Tips and tricks for barndominium foundations
The newest entrant to vernacular architecture in the state of Texas is colloquially referred to as the barndominium, or “barndo” for short. Barndominiums are steel-framed residential structures with an attached barn or shop, all under one roof. This style of building is particularly ubiquitous in extra-territorial jurisdictions, or ETJs. An ETJ is an unincorporated area where municipal building codes are not stringently enforced. This means no permit, no inspections, and no certificate of occupancy.
A lot of these buildings are built by experienced contractors who come up with a standard design and apply it to a variety of localities. But the simplicity and uniformity of the barndominium, coupled with the typical location a barndominium tends to be built, can lead to some wildcat construction.
Barndominiums are referred to as pre-engineered metal buildings in design professional jargon. A pre-engineered metal building, or PEMB, as the name suggests, is a steel building that is pre-engineered and sent as a kit to the construction site. Pre-engineered metal buildings are pre-engineered because they can be designed quickly by specialized engineers. The building is designed for the gravity loads and the lateral loads based on available sizes, and prefabricated to the exact dimensions that one would need for that particular project.
This style of building is not new. Pre-engineered metal buildings are ubiquitous all over the world, and function not only as residential buildings, but as commercial buildings, retail, offices, and even institutional buildings like schools and churches.
From a structural perspective, the trickiest part of a PEMB like a barndominium is the foundation. The building design tends to result in a horizontal or “kicking-out” force at the base of the columns, which must be resisted by the anchor bolts and the underlying foundation.
The popularity of barndominiums in extra-territorial jurisdictions, again, can incentivize wildcatting and cutting corners, especially on the foundation. But the foundation of a PEMB is a very crucial part of the long-term performance of the building.
Get a geotechnical report
Because of the nature of the locations of most barndominiums and the seemingly high cost of a geotechnical report, most builders and homeowners tend to forgo the soil investigation. This is not advisable, especially in parts of Texas that are notorious for having expansive soils.
PEMBs differ from the typical light-framed wood building in that the roof loads are not spread evenly to the foundation by a studwall with studs spaced every 16 inches. PEMB roofs are supported by rigid frames that can span as far as 25 feet and can be spaced in bays as far as 25 feet. This means that when all the load gets to the foundation, it is more concentrated than a typical light-framed wood building. Depending on the soil you have, it might be recommended to investigate piers if the shallow soil does not have adequate bearing capacity to support the load.
Add horizontal restraints at the columns
We then come back to the “kicking-out”. Because of the structure of the rigid frame, the roof loads can create a horizontal outward force at the base of the column. By the time the load gets to the foundation, it is concentrated at the column locations. The foundation must support that vertical load, and also the horizontal load from the “kicking-out” phenomenon. This mean that some method is required to restrain the columns at each frame and tie one side of the rigid frame to the other. This can be done using a grade beam, a tie rod, or designing piers to resist the lateral load. Depending on the width of the frame, the horizontal force can be so much that hairpins are required in the slab to arrest the horizontal motion at the anchor bolts.
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