a shipping container project. It is, literally, the element upon which the rest of your container home (or other container building type) is constructed.

But do you really even need a foundation for a shipping container? And if so, what kind of foundation is required: is it something special compared to other types of construction?

Below, we'll answer all your questions about foundations for containers to ensure your project is safe, secure, and will stand the test of time.

What is a Container Foundation and Why Do You Need One?

Whether you're building a container pool, a container house, or just placing an empty container on your property for storage, you need to safely and securely support it. The ground beneath us may seem simple and static, but it can be uneven, unstable, and difficult to predict.

The movement of moisture, changing temperatures, the decay of organic matter, and the growth of nearby vegetation all can cause the ground to rise, sink or slide. These changes are typically very slow, and it often takes months or even years to visually see their cumulative effects.

But don't let the slow speed fool you. Unless you build on solid rock, odds are high that the ground will eventually affect the position and level of your container home. And even if you have a foundation, don't assume that it is simple to build the right type of foundation in the right way. Just look at the number of foundation repair companies in your area'clearly, foundations are tricky!

A well-built foundation provides a solid, stable platform for your building, ensuring that its weight is evenly distributed across an area of ground that can adequately support it. A foundation can prevent costly aesthetic, functional, and even structural issues in the future.

And, in a worst-case scenario like a flood or tornado, a good foundation ensures your container will stay right where you put it. It really one of those areas where 'it pays to do it right the first time'. So let's get started on understanding the theory behind foundations so you can make one that works!

Foundation Theory: How Foundations Work

The basic function of a foundation is pretty simple: hold the building up straight. But understanding the way a foundation is able to do this across different geographies and project types is important to ensuring project success.

There are a few key principles that determine how well a foundation will work. Most of them tie back to soil mechanics, the study of how the ground behaves. Intuitively, you know that soils made of sand, mud, or gravel will all support you differently. One of the main purposes of soil mechanics, and more broadly, geotechnical engineering, is applying some math and science to those intuitive feelings to ensure you have adequate ground supporting your foundation.

Bearing Capacity in Foundations

Bearing capacity is ability for a particular soil to support the force of a load above it. It's typically measured as a pressure: exceed that pressure and the soil can no longer support the load.

If you've ever stepped on fresh powdered snow, you've seen a material that has almost no bearing capacity. The weight of your foot will sink right through the snow, stopping only at the next layer of material (ice, compacted snow, or soil) that has a higher bearing capacity.

An important thing to understand is how a bearing load is supported by soil. An easy way to think of this is by visualizing a pile of dirt. You'll notice the dirt pile isn't vertical like a tower; it's shaped more like a pyramid.

As you get lower on the pyramid, the size of the horizontal layer of soil gets larger. This means that the pressure on any individual grain of soil is highest right at the foundation, then gets smaller as you descend. In the same way, when you put bearing pressure on the ground, the force spreads through the ground in a cone or pyramid shape that's typically assumed to be about 60 degrees wide.

Factors that affect bearing capacity include soil density, cohesion, organic matter content, moisture content, and friction angle. If you end up needing a soil analysis from a geotechnical report, a company will measure many of these factors via soil boring or in situ testing on site. 

Luckily, while a geotechnical engineer needs to be familiar with all of these factors and the complicated equations that connect them, you do not. Even better, for most projects, you likely don't need a geotechnical investigation anyway.

Instead, scientists and engineers have broadly categorized types of soil with different methodologies, such as the Unified Soil Classification System (UCS). And if you know the type of soil you have at your project site (such as its UCS classification), in most cases you should be able to find out its bearing capacity without having to do any on-site measurements.

To start, you can use the United States Department of Agriculture (USDA) Web Soil Survey. This system enables you to find the type of soil in the specific area where you're building. If you want to know how to use this tool, keep reading to the end of the article where we'll give an example with screenshots.

After you have a soil type from the Web Soil Survey, you still need to know the bearing capacity. That's where the ICC Building Codes come in.

Both the International Residential Code (IRC) and International Building Code (IBC) give allowable bearing capacities for different types of soil. Table R401.4.1 from the IRC gives the presumptive load-bearing values of foundation materials.

Similarly, Table .2 from the IBC gives Presumptive Load-bearing values along with a few other metrics like lateral bearing pressure, coefficient of friction, and cohesion. Note that while the IBC version has this additional information, the load-bearing values are the same in both tables.

Even if your container project doesn't require building code compliance, these are still good numbers to use for your foundation design. However, you don't want to build directly to these limits. Instead, divide these bearing values by a Factor of Safety between two and three (depending on how sure you are of your soil type).

One thing we need to address is recommendations for our readers outside the US. Obviously, the USDA's Web Soil Survey is not going to be of much use to you.

