Adding a Cantilevered Bay - Continued

It was a relatively complex task, and it took me several hours with the architectural and engineering plans to understand all the requirements. Fortunately, the engineer was very helpful, and was willing to listen to my suggestions. After a couple of phone calls, we knew exactly what we had to do to make sure everything fit together properly.

The author guides one half of the welded steel frame onto a newly poured concrete pier (top). The anchor bolts had to be carefully placed, as the steel base plates allowed no slop (middle left). Leveling nuts below the plates permit final height adjustments (middle right); 2-by forms (bottom) await the nonshrink grout that will support the bay after it is framed and loads are imposed.

The Steel

The most critical challenge was getting the concrete knee walls and steel posts right. Concrete and steel are unforgiving under any circumstances, but they're especially unforgiving when you have close tolerances.

That was certainly the case on this job, because the floor of the small addition had to line up with the floor of the existing home.

To make sure this happened, the posts and knee walls had to be precisely the right height. And the foundation bolts in the knee walls had to be in exactly the right place, so that they lined up with the holes on the baseplates.

I spent a good amount of time measuring and remeasuring everything; rather than "measure twice and cut once," it was more like "measure a few-dozen times so we don't have to worry about refabricating the steel."

A thick LVL (top), bolted to a welded plate (middle), spans the distance between the two post assemblies. Dimensional joists set in hangers (bottom) run from the LVL to the band at the existing house wall.

Next I made a template for the foundation contractor to use in setting the bolts. This turned out to be time well-spent.

Danny Serusa, a local welding contractor, fabricated all the components of the steel posts in his shop, then delivered them to the job site and set them.

I had scheduled a half day to install the frames, but it actually took only 20 minutes. They dropped right into place.

There was some room for small adjustments. The plan called for a nut to be threaded onto each bolt before the goalposts were installed. The baseplates rested on these nuts, which allowed us to fine-tune the height of the posts with the turn of a wrench.

After the framing was completed and everything lined up, we filled the space created by these nuts with grout. The grout is what carries the structural loads.

Once the steel frame was set, the rest of the framing was conventional. The frame was well-engineered: By the time the entire weight of the addition was in place, it had settled by only about 1/4 inch.

Finishing Up

The remaining framing was nothing out of the ordinary. With the posts in place, we installed the LVL and joists, then proceeded to frame the walls and roof. Our planning paid off here, too. Since the posts had been so precisely fabricated, the wood framing required no padding or shimming. Everything went together as drawn on the plans.

We had a tougher time with the rest of the addition, because the whole house was 2 1/2 inches out of square — but that's another story.

The engineering proved sound. I didn't place the grout beneath the baseplates until after the framing was done and the walls and roof were in place. By that time, the addition had settled only about 1/4 inch.

I had worried that there would be some play at the end of the cantilever, but the steel-and-wood structure impressed me here, too. The outboard end feels stiff enough that you would never guess it's hanging in the air.

Peter Marzbanian owns Marzbanian Construction in Oak Bluffs, Mass