Concrete Slabs and Foundations for Metal Buildings in the Hill Country

Most metal buildings here sit on a slab-on-grade: one pour with a thickened edge, or footings and piers under the frame columns, with anchor bolts set to the frame drawings before the pour. Where the ground moves or falls away, piers or grade beams replace the slab. Cities differ. Kerrville asks for foundation and pier details, including beam details and placement in the slab, and sets a four-inch minimum on a ground-supported floor slab. New Braunfels wants a foundation plan and an engineered foundation letter on its city form for new construction and additions over 500 square feet. Fredericksburg asks for the foundation type on its application (pier and beam, slab or other), and none of the city pages read requires an engineer-sealed foundation plan for a commercial building.

A slab suits any building you will work inside: a shop with a lift or a floor to roll tools across, a barndominium shell that will carry finished living space later, a commercial building that has to clear a city plan review, an RV or equipment building where wheel loads and door approaches drive the design. A plain pole barn does something different. Post-frame carries its load on posts set in or on the ground rather than a continuous foundation, so an open hay or equipment barn can skip the slab, or take a partial pour under the working bays only. If the building will be finished later, say so before the concrete is designed, because the slab is the piece you cannot change afterward.

• A shop slab with thickened edges and anchor bolts set to the steel frame drawings
• A barndominium shell slab sized for the finished living space that follows
• A commercial building slab in Boerne, Kerrville, Fredericksburg or New Braunfels that has to pass city plan review
• An RV or equipment building slab with an apron at the door openings for heavy wheel loads

• Across much of Kendall, Kerr and Gillespie counties the soil is inches deep over rock. The 1981 Kendall County survey puts Brackett at 10 to 19 inches and Eckrant at 4 to 14 inches over hard fractured limestone; the 1975 Gillespie survey puts Tarrant at 8 to 16 inches, and the 1986 Kerr survey puts Tarrant at 6 to 20 inches and Eckrant at 4 to 20 inches. The Kerr survey also says foundations sit easily on that bedrock and that excavating it is the difficult part.
• Where the soil is deep it moves. The Comal and Hays survey calls high shrink-swell, corrosivity to uncoated steel, very slow permeability and low strength difficult to overcome on the Blackland Prairie clays. The Kerr County survey maps the Doss-Urban land complex throughout the city of Kerrville, where rock is within 20 inches of the surface and a moderate shrink-swell can crack and buckle foundations, driveways and sidewalks that are not properly reinforced.
• On the Edwards Aquifer recharge zone in Comal County, constructing a building and any clearing or excavation that alters the site are regulated activities. No construction may begin until TCEQ has reviewed and approved the plan, and TCEQ allows itself 90 calendar days for technical review. Kendall County is not named anywhere in that rule.
• Disturb an acre or more of soil and the TCEQ construction stormwater permit applies. Its pollution prevention plan has to be written and put into effect before any work begins. Below an acre the permit does not apply, unless the work is one piece of a larger common plan that reaches an acre on its own.
• If the building is commercial and the estimated construction cost reaches $50,000, the plans go to TDLR for review and approval first. TDLR counts setting formwork as construction having commenced, so the registration and the plan submission come before the concrete, not after.