However, if you have clarity about the soil type at your building site, you can still use the values in the tables above. If not, there may be something similar in your country that you can use. Or, you can try reaching out to local builders and/or building officials to see what information they have about the bearing capacities of soils in your area. While the specific location-based data may be harder to find, the overall concepts here are universal.

Skin Friction in Foundations

With bearing capacity fully covered, we now need to talk about a related concept known as skin friction. The easiest way to understand skin friction is to picture a stake, like the kind you use to hold up a camping tent.

A stake is long and slender, and when you drive it into the ground, hardly any support is coming from the very tip of the stake pushing against the ground below it. In other worse, very little of the stake's support comes from bearing capacity.

Instead, it's the soil pushing on the side of the stake that primarily holds it in place. As you drive a stake into the ground, you're displacing and compressing the soil around the stake to make room for it in the ground. This pressure, correspondingly, pushes against the side of the stake too.

The pressure increases the shear friction that we call skin friction, which makes the stake resist both pulling out and being driven in deeper. It's the same frictional force that holds a nail in a piece of wood, even though the sides of the nail are smooth.

Compared to bearing pressure, skin friction is a much more complex topic. Fortunately, pile foundations (discussed later in the article) are essentially the only foundation type that gets the majority of its load capacity from skin friction.

For foundations that primarily use skin friction for load support, you really need the assistance of geotechnical and structural engineers. As you go deeper into the soil, you often encounter multiple layers called soil strata, each with its own specifications and strength.

Due to these complexities, we don't recommend trying to design your own foundation based on calculating the skin friction. Instead, we just want you to be aware that skin friction exists, and know who to talk to in case you need to know more about it for certain types of foundations.

Foundation Settlement

Our next topic has a more indirect effect on the load capacity of your foundation. As you know by now, your soil is not a solid, monolithic material, but rather an aggregated collection of particles with different sizes, shapes, materials, and properties. And under certain conditions it can settle, meaning it occupies a smaller volume and has a lower elevation than it did before.

If your soil settles, it can be localized to a particular side of your project (differential settlement), or the same magnitude across the entire project site (uniform settlement). Uniform settlement is much more desirable, as it doesn't put bending or shear stresses on your foundation and container structures. Rather the whole system just sinks slightly into the ground evenly, ideally without causing problems (though utilities can become an issue even with uniform settlement).

Three of the most influential materials that may be part of your soil are organic matter, clay, and air. Why? Because more than other materials, these three soil constituents make the soil more likely to move in certain cases (and that's usually not good!)

Let's start with organic matter, which typically means things like decaying leaves, animal waste, etc. The problem with organic matter is that it slowly decomposes via biological processes. As this soil decomposition happens, the properties of organic matter change. And any time soil properties are changing, its ability to support a load is changing as well.

Clay is a type of soil based on a particular set of minerals with especially fine particle sizes. Clay can be particularly problematic because it tends to swell and shrink based on the amount of moisture it contains.

Finally, there is air. Due to the fact that soil is made of particles, there are tiny voids between particles where air (and as we'll discuss in a later section, water) can reside. With enough pressure or vibration, that air can be forced out of the soil, increasing the soil density and compacting it in a process called soil consolidation.

Failing to account for these factors can result in soil that contracts and a foundation that may crack or shift. Perhaps the most famous example of this is the Leaning Tower of Pisa in Italy, which was built on unstable soil with a large percentage of clay. This building site experienced significant differential settlement on one side of the tower, causing it to lean.

Similar to skin friction, the analysis of settlement is complex and best left to a professional geotechnical engineer. For the average homebuyer or DIY builder, you mainly need to know that settlement does exist.

In response, you need to be mindful of soil compaction (discussed later) as well as the presence of significant amounts of organic matter or clay. If your soil investigation does reveal the presence of these materials, you can either remove and replace it with better soil or use a foundation that reaches through the weak soil and gains support from stronger layers deeper underground. Either way, this is an area where we strongly recommend getting professional help.

Soil Expansion

Just as problematic as soil settlement is soil expansion. Soil expansion primarily occurs due to water acting in two ways.

First is the clay soil that we mentioned before. When clay dries, it contracts and shrinks. But when it is saturated, it expands.

Expansion due to clay and other fine particulates can cause a lot of problems for foundations. Section .6 Design for Expansive Soils in the IBC actually addresses this problem, but the summary is: get professional help.

Water also plays a role in soil that is below the freezing temperature. Prolonged exposure to freezing temperatures can literally freeze the water in the soil's voids, leading to frost heave. This condition can lead to significant soil expansion that can severely damage foundations that weren't designed correctly. Once again, we'll share more about how to deal with this in a later section of this article.

