Steel Building Foundations

Pre-Construction Work

Construction of a steel building or metal structure is similar to construction of most other buildings and starts with a strong foundation.

Before the steel building foundation is poured, the land must be professionally surveyed to ensure the building site is level and that the builders know the exact boundaries of the plot.

Once the building site is surveyed and marked, the grading or physical leveling of the building site can begin. Grading involves making the height and shape of the land conform to the surveyor’s stakes.

When the building site has been made level, excavation for the foundation can begin. You will need to consult with your steel buildings manufacturer or salesperson regarding the type of foundation excavation your will require. Most simple steel buildings only require minor excavation while more complex metal buildings will require deeper general excavation.

Minor excavation can be done with hand tools such as a pickaxe, shovel and steel rake. A 2” – 3” deep base for a poured concrete foundation should suffice.

Major excavation is likely to involve the use of construction equipment such as a backhoe or bobcat shovel.

Remember to retain some of the removed earth for final grading and leveling after the steel building is completed. You will also likely need some earth for backfilling, or the in filling of small holes or voids left after construction.

Steel Building Foundation

The foundation is the most important part of a prefab steel building and the part that can cause the most problems in the building over time. Leaky basements, unsettled or shifting walls and eventual structural damage can come from a poorly laid foundation.

Foundation Types

Floating Slabs (or floating foundations) are concrete slabs with a continuous grade beam. Spread directly under a column or reinforced along the bottom, continuous grade beams carry the vertical load of the columns.

Pier, Footing and Grade Beam consist of a square or rectangular footing and a grade beam wall. A driller pier can be used in place of the square or rectangular footing. Piers and footings carry most of the vertical loads.

Selecting the right mixture for concrete is important for the long-term maintenance of your steel building. Concrete should be strong and durable. Making concrete is relatively simple.

The basic ingredients of concrete are:

  • Water
  • Aggregates (sand and gravel)
  • Portland cement (Type 1 portland cement is most commonly used)
  • Admixtures (fly-ash or a water retardant mixture). Admixture is used to strengthen or cure concrete as it is poured.

There are two ways to measure the strength of concrete.

The Compression Strength is defined in terms of X# of pounds per square inch (psi) in 28-days. (28-days is the normal amount of time it takes for concrete to fully set.)

The Tensile Strength notes resistance to stretching or expansion of the concrete. Use steel reinforcing rods or bars to increase the tensile strength of the foundation if necessary.

The typical strength of concrete is 3000psi after 28 days. To achieve this strength, a mixture using these approximate proportions would be desired:

  • Cement: 94 pounds (43 kilograms)
  • Sand: 185 pounds (84 kilograms)
  • Coarse Aggregate: 360 pounds (163 kilograms)
  • Water: 5.5 gallons (21 liters)

Mix these ingredients in a rotating drum-cylinder or a ready-mix truck to insure the concrete is thoroughly blended.

Pouring the Foundation

Before the concrete can be poured, the Forms must be placed. Forms are used to hold the concrete together while it dries and to dictate its shape. Usually made from wood or metal, forms must be strong enough to bear the weight of the concrete as it hardens. They must also be rigid enough to withstand the pressure of the concrete without deformation, bending or deflection. They should also be tight enough to keep liquid concrete in and rain water out.

As the forms are being laid for the steel building foundation, be certain that there are sufficient drainage troughs for water runoff.

Pour the concrete evenly making sure the aggregates have not settled towards the bottom of the mix and that all reinforcing steel bars have been fully covered and that there are no voids or air-bubbles. Liquid concrete is rarely allowed to sit still. While waiting to be poured, concrete should be kept churning in the rotating drum. On professional construction sites, concrete is often vibrated by an electric or pneumonic vibrator, while being transported from drum to foundation.

When pouring deep foundations, water often seeps up to the top due to the immense pressure of the sand and aggregate. This water is called Latinate and should be removed as it comes to the surface.

Screeding: When pouring floor surfaces, the concrete must be screeded before it is finished. Screeding is the method of leveling a floor by pushing away excess concrete and using a template to push concrete into lower or under filled areas. When the floor surface is leveled, it must be finished.

The first step to finishing the concrete floor is to use wood or metal floats to compact the concrete and push larger aggregates back down into the mixture. The cement is still quite wet at this point. After the concrete has been compacted, steel towels are used to smooth out the surface and to compact it for a harder finish.

