Foundations for Steel Buildings
Regardless of what type of steel building you choose, you will need to consider steel building foundation options before constructing your building.
This article will help guide you through the foundation selection process. This selection should be carried out carefully for any type of steel building.
Durability and sustainability of the building are very much dependent on how well the foundation is designed and how well it is constructed. An ideal foundation should transfer the dead and service loads of the buildings effectively to the soil beneath without incurring any structural damage. This includes avoiding settlement and/or tilting of the walls and building frame.
Initial Building Foundation Considerations
Before initiating the selection process, I’d recommend you write down the answers to a few variables below. There may be some additional considerations that your building expert will ask you about, but there are a good starting point:
- The type of soil and its bearing characteristics. As the soil having immediate contact with the footing would be bearing a greater share of stress. So ensure to excavate any dirt poor soil and replace it with a high-quality soil if required.
- The loading conditions depending on the type of usage. You must be clear about how many vehicles would be parked on the foundation and any other heavy items being stored on it.
- Wind, Snow, and Earthquake conditions. As the steel building would be anchored with the foundation, any lateral or uplifting load is directly transferred to the building. Because of this if you live in an area prone to earthquakes, then this is something to let your building rep. know. Also if you happen to be in an area with heavy winds, or snow loads then those need to be factored in since any pressure put on the building is transferred down to the foundation itself.
- The underground local frost line: The foundation for occupied structures should have to go below the maximum expected frost penetration depth. The reason is simple; when the soil below a foundation freezes, the water expands and 'heaves' the ground upward. This can exert uplift stresses and deteriorate your structure.
- The area of the building and the size of the plot: The land can be surveyed for grading and area. This may seem simple, but be sure to get the tape measure out at the very least and ensure you have enough space. Not only space for the building itself, but for a perimeter around it as well.
Steel Building Foundation Options
Slab Foundations (Floating Foundation)
Slab foundations are also sometimes called floating slabs or floating foundations. This is because of the fact that it literally floats over the undisturbed soil. If large enough, it can span patches of poor bearing soils. This feature makes it an ideal choice for softer soils, like in coastal areas that could sink or settle unequally.
If you're working on a budget then a slab foundation is likely your best choice. As an economical solution, this floating type of slab foundation is very popular and common. It can be poured easily and quickly. If you’re planning to construct a shed, a garage or some extension work which is not wider than 60 feet than this is the most suitable option. Unlike other types, it does not require any extensive excavation and also fewer labors are adequate.
Often, on the exterior perimeter of the slab foundation, a thickened reinforced edge beam is added. It's referred to as a grade beam. The thickness of the slab would be decided by the foundation engineer, keeping in view the above-stated inputs. Typically a slab in between 4“ to 6” would be enough.
Reinforcements are embedded in the concrete. Typically this includes #3 rebars with a center to center spacing of 24 inches plus a reinforced steel mesh. If you intend to store a particularly heavy load on the steel building foundation then you must use concrete of high compressive strength i.e. about 4000 psi. For normal usage, 2500 psi could do the job.
If you have a problem fixing the reinforcement mesh in the concrete slab; you can use fiber-reinforced concrete which has considerable tensile strength when compared with that of un-reinforced concrete.
There are two main ways to go about a Pier Foundation. You can either construct your steel building directly on pile or pier, or you can embed the pier in the floating concrete slabs. It looks similar in style to the foundation of decks for the building. In slab foundation type, the top of concrete slab serves the purpose of the floor while in this type the floor is left as dirt or rough gravel.
This type of foundation is ideal for agricultural sheds and open pavilions where an enclosure is never needed. The most common type of pier foundation has a grade beam that ties together all the piers which help in resisting horizontal column reactions. Pier foundation is comparatively more expensive than slab foundation but it offers resilience against wind uplift pressure and seismic stress reversals.
Perimeter Wall Foundation
Referred to as standard foundation, this type is used for steel buildings wider than 60 feet. You can also call it a T-wall foundation, perimeter wall foundation or frost wall foundation. The strip which is wider than the width of the perimeter wall provides stability by distributing the load over a larger area. The columns of the steel buildings are anchored through embedded bolts in concrete pedestals in the strip wall. This type of footing should run below the maximum anticipated frost line.
You can cast this foundation in three stages; first, the strip is cast followed by concreting the perimeter wall. After curing, the plinth of the building is backfilled and the floor is afterward cast. This type of foundation requires labor force and is time-consuming as you’d require fixing formwork and waiting for the curing of the concrete.
Miscellaneous Foundation Options
The three classes of the foundation, described above are the most common. There are some additional accessories or options you can consider, however. These are specific to steel buildings in most cases.
This foundation option is for arch style or quonset hut kits. It's a metal frame which is embedded into the concrete foundation. Your arch panels can then be bolted onto these baseplates instead of being embedded directly into the concrete itself.
This can help lengthen the lifespan of your steel building since the building panels will have less contact with any runnoff water on the ground. It also makes your building more portable since you can easily unbolt your building and move it elsewhere.
This option of the foundation is for the temporary nature of buildings that are to be shifted from one place to another. It offers all the prerequisites of foundation for a temporary structure including easy, fast and cheap installation. It usually comprises of a 14 gauge steel sheet that is affixed with the concrete perimeter. The steel structure is bolted with this temporary base plate foundation.
This is actually a broader classification of foundation options for steel structure; it includes all ground mount or concrete mount foundations including Slab foundation, Pier Foundation and Perimeter foundation described above.
Whatever steel building foundation options you're considering, the best course of action is to consult a professional. A steel building expert will likely have pre-drawn engineered plans for a foundation similar to one which will fit your needs. If not then they'll have an engineering team available to help draw up plans.
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