Types of Shipping Container Home Foundations

There are several different types of shipping container foundations you can use, varying in their functionality and material. We categorize them by their expected service life: temporary, semi-permanent, and permanent:

• Temporary Foundations: These are the easiest and cheapest method, but the riskiest. They get your container level and up off the soil to help with corrosion, but the container is still able to move freely.
• Semi-Permanent Foundations: These will fix your container in place securely and provide all the benefits of a permanent foundation while being removable.
• Permanent Foundations: The type of foundation you normally think of. Once built, they aren't moving without heavy equipment and demolition.

Below, we'll go over the common types of foundations for a shipping container, noting the service life categorization for each.

Wood Beam Foundation (Temporary)

A wood beam foundation is literally just placing the shipping container on top of some large pieces of wood. Most commonly, you put the container on railroad ties, though other types of timbers and lumber can be used. Railroad ties have the chemical treatment to endure prolonged ground contact, and the size to distribute the weight over a large area.

Gravel Foundation (Temporary)

A compacted gravel bed may seem like it is not much different than placing your container on the ground. But by setting a container on gravel, you allow water to drain through so the bottom frame rails aren't in contact with moist earth. This helps prevent rust and corrosion.

Plus, gravel will settle less than added fill dirt, so the container will stay level until it is moved to a more permanent location. Although, with proper container anchoring, you could consider a gravel foundation for a more permanent placement.

One important note about the type of gravel you use for a gravel bed foundation. What you really want is crushed stone with jagged edges that interlock together for better strength. Common gravel, especially river gravel, is usually smooth and not nearly as strong of foundation material.

Concrete Block Foundation (Temporary)

While at first glance this foundation type may appear similar to some of the later options, it's actually a different category. Here, we're talking about placing a container on concrete blocks that are set right on the soil. Regardless of whether you purchase prefabricated blocks and stack them, or build your own in forms on-site, the results are the same.

Since the blocks aren't attached to or embedded in the soil in any way, the only thing holding them in place is weight. So even though the presence of concrete may make this type of foundation appear permanent, it is clearly not. Furthermore, stacking the concrete blocks as you see in the picture makes them even less sturdy.

Helical Pier Foundation (Semi-Permanent)

This particular type of container foundation goes by several names: soil screw, screw pile, helical pile, helical pier, screw anchor, helical anchor, and others! Whatever you call it, the result is the same. A large metal screw (typically several feet long and several inches thick) is twisted into the soil with hydraulic machinery and can immediately support loading. No waiting on concrete to set and no dealing with forms or excavated dirt.

Depending on the size of the helical steel plate that forms the threads and the length of the screw, a screw pile can support a surprising amount of weight. They work through a combination of both the bearing capacity of the screw's helixes and skin friction of the screw's shaft.

Screw piles also provide a huge amount of uplift resistance compared to many other foundation types, which is helpful for securing a container during a storm. And when it's called an 'anchor' versus a screw, pile, or pier, it typically means the intended use is for pull-out resistance.

The best part of helical piers is that they can be removed and reused. Just unscrew them from the ground, move them to a new location, and screw them back in. They leave behind only a narrow hole the size of the screw pile shaft, so with a few minutes of work you could make it appear that they were never even there. A few examples of screw pile manufacturers are Heli-pile, Helical Anchors Inc., and Techno Metal Post.

Specialty Pin Pile Foundations (Semi-Permanent)

Pin piles are a form of micropile, essentially a pile that is much narrower in diameter than typical. Pin piles are usually so small that you need several of them to give the capacity of a single regularly sized pile or pier.

But now, there are a few manufacturers that make a system of pin piles and pile caps that work together to form an integrated foundation solution. Three or more steel pins are driven with a pneumatic hammer through the head/pile cap, angled slightly off vertical. On top of the pile cap, you can attach whatever structural member you need. Examples of these specialty pin pile systems include Diamond Piers and Sure Foot.

Pile Foundations (Permanent)

Pile foundations, sometimes called friction piles or driven piles, are used when the soil near ground level has a low bearing capacity. They are long, slender foundation members that work primarily through skin friction from the surrounding soil. However, piles may get a portion of their strength from end bearing as well if they reach a lower soil stratum with greater bearing capacity.

There are a number of materials used to make piles, including wood, steel, and concrete. Regardless of the materials, piles are typically pushed or hammered into the soil using heavy equipment like a pile driver.

Alternatively, piles can be concrete cast in situ, meaning the concrete is poured inside an excavated hole in the ground. This typically involves boring a hole, temporarily lining it with a steel casing, then adding concrete as the casing is removed, as in a Franki pile. 

Given the requirements for expensive, specialized equipment, piles are typically used for larger commercial projects and aren't DIY friendly. An exception to this is the much small pin piles shared in the section above.

