PEMB Foundations
The Ultimate Guide to Choosing the Right Foundation for Your PEMB Project
What’s the best type of foundation for your pre-engineered metal building?
To answer that, you’ll need to answer some preliminary questions:
1What is the purpose of the structure?
2How big will the structure be?
3What kind of load will it need to bear?
4What’s the climate like in the area?
5What’s the terrain like?
6What type of soil does the site have?
7What do the local building codes require?
It’s a lot to think about. But getting this decision right will be critically important for the long-term stability of your PEMB.
People who choose to erect a PEMB aren’t always thinking about the foundation; they’re thinking about the building itself and what they intend to do with it. As a result, the foundation may not get the attention it requires. That’s when mistakes happen. And obviously, once the building is up, mistakes in the foundation can be extremely hard to remedy.
Starting at the Bottom: Testing the Soil
Soil tests can reveal a number of things that are critical in selecting the right type of foundation, including:
➜ COMPOSITION OF THE SOIL
➜ Moisture content
➜ Loadbearing capacity
➜ Drainage issues
There are several types of soil tests that can help inform the choice of a foundation. Their technical nature calls for the involvement of a soils engineer.
A soil compaction test (also called a soil density test) will determine if the soil is firm enough to withstand the expected load. If it’s not, and the soil settles under the weight of the structure and its contents, that can lead to a number of problems, including:
✘ Foundation cracks. Besides compromising the stability of the entire structure, cracks in the foundation can allow water and pests such as mold and termites to enter the building. These, in turn, create their own sets of problems that must be remedied, often at great expense.
✘ Walls out of plumb. Doors and windows may not open and close properly. The stability of the building may be affected.
✘ Uneven floor surfaces. This can lead to costly accidents or damage to the building’s contents, as well as reducing the building’s resale value.
Water in the soil can be pushed out under the weight of a building, causing the soil to settle. Conversely, if moisture accumulates after the structure is built, it can cause the soil to swell, also disrupting the foundation. Verifying that the soil has adequate drainage capacity is thus very important.
Certain types of soil are particularly prone to settlement:
SANDY SOILS are composed of large particles that compact easily when pressure is applied.
SOFT CLAY is also prone to compaction, as well as swelling when moisture is introduced.
FILL SOIL from another location may be inadequately compacted to sustain a building’s weight.
In regions that experience extreme cold, the soil can be subject to frost heave, as water in the soil freezes and expands. This swelling can severely damage the foundation and must be taken into account early in the construction process.
In the far north, ground a few feet below the surface is subject to permafrost. This poses a challenge for builders, as the weight and heat of a structure can cause the frost underneath to melt, rendering the building unstable.
In uneven terrain, soil erosion can undermine the foundation, causing all the problems described above. Thus, extra caution is needed when a building site is hilly or sloped.
Other Factors Affecting the Choice of Your PEMB Foundation
The climate of a region is also important to consider in choosing a foundation. Areas subject to strong winds call for stronger foundations. Metal buildings are relatively lightweight and therefore more vulnerable to windstorms than concrete structures. And areas subject to heavy rainfall can cause problems for a foundation, as water accumulates in the soil, rendering it unstable.
The seismic characteristics of a region are also important to consider. Clearly, areas prone to earthquakes require stronger foundations.
These factors are usually reflected in local building codes, which typically specify the acceptable type and depth of foundations for all types of structures. To ensure a building’s soundness, and to avoid penalties and legal issues, these local requirements should be scrupulously observed.
With all this as background, here are the main choices available for a PEMB foundation:
Three Types of PEMB Foundations
Slab-on-Grade Foundation
Slab-on-grade (also referred to simply as slab). This is a flat concrete slab poured directly on the ground, matching the footprint of the building. The depth of the slab may be greater at the edges to enhance its stability. It may also be reinforced with steel rebar and wire mesh. Often, the slab also serves as the floor of the structure. The simplicity and relatively low cost of this design make it a popular choice for smaller metal buildings. Being in direct contact with the ground, a slab foundation is particularly vulnerable to soil settling, which can lead to cracks and other related problems. Additionally, wiring and plumbing that lie in or under the slab can be hard to reach without damaging the slab itself.
Pier Foundation
Pier. In this design, concrete piers are sunk deep into the ground to support each of the frame columns. This approach is popular for structures that aren’t fully enclosed or don’t require a floor, such as barns or riding arenas.
Perimeter Wall Foundation
Perimeter wall (also known as stem wall). A cement structure is placed around the perimeter of the building footprint, providing support for the entire structure. In the commonly used T-shape design, a wide concrete footer is first laid beneath the ground surface (below the frost line to avoid expansion and contraction of the soil). The stem wall, made of poured concrete or block, is placed on top of this footer, and the building itself on top of that. The building load is thus transferred from the walls and roof of the structure to the footer. This design allows access under the building for pipes, ducting, and wiring. Its excellent load-bearing capability makes it popular for situations that require exceptional strength and stability.
There are many variations to each of these designs, and they can also be used in conjunction with each other. The appropriate choice for any particular structure depends on the many factors mentioned above.
Laying a proper foundation is critical to a building’s stability and longevity, so it should always be done using the services of a structural engineer.