If the concrete floor surface is going to be wet or is outdoors, a metal rake is generally used to contour the surface in order to prevent slippage.

Curing the Concrete

Concrete hardens due to a chemical reaction between Portland cement and water. Temperature and moisture have a direct bearing on the strength and durability of the concrete.

The best conditions for strong concrete are dry and between 50 degrees Fahrenheit and 90 degrees Fahrenheit. The concrete must be exposed to these conditions during the first 72-hours of drying for maximum strength. After the first 72-hours, the concrete should be strong enough to permit further construction on the building.
If you are building in a cold weather region, take proper precautions to ensure your concrete can set at a proper temperature. Concrete that does not set properly can lose up to 50% of its cohesiveness and strength over time.

There are two components to a foundation: the walls and the footings.

  • Foundation Wall is a wall with a portion of it below ground level. These are load-bearing walls that serve as supports for other walls and columns.
  • Foundation Footing is a structural unit used to distribute the weight of the building to load-bearing materials.

Steel and metal buildings do not present a great deal of vertical load but they do need to withstand very high horizontal loads which tend to push outwards. If not properly accounted for, horizontal load weights can cause structural failure of the foundation and framing members.

The two common ways to distribute or resist horizontal load are:

  • Steel Tie Bars.
    Reinforcing bars are connected to anchor bolts in order to tie the building columns together and evenly distribute the load. In cases where the horizontal load is not as high, spread ties (or hairpin ties) can be used to transfer the load directly to the rebar used in making the cement floor.
  • Increasing the size of the Footing.
    This is the most expensive option. By increasing the size of the footing the design counteracts the horizontal load force thus preventing shifting of the foundation.

Laying the Floor

In steel or metal buildings, floating slabs or slabs on grade are most often used making the base floor. The floor can be poured while the foundation walls are being poured or after the foundation walls are in place. When built, use steel rebar to give the floor added strength and prevent cracking over time. If the building is going to be used for heavy machinery or for vehicles standard reinforcing bars are almost always necessary.

Before pouring the floor, lay a sheet of polyethylene sheet material on the surface you are pouring the floor onto. This will prevent water vapour from seeping up into or through the concrete floor.

The thickness of the floor determines its ability to hold heavy loads. Many jurisdictions have specific requirements regarding floor thickness and load-bearing ability. Before building, we recommend you check your local building codes.

Floor Joints

Joints are required between separate construction components or between different concrete pours. For floors, an expansion joint is used to control the point where normal expansion or contraction of the concrete will take place. Placed where a floor slab abuts a wall or is pierced by a steel column or pier. During the curing process, the concrete used in the floor will contract or shrink. Expansion joints prevent floor cracking during the curing process and also protect the floor against temperature related contraction and expansion.

Waterproofing the Floor

Apply waterproofing chemicals towards the end of or just after the curing process.

Pre-Assembly of the Steel Building

There are a number of things one needs to consider in the pre-assembly phase. Fortunately, most of these things are basic common sense.

CONSTRUCTION PHASE

Site Access

Your steel or metal building will be very heavy when shipped and packaged. It will be transported on a large, flatbed truck. It might be packed as high as a typical eighteen-wheel truck loaded with a shipping container. On the day the building is delivered, you will want to ensure the truck carrying it has unobstructed access to the site. Check for overhead wires as well. If the access road is not finished, be sure the truck will not be mired in mud, as it will be carrying an extremely heavy load.

Make sure there is an adequate amount of room to work in. The truck needs to be off-loaded so be sure there is a place to put the materials while building. Since steel structures are modular in design, you will want to have a large area to put individual pieces of the building.

When planning and designing the building, access to utilities was considered. What was theoretical a few weeks ago is now very real. You should consult with your local utility providers and have an inspector on-site before construction begins.

Make sure your crew is familiar with the site they are going to work on. A good contractor makes a work-safe plan for every phase of construction. Steel and metal buildings are unique. You should be certain your building crew is familiar with and comfortable working with the materials. Before assembly, the crew and contractor must be fully familiar with the design drawings.

Please note, each steel and metal building is unique. Design plans and building materials are created for specific projects only. The design plan should be strictly adhered to.

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