One of the most common places you'll see piles is near the coastline. If you've noticed a beach pier or even a structure (such as the lifeguard station in the picture above) built above the water on poles, you've seen piles.

While the piles in the picture above extend above the surface of the ground, often piles are terminated below the soil surface. Then, a concrete cap is poured on top of the pile to give a uniform bearing surface for the structure above.

For this reason, it can sometimes be challenging to recognize a pile foundation after construction has moved forward. Often all you'll see are the concrete caps, while the piles are buried beneath the soil.

Pier Foundation (Permanent)

Pier foundations are actually quite similar to pile foundations in appearance and the two are commonly confused for each other. The differences between the two are in shape, function, and installation.

First, a pier is both thicker in diameter and shorter in height than a pile. Second, a pier works primarily via end bearing, not skin friction. Finally, piers are almost always cast in place by placing wet concrete in an excavated hole.

To elaborate, most pier foundations are constructed by boring a cylindrical hole in the soil with an auger, then placing concrete in the hole (usually with reinforcing steel embedded). The purpose of a pier foundation is to bore through soil with lower bearing capacity until you reach strata containing better material that can support a heavier load.

As with piles, piers can also extend above ground level and be used to elevate your building. In this way, they serve as defacto columns as well, helping get your container off the ground without having to use separate metal supports.

Piers are the most popular shipping container foundation and the type we recommend for most people. They are relatively inexpensive, DIY friendly, and quick to construct. If you really want a workout, you could even dig the holes by hand with a post-hole digger!

The two main downsides to pier foundations for containers are that you do require soil with good bearing capacity and that the piers have little uplift resistance. But both of those downsides can actually be addressed as this next foundation type shows.

Concrete Footing (Permanent)

Sometimes called an isolated spread footing or a bell footing, a footing foundation takes isolated point loads and spreads them out over a larger area. Think about the way the bottom of your foot is much larger than the bottom of your leg, just above your ankle. The increased cross-sectional area gives stability while reducing ground pressure (and thus, a lower bearing capacity soil can be used).

Footings are typically used in conjunction with concrete piers. Essentially, it's just enlarging the bottom of the pier so that it is pressing down on a larger area of soil.

To construct a footing, you need to excavate a hole wide enough for the appropriate footing width/diameter. Then, you set up forms for the footing itself as well as the necked down column that goes up to ground level. After you pour the footing and pier and the concrete has cured, you backfill soil on top of the finished footing.

For more Flat Packing Container Houseinformation, please contact us. We will provide professional answers.

Due to this backfilling, it's basically impossible to tell after the fact if a pier has a footing below it, or is just a straight pier. However, the backfilled soil serves more than just an aesthetic purpose. A pier attached to a backfilled footing adds considerable uplift resistance, whether due to wind, flooding, or frost heave. 

You may be wondering why you'd go to the trouble of excavating a hole large enough for a footing if you're just going to backfill much of the dirt around the much smaller column afterward. It's a good question, and you could just build a traditional straight pier, sized the same as the footing you were going to build. The difference is that a footing will save you a lot of money on concrete due to the thinner diameter pier above it.

Assuming you want to use a footing foundation, there are actually a few shortcuts to help you build one. First is using plastic concrete footing forms that speed up the process of building footings and ensuring consistency. The products work with tubular concrete forms (Sonotubes) and are available from companies like Redibase, The Footing Tube, Square Foot, and Big Foot Systems.

Another shortcut for constructing a concrete spread footing is using a bell auger. After digging a traditional straight hole with a regular auger, a special bell auger can be lowered into the hole and scrape out additional soil to expand the size of the footer's footprint at the bottom of the hole. The bell auger works by keeping a narrow profile, then expanding once it touches the bottom of the hole.

Finally, there are even precast footing and piers available if you don't want to deal with mixing concrete. Instead, you just stack sections of precast cylindrical concrete blocks that are tied together with a metal threaded rod in their centers.

Concrete Slab Foundation (Permanent)

Slab foundations (also called slab-on-ground foundations) are popular for traditionally constructed houses because they not only support the structure of the home, they give you a smooth concrete floor. This prevents you from having to add floor joists and a subfloor to your home. But a container already has a plywood subfloor built-in, so this benefit is wasted for a shipping container. 

Furthermore, a slab requires a lot of extra concrete, and that means extra money. While you don't have to dig as deep as with other types of foundations, you still need to dig.

Stronger slabs are typically thicker on the edges, where the most weight is bearing down. So, you'll need to effectively dig a trench around the perimeter of your slab foundation.

