Steel post and frame buildings are great when a regular shed is too small to meet your needs.
Many people that live in rural areas will use them for storing a variety of items and depending on what you are storing the need for a concrete slab and perimeter foundation may not be part of your plan.
Larger buildings can be used to home animals like horses or can store farm equipment.
If you will be storing a car, truck or maybe even a tractor and using the building primarily as a garage then you want to install a concrete pad to reduce the chance of oil and chemicals entering the ground.
If that is not part of your plan then you can go with a dirt floor and use a perimeter post foundation to support your building and tie it to the ground.
Building the Caisson Foundation
The first thing that you need to do is survey the area that you plan to position the building on and mark a 1 foot wide perimeter where the walls will sit on the ground and make the ground level.
Using a rotating laser level or a water level (see our other howtos) remove the topsoil and plant matter and fill the area with gravel making sure that all sides of the building will rest evenly on the ground. This may take more work then you expect even on relatively level land so a bobcat could be your friend.
Layout of the Building Perimeter
It is important that you talk with your building kit supplier and ask how they supply the materials for your building.
Some companies will build the building in the factory then mark the pieces and disassemble for delivery. Other manufacturers will simply supply enough materials so the customer can build a specific design.
If they actually build the building before shipping ask the foreman to give you AS BUILT dimensions of post locations. If not then you can work within the design to match the plans.
Using a string line mark the location of each wall around the level footing that you just prepared.
The string will sit 4 inches away from the actual wall surface and then you will measure in from that string to position the wall. Don’t put the string exactly where the wall is because once the wall is up you can not check its position if the wall is in the way.
The outside steak where your string is attached should be at least 2 feet from the exterior wall that way you can remove the string line just prior to building or setting the building in place and still keep the steaks in place. Later you can reattach the string line and check your work.
Locating the Caisson footings
The number of caissons that you will need to install will be based on the dimensions and design of your building.
Post Framing usually spaces the posts within the walls of the building at 4 or 5 feet on center. You will need the manufacturer of your building kit to provide specific dimensions.
You must dig a hole under each post that is at minimum 30 inches deep and 10 inches in diameter.
To provide accurate measurements you can use sono tubes which are cardboard forms that are inserted into the holes you dig to accept the concrete.
If the soil conditions in your area require it you may also need to install 1 foot of footing below the tube before your building official inspects your sono tubes prior to them being filled with concrete.
Code will also require that the corner of the building have a footing anchor within 1 foot of the corner.
Digging the Caisson Holes
Using a gas powered auger is a great way to reduce the effort in digging post holes.
If you can not get hold of a gas powered post hole digger then you will need to do it by hand. A simple shovel won’t cut it because you will need to dig too deep and too wide a hole to get 30 inches down.
Adding Your Anchors
Once your holes are dug and you are ready to fill them with concrete you will need to have your post anchors ready to insert in your concrete.
Many anchors will require setting of a 10 to 12 inch 1/2 inch galvanized steel anchor in the concrete. If your concrete is stiff this can be difficult so be ready to give the anchor a couple wacks with a sledge hammer to place it.
Once the anchor bolt is down far enough place your anchor on top of the concrete and attach your washer and nut.
Checking the post anchor locations
Final adjustment can be made to the anchor at this time by restringing your perimeter string line and measuring to the center of the anchor.
A scrap piece of 4×4 post can aid in this measuring along with a 2×4 that can be marked to provide the distance between the string line and the post for quick checking.
After the anchors set a very last adjustment can be made during building by sliding the anchor in its adjustable mounting plate to get you exactly where you need to be.
Building inspectors can be less then informed about the use of perimeter anchors and may want you to install a full footing around your building.
If this is the case it will cost you much more in time, labor and materials so ask the manufacturer of your building to include a setout that describes the building code references used in your design and a number that the official can contact for architectural information.
In other words you are not building a Residential building and should not be required to meet those standards … unfortunately you do end up getting the new guy on small projects that doesn’t know his stuff.