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 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
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
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
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.
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