Aside from the cost, there are some other unique factors of slab foundations that may affect your container project. On the plus side, a slab keeps underneath your container sealed so there isn't room for vermin or insects. 

But this benefit can also be a drawback. With a slab you'll need all of your below container utility lines to penetrate the slab, causing more planning and an inability to adjust them later. If you ever wanted to add a sink for example, it would be very difficult and expensive with a slab, but comparatively simple with piers.

And in northern climates, a slab won't be ideal for handling frost heave. You'll need to do a lot of digging to get the perimeter of the slab below frost depth, and might be better off building a concrete basement instead.

Concrete Strip Foundation (Permanent)

A strip foundation (also known as a trench foundation or a continuous footing foundation) is basically what would result if you placed a bunch of piers with footings all next to each other. You end up with a linear footing that sits under a narrow, linear beam above.

A strip foundation can go around the entire perimeter of your container if you want, sealing the resulting crawlspace off from the outside. Or, you can two strip foundations that support the container at each of its two short ends.

Because a strip foundation contacts the ground along a line rather than at a point, it has a much larger ground contact area. For this reason, it works well on soils with less bearing capacity, where a pier and footing would need to be quite big.

How to Design a Container Home Foundation

With a good understanding of why you need a foundation at all as well as the types of foundations that are possible, we've now arrived at the third major step: designing the foundation. In this section, we'll describe the different factors that affect foundation design.

Understanding Foundation Loads

The weight of your container and everything in it is pushing down on the ground due to gravity. Your foundation stands in between, ensuring all these forces are more evenly distributed into the soil. But knowing how to size your foundation depends on understanding the forces acting on it.

• Dead Loads: Permanent loads that include the weights of materials used in construction and all fixed equipment. Things like floors, walls, roofs, plumbing, electrical, and HVAC equipment. Remember that things permanently affixed to your roof, like a green roof or solar panels, must be included in the dead load.
• Live Loads: Loads that are not permanently applied to a building, but that are likely to occur. Things like the weight of occupants and their possessions. Loads related to construction or from environmental factors (shown later) are NOT live loads. On a roof, a live load has to include the weight of maintenance workers, for example.
• Flood Loads: Loads related to floating, lateral movement, or collapse caused by floodwaters. Understanding flood loads requires analysis of the flood zones of your property and the design of your structure.
• Wind Loads: Loads from the pressure imparted due to wind, typically laterally against the building. Wind loads vary greatly based on your regional geography in addition to the placement of your structure on the land related to trees, hills, etc.
• Snow Loads: Snow loads are the weight imposed by accumulated snow on the roof of a building.  This can range from zero in tropical areas to well over 100 pounds per square foot in northern climates. 
• Seismic Loads: For structures built in areas that have historically experienced earthquakes and other seismic activity, the loads caused by these natural events must be included and analyzed as part of your foundation design.

An engineering analysis of a potential foundation will account for all of these loads (and potentially others). The loads are combined using equations based on different civil engineering theories including Strength Design, Allowable Stress Design (ASD), and Load and Resistance Factor Design (LRFD). It's unlikely that all of these loads will apply maximally at the same time, so these equations give engineers a way to aggregate the various loads into one number that will determine the overall load that your structure will impart on your foundation.

Obviously, this can get quite complicated and is beyond the scope of what a DIYer could determine. Thankfully there are some shortcuts and assumptions that you can use for a lot of standard container building cases.

Part of the reason we recommend using a factor of safety of at least two in your calculations is so that you don't need to calculate all of these loads more exactly. For smaller projects, it's often simpler and cheaper to assume the loading is a bit higher than necessary rather than paying for all the analysis to determine exact numbers (unless this is required by your authority having jurisdiction).

For instance, as a rule of thumb, we like to assume that the dead loads of a container home are approximate twice the empty weight of a shipping container. If you have a large secondary roof, significant solar panels, etc. that significantly alters the weight of your container home, this assumption will need to be increased.

To account for live loads, the IRC in Table R301.5 Minimum Uniformly Distributed Live Loads specifies, among other things, that sleeping areas have a live load of 30 pounds per square foot, and other residential areas have a live load of 40 pounds per square foot. These assumed live load pressures when multiplied by the sizes of your rooms will give you the live load forces.

The environment loads (floor, wind, snow, seismic, etc.) are much more difficult to account for since they vary so much. Section R301.2 Climatic and Geographic Design Criteria in the IRC gives numerous tables and maps that help to determine some of these environmental loads. But they ultimately require additional analysis that is best done by a professional, if you have significant exposure to these environmental factors.

Accounting for Frost Heave in Foundation Design

Almost all soil has at least a tiny bit of moisture or water in it. Soil is made of particles, and the interstitial areas between the particles are great places for water to collect and bind the soil together. If the temperature of the ground gets cold enough, for long enough, and there is enough water in the soil, that water can freeze. And as you probably learned in elementary school, when water freezes, it expands.

