Preparing a Steel Building Foundation
All buildings start with a strong foundation, and pre-fabricated steel buildings are no exception. While you are waiting for your steel building to be manufactured and shipped to you, you or your builder will need to prepare that foundation. Prior to that, you will have to have the land surveyed professionally so you can be sure the ground is level. The ground must be marked so that your builders know the boundaries.
After the surveying and marking takes place, you can begin grading the building site. This process ensures that the land is perfectly level. You can then excavate for the foundation. If you are not sure what kind of excavation you need for your building’s foundation, then ask the manufacturer of the building. Some buildings require deeper, more extensive excavations than others. The simpler your structure, the less intensive the excavation usually is.
If a minor excavation is all your building needs, you can probably do it with some simple tools like a shovel, pickaxe, and steel rake. You probably only need to go down 2-3”. If you need a major excavation, you will likely recover a bobcat shovel or backhoe (and professional help). Don’t dispense with the earth you remove. Keep some of it so that you have it available to finalize the grading and leveling of the land when you are done. Construction also tends to leave small holes behind which must be filled; this process is called backfilling, and the dirt is good for that as well.
Types of Foundations for Steel Buildings
If you lay a strong foundation, your building will be strong too. It is important to get the foundation right, or your whole building can suffer. Poor foundations can lead to all kinds of problems, including shifting walls and leaky basements. Eventually structural damage can occur. You can prevent all that by laying your foundation properly.
There are several types of foundations. One common type is known as a floating foundation, or floating slab. This is a concrete slab which has a continuous grade beam. These foundations are ideal for supporting columns. If you need to support a very heavy load, however, you may want to consider a pier or footing grade beam. There are other specific instructions for heavy load-bearing foundations and floors; we will go into more detail about that in just a bit.
Creating Your Concrete Mixture
Your foundation is only as strong as the concrete you use. You have to get the mixture of ingredients just right to make your concrete adequate. You will need to mix water with Portland cement, aggregates like sand and gravel, and admixtures. You can check the strength of your concrete by measuring it. You will want to find out the Compression Strength and the Tensile Strength. The Compression Strength is measured in terms of the number of pounds per square inch (psi) in 28 days. This is how long it usually takes for concrete to set completely. Tensile Strength is a different measurement; it tells you how resistant the concrete is to expansion. You can boost the tensile strength of your foundation by inserting reinforcing rods made out of steel.
What should your concrete’s Compression Strength be? You are looking for a measurement of around 3000 psi after 28 days. Your concrete compression strength should measure 3000 psi if you mix using the following proportions: Cement: 94 pounds, Sand: 185 pounds, Coarse aggregate: 360 pounds, Water: 5.5 gallons. Obviously you will need to recalculate according to your needs. These ingredients can be mixed together in a rotating drum-cylinder or using a ready-mix truck. Make sure that you achieve a thoroughly homogeneous mixture.
Pouring Your Concrete Footings
Once your concrete is mixed, you can start thinking about pouring it. You aren’t ready yet though; you need to place your Forms. Forms are made out of wood or metal and hold concrete together while it is drying, and help mold it to the proper shape. The concrete is heavy and will exert plenty of pressure, so these Forms need to be quite strong. That way they will hold the shape of the concrete instead of losing their own. They need to keep rain water out as well; you will have to integrate drainage troughs so that runoff has somewhere to go.
After your forms are laid, then you are ready to pour your concrete. While you are waiting to pour it, keep it moving in your rotating drum. If you let it sit too long, it will start to set. Check your mix to make sure the aggregates haven’t settled, and then pour it evenly. If you have steel bars reinforcing your foundation, ensure that they are covered. Check for air bubbles or “voids,” and fill any you find. You may notice water on the surface of your concrete as you pour, especially if you have a deep foundation. The pressure of the sand and aggregate forces this water to the top. It is called Latinate. Remove it as you see it.
