Lynne is building a new house on the shore of Lake Champlain, on a strip of land squeezed between the road and the water’s edge. The skinny, sloping site inspired a floor plan with a small footprint and a tall profile, including a walk-out basement. Unusually, the garage is above the basement rather than on grade. Lynne hired me to design that garage.
A regular slab on grade requires no structural design… just a reinforcing mesh to prevent cracking and maybe some additional steel below loadbearing walls. But this slab doesn’t have soil support underneath. From each outside wall to a central loadbearing wall – two spans of 10 feet – the concrete needs to carry its own weight, plus the weight of a vehicle on top.
Incidentally, one load I checked is that of a jack supporting the vehicle, which the code assumes is 3000 pounds spread over a 4½-inch square area. No problem for Lynne to do car maintenance from the comfort of her garage!
I figured out how much load the slab carried at midspan (where it sags), as well as over the central loadbearing wall (where it flexes over, a condition known as negative moment). Then I came up with a reinforcing pattern that would carry the load required and be easy to build. I had to stay within the 5-inch thickness available. I also researched and specified a corrugated steel deck on which to pour the concrete, and I designed temporary walls to support the deck at midspan while the concrete is wet.
After the contractor, Randy, placed the walls, forms, and reinforcing, I visited the site to see if it looked OK before the “point of no return” when the concrete got poured. Randy complied with my requests, and the deck was placed successfully the following week. Now we’re just waiting for the concrete to reach its 28-day compressive strength.