NOTE: Foundations and anchor bolts are NOT included as part of the metal building kit package.
A well-designed and constructed foundation is crucial for any building, but it is often given much less attention than the above-ground metal structure that comes in the kit. Many prospective buyers are unsure how to proceed with foundation design and construction.
Foundations are critical to building performance. A poor foundation can lead to the walls being out of plumb, floor settlement, and damage in earthquakes and wind events. These issues, in turn, can lead to safety problems, very expensive repairs, or the building having a much shorter lifespan than it could have had. Sometimes, it is less expensive to demolish the building and start from scratch than to attempt to fix poorly designed and built foundations.
Foundations for pre-engineered metal buildings are different – and generally more complicated – than those for conventional structures.
Therefore, you must engage the services of a licensed structural / foundation engineer experienced in their design.
You will need to find an engineer locally to design these foundations and a reputable contractor to build them.
Here are some critical points for having the foundations for metal building systems designed and built:
- All metal buildings require a concrete foundation and, typically, slabs on grade. (Some agricultural buildings and sheds might not need concrete slabs.) The type of foundation and slabs will be dictated by the intended use of the building and its size and location.
- Have the soil analyzed to ensure you have good support for the foundations; bring in better quality fill if necessary, as discussed in our construction site preparation article.
- Be sure to comply with all applicable building codes and regulations. Your local building code will specify the minimum foundation depth in your area (for frost protection), the minimum thickness and strength of concrete, etc. The code and local regulations might specify the maximum size of a building that may be erected without securing a building permit. For example, the 2021 International Building Code (IBC) requires a permit even for constructing small structures like tool and storage sheds and utility buildings with floor areas larger than 120 square feet.
- In most cases, the foundation should be designed by a licensed structural engineer, as already stated. Except for small metal buildings, such as storage sheds less than 120 square feet, designing and building foundations on your own is NOT a good idea. Any foundation failure or distress would be very difficult and expensive to fix, so this is not the place to save on professional services.
- The structural engineer will know the best practices for designing metal building foundations. Unfortunately, not every engineer knows the specific challenges of designing these foundations. Steel building foundations are different from conventional building foundations. Request that the engineer follows the practices in the “Foundation and Anchor Design Guide for Metal Building Systems” by Alexander Newman (McGraw-Hill, 2013). Discuss the best type of foundation for your project with your foundation engineer.
- The type of foundation for your metal building will depend on the factors listed below. Several types of reliable metal building foundation systems include moment-resisting footings, footings with tie rods, mats, and deep foundations (drilled shafts or piles). There are several unreliable but cheap foundation types, such as those with hairpin bars or haunched slabs. These may be common, but you should avoid them because their use might not comply with the codes.
NOTE: The engineer will need the metal building engineered drawings provided by your building kit supplier to work with.
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Metal Building Foundation Design
Factors That Affect Foundation Design
The soil at your site will need to be investigated. You need a good foundation for your foundation! The underlying soil characteristics are a critical factor in the stability and longevity of your pre-engineered building. The process is described in our Site preparation article.
The frost line must be known. Your foundation will need to extend below that line.
Wind Loads. Unlike other construction types, metal buildings are very lightweight. They need to be held down by substantial foundations; otherwise, a severe wind storm could compromise the structural integrity of the building. Think of a metal building as a huge sail kept in place by a few sturdy anchors. In many cases, the foundations need to be heavier than those for other buildings because of the required “ballast” that resists wind uplift and lateral forces.
Snow Loads. The snow loading at your site will also affect foundation sizes, this time for downward loading.
Seismic Loads. Building codes require certain specific foundation designs in zones of high seismicity.
See our article Building permits, codes, and loads for more information.
Foundation types for larger metal buildings might differ from those for smaller ones because larger buildings would exert higher loading on their foundations. Naturally, your foundation engineer will need to know the dimensions of the building you plan to buy and the locations of anchor bolts for the attachment of the steel columns.
The possibility of future expansion or adding weight
If you have plans for future expansion or adding loading on the steel building, it is wise to design the foundations to accommodate those plans. It would be much more difficult and expensive to enlarge the foundations after the fact.