Frost heave is the name building professionals give to the phenomenon of a foundation being literally pushed upward by the freezing of soil. As you can imagine, having your foundation pushed upward is not good for the structural integrity of your building. So it's vitally important to understand under what conditions frost heave might affect you, and if so, how to deal with it.

The primary determining factor for frost heave is the frost line. The frost line is a soil depth specific to your geographic area above which there is a chance of the ground freezing and causing frost heave. The frost line can vary from 0 inches in parts of Hawaii and Florida, to as much as 100 inches in parts of Alaska.

The map below gives you a rough idea of the frost line in different parts of the United States. However, factors like climate, elevation, and soil type can drastically affect these numbers. Therefore, it's best to find the frost line from local building officials, the water department, local building contractors, or other professionals that know the details of your area.

Once you know the location of your frost line, you really have two ways to account for it in your foundation design. The most common method is via foundation depth.

Essentially, your foundation footers, AKA the lowest points of your foundation, should be at least 12 inches below the undisturbed soil depth (Per R403.1.4 Minimum Depth in the IRC) and at least six inches below the frost line. However, in some northern climates, this can be quite deep!

As an alternative, there is the Frost-Protected Shallow Foundation (FPSF). You can read more about it in Section R403.3 of the IRC, but essentially it involved placing foam insulation around the concrete of your foundation to insulate it from the colder soil. In exchange, your footers don't have to be as deep.

How to Attach Shipping Containers to Foundations

It is not enough for your container to simply rest on top of a foundation, it needs to be attached to it. Attaching a container to a foundation will ensure the container can't slide laterally or be lifted off the foundation.

There are a few common ways to anchor a shipping container to a foundation. Deciding between them depends on if you want your container permanently attached or semi-permanently attached. Either way, you're going to need some metal embedded in your concrete foundation.

The most popular way to attach containers to the foundation is by welding the bottom of the container (at a minimum, the corners) to a large steel plate. The steel plate has long metal rods (shaped like an L, J, or with a larger piece at the bottom) welded onto its bottom, and the whole assembly is placed into the wet concrete.

This gives a smooth upper surface for the top of the steel plate so the container can be moved into the correct position without hitting any fasteners. Therefore, you don't have to be quite as exact with your measurements. As long as you have the center of the plates in approximately the right place, if your measurements are off by an inch or two, the container will still be on the steel plate with a clear line of force through the foundation.

Alternatively, you could pour the foundation and after curing, drill holes into the concrete. Then, you embed the steel plate's anchor rods using either a mechanical expansion anchor or epoxy. In this case, the rods have to be straight (no L or J bolts), and you're relying on the mechanical or chemical bond in the drilled hole.

Another way to attach your containers is using bottom twist locks. Yes, these are the same mechanisms that are used to secure containers on huge cargo ships, but they can also be used for a foundation as well.

In this case, the steps are similar because the twist lock itself will also mount to a steel plate (either welded or with a dovetail fitting). You could embed J or L bolts into the wet concrete and then attach the plate with nuts after the concrete cures. The twist lock is elevated an inch or two above the plate, so you don't need to worry about the container hitting the nuts. You could also drill holes in the cured concrete and epoxy in the anchors as discussed previously.

Either way, with a twist lock, the container is very securely attached to the foundation. But you still have the option of unlatching the lock and moving the container, should you ever need to.

There are two main downsides to using bottom twist locks. First is that your measurements have to be exact, otherwise the container won't actually fit onto all four twist locks! Second is that twist locks only mate with the container corner fittings, so if you want additional points of contact between the container and foundation, a twist lock can't be used.

If you like the idea of twist locks but aren't so sure about getting the measurements exactly right, consider side twist locks. With a side twist lock, you can set the container onto the bare concrete foundation, then insert the twist lock on the side of the container corner fitting and attach it to the foundation with a drilled hole and epoxy anchor, for example.

As we discussed above regarding temporary foundations, some people choose to place the containers onto the foundations, where they are just held in place by their massive weight. In most cases, this is probably fine temporarily, but you should know that floods and tornados can move a container. So, we definitely recommend a more secure attachment.

Number of Foundation Attachment Points Needed for a Shipping Container

The obvious next question (and one we previously alluded to) is how many of these attachment points does your container home need. It's a complex question, but we'll give you our thoughts.

On one hand, you know that an empty shipping container can safely hold tens of thousands of pounds of cargo, a much higher loading than a residential use would require. However, using a container for a residential purpose almost always involves some modifications to the container.