When you are pouring a floor surface, you have to screed your concrete. Screeding is a leveling process which involves two actions. First, you need to scrape away excess concrete. Second, you have to fill any low areas you find. You are then ready for finishing. To begin, you must compact the concrete using wood or metal floats. Some of your aggregates may float up to the top, and you will have to push them back down. Your concrete at this point will still be wet. Use steel towels to smooth the surface and compact it some more. You will need to use a metal rake to contour the surface of your foundation if it is outside and there is a chance of it getting wet. This will prevent slippage from occurring.
Curing Your Concrete
Portland cement and water react on a chemical level, which is what causes the mixture to harden. The weather conditions during the curing phase have a direct impact on how strong and durable the concrete will be. So this is something you should keep in mind before you mix and pour your concrete. Aim for a day which is going to be dry. The temperature should be somewhere between 50 and 90 degrees Fahrenheit. It is best if you know these conditions will persist for at least 72 hours. After 72 hours in these conditions, the concrete should be set, and you should be able to proceed to the next step in construction. If your concrete does not properly set (this is a particular concern in a cold climate), it can lose up to 50% of its strength as time goes by. So wait if you have to, and plan ahead! It is worth it.
Your concrete foundation has two main components. The first is the foundation footing, which provides support for load-bearing materials, and helps to distribute the weight. The second is the foundation wall, which rests partially below-ground. These walls help to support other walls in your building as well as columns.
Be sure to account for both vertical and horizontal loads; the latter tend to be more of an issue with steel buildings. The best ways to distribute the horizontal loads so they do not become a problem is through the use of steel tie bars and by increasing the size of the footing. You can connect steel tie bars to your anchor bolts if you have a high horizontal load to deal with. For less of a load, you can opt for spread ties instead. These can be used to transfer the load to the re-bar. Boosting the size of the footing is an efficient way to counteract your horizontal load, but it is also a pricey method.
Prep the Slab
When you are making the concrete slab floor for your steel building, you will probably use floating slabs or slabs on grade. You can pour the floor itself either at the same time as the foundation walls or afterward. Steel reinforcement bars will boost the strength of your floor. You do not always need rebar, but it is a wise move if you will be operating heavy machinery or vehicles on the floor or the floor will be bearing any other particularly heavy load.
Lay a sheet of polyethylene material on top of the surface you are going to pour the flooring onto. The sheet is necessary to stop water vapor from seeping through the concrete floor. The thicker your floor, the better it will stand up to heavy loads. Your jurisdiction may even have rules for this, so check with your local building department before you make your plans.
If you are going to have different concrete pours or construction components of the slab, you will need joints to control expansion and contraction. These are placed where floor slabs adjoining walls or where piers or columns penetrate the slab. Your expansion joint will protect your floor from cracking while your concrete cures. Afterwards, the same joints will protect your flooring from the expansion and contraction caused by rising and falling temperatures in your climate.
Once you have finished the curing process or you are drawing close to the end, you can waterproof your floor by adding chemicals formulated for that purpose. Then you should be ready to move on to the construction phase. Again, wait 72 hours or more for your concrete to set before you begin building, or you will compromise your entire structure.
Before You Begin Construction
Make sure your site is all ready before you start building.
- Ensure that the delivery truck will have access to your construction site. You can expect an eighteen-wheel flatbed truck with a large shipping container.
- Check the site road to make sure it will be safe for the truck and there are no overhead obstructions.
- The materials will then need to be unloaded, so make sure there is room to put them on your site. Some of the components for your building will be quite large, so be aware of that in advance.
Talk to your utilities provider and get an inspector out to your site so that you can start construction. Hire a building contractor that is familiar not only with pre-fabricated steel buildings, but your project in particular. That way they will be able to take all the proper precautions to ensure a safe workplace, and will also know exactly what your project entails. This will make for more efficient construction.
Whether you or a contractor is erecting the building, make sure you stick to the exact design specifications. Your materials were created for your specific project, and may not work for an altered design. Now that you are all read up on creating a strong and sturdy concrete foundation, you are ready to proceed to the next section, where we will offer you tips for a safe and sturdy assembly of your pre-fabricated steel structure.