Who is Responsible for Foundation Design?
As noted above, the foundation design should be performed by a licensed structural engineer experienced in designing metal building foundations.
Designing and placing quality foundations is something that requires special knowledge and skill.
Are Foundation Plans Required for a Metal Building?
Yes, you will need foundation plans drawn up by a licensed structural engineer. The metal building kit comes with detailed plans -- but NOT foundation plans. A foundation plan, sealed by a professional engineer, is typically required as part of your building permit application. (See our article Planning a metal building project.)
Your concrete contractor also needs the foundation plan and foundation details to prepare an accurate quote for the job based on the foundation system required for your building.
Foundation Design Process
The building supplier typically provides a list of the building reactions (the forces exerted by the primary frames) under dead, snow, and wind loads (plus other loads, as needed). The foundation structural engineer uses these reactions to select the most appropriate type of foundation and ensures the foundations can safely resist the loading imposed on them.
There are two main differences between the foundations for conventional buildings and those for metal building systems:
- As already mentioned, the need to resist wind uplift requires relatively heavy foundations.
- Most metal buildings use rigid frames for primary framing, and those exert not only vertical forces on the foundations but also horizontal forces that tend to push the foundations sideways. This is not a common problem in conventional post-and-beam construction.
Thus the foundations for your building might be much more substantial than – and different from – the foundations for, say, a large house or a two-story office where large concentrated uplift and lateral loads are either absent or overcome by the weight of the building.
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Metal Building Foundation Options
There are several types of metal building foundation. Some of them are more expensive but perform better. Others are cheap but unreliable and should be specified with extreme caution. Also, what might work for a small building, would not be appropriate for a large commercial steel frame structure.
The most common steel building foundation types are:
Tie Rods + Column Footings
The foundations at the opposite sides of the rigid frame are connected, which “extinguishes” the horizontal column reactions at both ends. Tie rods range from cheap and less reliable, such as a couple of reinforcing bars placed in a thickened slab, to more expensive but more reliable designs, such as concrete grade beams. The column footing should be heavy enough to resist wind uplift.
Hairpins + Column Footings
These are similar to tie rods but rely on the slab on the ground to transfer the horizontal forces. While hairpins have been widely used in the past, today’s codes make them less economical. Because the slab now takes a structural role, it must be designed as an elevated structural slab (be reinforced with rebars, not wire mesh, have no control joints, etc.). Also, if the slab is cut in the future for utility installation, the system no longer functions. This system should be used with caution.
This is the most versatile solution for steel building foundations. It is also widely used in non-building structures. The foundation works similar to a cantilevered retaining wall and has a wide base that resists overturning and sliding.
Slab Foundations with Haunch
This system, also known as a downturned slab, has been widely used in house building. It works similarly to the moment-resisting foundation, and a proper design results in the haunch size similar to the footing of the moment-resisting foundation – not the small narrow haunch typically expected by builders. This design should be avoided for metal buildings, except maybe for the smallest ones. This system is impractical in areas with deep frost lines.
Concrete Mat Foundations
Mats are thick slabs reinforced in two directions at the top and bottom. They could be heavy enough to resist wind uplift, and their reinforcement acts as distributed tie rods. Their cost is relatively high.
Pile foundations are costly and are typically used only in poor soils, where shallow foundations listed above are not economical or practical.
Deep Pier Foundations (a.k.a. drilled shafts or caissons)
As piles, these deep foundations can be used at sites with poor soils. They are reasonably economical and are popular for both steel building foundations and nonbuilding structures like highway signs and poles. Because deep piers do not depend on slabs on the ground, they are appropriate for agricultural buildings. Deep piers can be used in combination with perimeter grade beams.
Regardless of which system is chosen, the foundations should be heavy enough to resist the forces of wind uplift, not just be designed for downward loading.
Foundations for Small Metal Buildings
This category includes residential metal garages, storage sheds, and similar structures, typically not exceeding 30’ x 40’ in size. As noted above, even these buildings (if at least 120 square feet in area) will generally require a building permit. They will also most likely require a set of foundation drawings prepared by a licensed structural engineer.