Every time you cut out windows and doors or make interior openings to join adjacent containers, you weaken the structure of the container. We always recommend adding structural steel back to the perimeter of these new openings to make up for what's been removed, but still, you've fundamentally changed the container. Next are other modifications, like stacked containers that don't have a vertical load path to the foundation due to perpendicular containers. 

Finally is the deflection. Every container is going to move under loading, whether it is inches, millimeters, or microns. But if it moves too much, even if it is safe, you might be able to perceive it as a sort of springiness in the floor. Know that this is unlikely but still worth noting.

Under maximal loading, the bottom of a container's side rails are allowed to deflect nearly one inch vertically. With a modified container, this much deflection, or more, might result from an even lighter loading. Remember though, that discernable deflection must come from live loads that are added and removed; dead loads will permanently cause the container to move during construction but it will move to a stationary deflection point.

With all that said, it's unlikely you're going to be able to feel a container move under everyday use. Still, for longer containers (40ft in length or more), it's common to use an intermediate set of foundation attachments halfway down the long side of the container. These additional attachment points will reduce deflection and add strength.

To summarize, for containers greater than or equal to 40 feet in length, four foundation attachment points are probably sufficient for most projects, but six is even better without much additional cost. If you have extensive stacking, container modifications, or other extensive modifications, you may want even more foundation attachment points.

Container Foundation Cost Analysis: Design versus Material

You've heard us mention several times now that a lot of the complex structural analysis required for a container foundation needs to be conducted by a professional engineer or architect. As you can probably guess, this can easily eat up a lot of your budget on smaller projects.

Though you may not have thought of it in quite these terms, you have a decision to make. You can invest in hiring a professional to design a high-precision foundation that will take all the specific loads into account. Or, you can make some conservative assumptions, use a large factor of safety, and end up with a foundation that will probably be structurally sufficient but maybe 'overkill'.

The fact is that it's almost impossible to have a foundation that is too strong. The real risk you run is overspending on construction. But on smaller projects, the cost of a bit more form boards and extra bags of cement is less than the cost of a full geotechnical and structural analysis.

In other words, if you just assume the worst case and overdesign/overengineer your foundation, you should be fine. But, if your container home design is complex to the extent that you really need an engineer to assist with the design of the building above the foundation, it's not going to be much more added expense to have them go ahead and design the foundation itself as well.

Container Home Foundation Construction

With a container foundation design complete, you then have to build it. If you hire a company to do this part, most of these problems will be addressed by their employees. But it's still a good idea to know what to look out for. 

Soil Compaction

If the secret to a good house is a good foundation, then the secret to a good foundation is a good base material. We've already shared that you need to excavate your foundation to get past the surface soil and get below the frost line if you have one.

But anytime you dig soil, you disturb the layer of soil just below what you've removed by introducing air. This soil will eventually compact again, either from the weight of your home settling or from you compacting it prior to pouring the foundation. The latter is obviously preferable!

At the beginning of this article, we shared a graphic that showed how the bearing force on the soil was highest directly under the foundation, then lowered as you went down in depth and the load was distributed over a larger amount of soil. 

Concrete Type

There are several types of concrete, also known as mixes. Concrete is a mixture of water, cement, sand, and aggregates (gravel and stones), plus some chemical admixtures. Varying the amount of each of the components affects the properties of the resulting concrete, such as its strength and cure time.

If you hire a professional to design your foundation, they will specify the concrete mixture and strength required based on the loading and geotechnical report. Otherwise, you can look at the IRC's Table R402.2 Minimum Specified Compressive Strength of Concrete for an idea of the strength of concrete you'll need based on your location and weathering potential.

If you are mixing small quantities, then you can either do this by hand or by using a portable cement mixer. For anything more than about a cubic yard or two, consider having the concrete delivered directly to your site, ready to use.

Example Foundation Design

It's time to put everything from the article together and design a sample container house foundation. We're going to assume this container home is built with one 40-foot container and has no secondary roof, green roof, etc.

The home will be located at a site we've randomly selected in Central Texas. For our foundation, we're going to use concrete piers.

To start, we login to the Web Soil Survey and zoom into the general area of our building site. Then we Area of Interest (AOI) toolbar button to draw a shape around the specific area we want to build.

Next, we switch over to the 'Soil Map' tab, where we can see the type of soil in the area. In this case, it is 'Singleton fine, sandy loam, 1 to 3 percent slopes'. That gives us some information, but we need more detail.

So now, switch over to the 'Soil Data Explorer' tab, then the 'Suitabilities and Limitations for Use' subtab. Within the 'Building Site Development' dropdown menu, there are several development types to choose from. Since we're building a residence and aren't planning on adding a basement, we select 'Dwellings Without Basements'. 