The most appropriate foundation systems for these buildings are:
- Tie rods + column footings
- Hairpins + column footings (use with caution)
- Slab with haunch (use with caution)
- Drilled shafts.
Most of these buildings will also need a slab-on-grade foundation.
Foundations for Agricultural Metal Buildings
This category includes barns, riding arenas, and similar structures, typically with soil floors. As noted above, even these buildings (if at least 120 square feet in area) will probably require a building permit. They will also most likely require a set of foundation drawings prepared by a licensed structural engineer.
Because these buildings generally lack a floor slab, the most appropriate foundation systems are:
- Moment-resisting foundations, and
- Drilled shafts.
Foundations for Large Commercial Metal Buildings
This category includes warehouses, industrial, commercial, and community buildings of moderate and large sizes. Because the loading on the foundations is higher, these structures require the most reliable solutions. A set of foundation drawings prepared by a licensed structural engineer will most likely be needed.
The most appropriate foundation systems for these buildings are:
- Tie rods + column footings
- Moment-resisting foundations
- Drilled shafts.
Most buildings will need a slab on grade and perimeter foundation walls or grade beams extending below the frost line. The building columns will bear on column pilasters (see photo below). All of these items are designed by a structural engineer.
Anchor Bolts for Metal Buildings
Anchor bolts serve two functions:
- They stabilize the building columns during steel erection, and
- They help transfer wind and earthquake forces from the frame to the foundation.
The type, diameter, length, and number of anchor bolts depend on your building. These anchors are of critical importance and need to be designed by a licensed structural engineer.
While in the past, two anchors per column were considered sufficient, today, OSHA requires a minimum of four. Also, hooked anchor bolts were common in the past, but today’s best practices call for anchors made of either all-threaded or partly threaded rods with one or more nuts at the top and at the embedded end.
Accuracy in Anchor Bolt Placement
Pre-engineered buildings, and other steel-framed structures, have tight installation tolerances. If the anchor bolts are not placed in the right location (even by an inch off), the columns won’t fit on the foundations. The installation should be carried out by highly qualified individuals, such as surveyors and field engineers. This is NOT a job to leave to general laborers!
Anchor Bolt Layout
From your building supplier, you will receive detailed plans (drawings and numerical specs) for the type and placement of bolts. You will need to pass these to your foundation designer and contractor so they know exactly where to place the bolts before placing the concrete foundation.
Metal Building Foundation FAQs
Do metal buildings need a foundation?
Yes, all metal buildings require some type of concrete foundation for the building to be anchored to. The type of foundation is dictated by the loading on the building columns and the soils at the site.
Do metal buildings need a slab on grade?
Most metal buildings need a solid concrete floor – a slab on grade (sometimes a structural slab spanning over the soil between pilecaps). Some buildings, such as agricultural storage structures, need only a dirt floor.
When should the foundation be placed?
Winter is NOT a good time to be laying a foundation! Spring, fall, and summer are best. Placing concrete in winter can cause freezing damage. Concrete needs to be above 50°F for several days to cure properly.
If working in winter is unavoidable because of the urgency of the project, it might be possible – with extra work and costs. But if the project is not urgent, it’s best to do the paperwork, planning, and buying materials in winter, so you’re ready to go when it’s spring.
How do you seal a metal building to the concrete foundation?
Some types of agricultural or other buildings are open-sided or semi-open. But for most buildings, you will want to prevent water, debris, and insects from getting inside.
If the building has exterior metal siding, the bottom of the siding should have a base trim in combination with foam closure strips for a tight seal between the siding and the foundation.
Do you need a new foundation if a foundation is already in place?
YES! As explained above, foundations are designed for the particular set of forces the steel structure exerts on them. What are the chances the existing foundation was designed for the same exact set of forces? PROBABLY ZERO.
In any event, if you really want to preserve the existing foundations, there would be significant costs of evaluating them and then planning and implementing any needed modifications. These costs could easily exceed the cost of new foundations many times over.