We're gives a rating in the table based on the proposed usage and soil type in the area of 'Very limited' with the rating reason of 'Shrink-Swell 1.00'. This tells us that our building site isn't ideal because the soil has a risk of shrinking and swelling. This doesn't mean we can't build here, but that we need to be extra mindful of soil compaction, concrete reinforcement, and moisture control around our foundation after construction.

Finally, we go to the 'Soil Properties and Qualities' subtab, then the Soil Qualities and Features dropdown menu, then select the 'Unified Soil Classification (Surface)' item. Here, we learn that our soil has a UCS rating of 'ML'. While there is way more data to explore in this system, this is all we need for now.

We can find out that 'ML' in Table R401.4.1 from the IRC corresponds to a bearing capacity of pounds per square foot. Now, we need to figure out the loading from our container home.

Previously, we said that the assumed dead loads for a standard container home are twice the empty or tare weight of the container it's built from. From our Container Dimension article, we know that a standard 40-foot container has an empty weight of about 8,400 pounds (and while we're looking, we'll also record that it has a gross square footage of 320 SF, taken from 40 feet in length times 8 feet in width). So, we'll double that to get a dead load of 16,800 pounds.

Next, we'll calculate the live load. We previously discussed how Table R301.5 Minimum Uniformly Distributed Live Loads tells us that sleeping areas have a live load of 30 PSF, and other residential areas are 40 PSF. To make the calculations simpler, and so that we can make all of our concrete piers the same size, we'll just assume the worst case and use the higher loading of 40 PSF. With 320 gross square feet multiplied by 40 PSF, we get 12,800 pounds of live load.

To make this DIY-friendly, we'll ignore other environmental loads (wind, snow, flood, etc.) and instead increase our factor of safety to account for these. So, our equation would be Combined Load = (Dead Load + Live Load) x Factor of Safety, or Combined Load = (16,800 + 12,800) x 2 = 59,200 pounds. 

Now that we know both the total combined loading and the soil bearing capacity, we can easily calculate the total size of foundation footers needed by dividing the two: 59,200 pounds / PSF = 39.5 SF. If we round that up to 40 SF and know that we want to have four piers, that gives us a bearing area for each pier of 10 SF.

To get a pier footer with 10 SF of bearing area, we could build square forms approximately 3.2 feet (or 38 inches) long on each side. On top, a cylindrical pier with a diameter of 12 inches would work fine.

From the frost depth map we shared earlier, this location is right on the line between the 6-inch deep and 36-inch deep regions. We'll go ahead and assume the worst case (36-inch deep), although you could call a local building official and try to get a more exact number. So, this tells us that our footers need to be 36' below ground level. A good rule of thumb is that a spread footing should be at least as thick as 1/3 of the footing width (or 6 inches thick, whichever is greater), so we'll go with a thickness of 1/3 of 38 inches, or 13 inches.

If these footers seem too big, or you'd like to use some of the premade plastic footers we discussed earlier, you could use a larger number of footers that are smaller in size (and simply space the extra piers down the long sides of the container evenly). For instance, we could use six of the Square Foot SF32 plastic footing forms, which each have a bearing area of 7.11 SF (32 inch x 32 inch). Six of these forms give us a total bearing area of 42.67 SF, over our requirement of 40 SF.

To be more precise with these pier/footer sizes, a structural engineer would combine live, dead, and environmental loads using Section Load Combination of the IBC. This would most likely result in a smaller foundation with less concrete required. In this example, we're over-designing in exchange for under-analyzing.

An engineer will also give you precise sizes and locations for your steel reinforcement (rebar). In this case, we'd recommend something similar to this illustration, with vertical rebar in the pier that flares out into the footing.

And once again, we're also making some huge assumptions here:

• We're overlooking geographic areas with huge amounts of snow accumulation, hurricane-force winds, flood zone potential, seismic design requirements, etc.
• We're ignoring unique container home designs like large roof overhangs, cantilevers, stacking, etc.
• We're not considering localized soil features with large amounts of organic material, previously disturbed soil that isn't compacted well, and other factors that cause drastic settlement issues

In extreme cases like these, please consult with a professional engineer, because it's possible our analysis could produce a foundation that is undersized for these extreme requirements. Our example here is just for illustration purposes and any analysis you perform is undertaken at your own risk.

Summary

Now you're clear on why you need a foundation in the first place, and how to select the proper one for your build.  We made our selection of foundation types based on those that are most commonly used and the most DIY friendly, since many of you reading this article are self-builders!

You should also understand how to mix the cement for this type of foundation. Make sure you pay special attention to the advice regarding how to lay in extremely hot and cold climates as this can make or break your container's foundation.

The next step now is getting your containers delivered and installed on your foundation.

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