stair's concrete treads are threaded on a steel column and locked
in place by steel balusters. Each of the 300-lb. steps was cast
individually in a mold lined with plastic laminate.
circular staircase was inspired by the stone stairways built in
Europe during the Middle Ages. Designer Paul Tuttle wanted to create
the sense of timeless solidity that massive stone steps evoke, and
the two-story greenhouse in the Douglas residence overlooking Santa
Barbara, Calif., gave him his chance. The room needed both a stairway
and a sculptural focus, so Tuttle captured the medieval aura with
the l0-ft. high, 7-ft. dia. concrete spiral stair shown in the photo
before I had seen the design, Tuttle asked me if I would be interested
in building this staircase. My first impulse was to say no. A poured-concrete
spiral stairway seemed impossibly difficult. But once I saw his
drawings, I got excited by the challenges its construction presented.
Along with two of my associates, I agreed to build it. But I still
wasn't really sure I could.
- The first thing we had to do was to determine the number of steps,
the rise of each step and where the beginning and end of the spiral
would fall. We eventually decided on a landing and 15 steps of 7
½ -in. rise. Each one is a 25° segment of a circle.
we made a full-scale mechanical drawing of one of the steps, including
sleeves for holding the balusters, a cutout for toe space, indentations
for carpeting and a sleeve for the center column. This drawing (left)
proved to be an indispensable reference in all stages of the project.
the concrete form - After we figured out what one of our concrete
treads had to look like, we had to build a form that would duplicate
it 15 times. The form had to be durable, yet flexible enough to
be taken apart after each pour and reassembled for the next one.
Carpenter John Bunyan and I thought about our options and eventually
came up with the one shown below.
form's main components were the base, the sides, the pivot end and
the outer end. The base was a 3-ft. by 4-ft. piece of 1/2-in. plywood
that we covered with plastic laminate (for more about working with
plastic laminates, see FHB #9, pp. 39-41). We made the form's sides
out of fir 2x8s, with 2x2 blocks screwed and glued to their ends.
We made the tight-radius pivot end from five 2x6 blocks laminated
together with yellow glue. We cut out its 4-in. radius arc on the
bandsaw. The outer end was made up of five 2x4s. It had a 21-in.
arc of 3 1/2 -ft. radius cut out of it. We let the tails run long
on the end pieces so that they could be bolted to the end blocks
on the sides of the form.
Once we had the two sides and the tight- radius end piece bolted
together, we lined its inside face with plastic laminate. This part
of the form stayed together as a unit throughout the casting process:
We lined the outer end, and then bolted it to the side pieces. We
put the entire assembly on the base, and screwed the two together
with 26½-in. screws.
we screwed plywood pieces to the bottom and lead edge of the form
to create an indentation for carpeting. The shape of these pieces
had to be carefully worked out so that the carpeting would flow
from one step to the next without any offset. We planned for the
bottom of the form to be the mold for the top of each step, so we
could place the largest of the ½ in. let-in pieces for carpeting
on the bottom of the form. Pouring the steps upside down also let
us trowel the bottom of each tread-the largest and most visible
expanse of concrete on each step. Finally, a 2½-in. square
piece of wood 28 in. long was covered with laminate and screwed
to the top edge of the form. This would form the indented toe space
on the bottom lead edge of each step.
last parts of the form were the registration pins for the steelwork-two
1 1/8 in. tubing sleeves to support the rebar, and the 4-in. pipe
sleeve that would fit over the central column of the stairway. We
screwed two 7 ½ -in. lengths of I-in. wood dowel to the base
near the corners at the wide end of the form (drawing, above right).
At the other end, we attached a 7½-in. piece of 4x4 with
chamfered corners. These pegs, carpet inserts and toe board were
screwed in from the outside of the form so they could be easily
released when we stripped the form from the cured concrete.
steel work and balusters - We decided to cast a short length
of 3-in. pipe into the slab-floor footing to act as an anchor for
the 3½-in. central-column pipe. Each step would have cast
within it a sleeve of 4-in. pipe. These sleeves would slip over
the center column and become an integral part of the re- bar assembly
in each concrete tread. The re- bar grid would also include the
two 1 1/8-in. tubing sleeves into which the railing balusters would
slide and be secured, as shown in the drawing above.
sleeves in each step are attached to one another by a matrix of
3/8-in. rebar. Fifteen grid assemblies were required, one for each
stair, and each one had to match all the others exactly in order
for the balusters and central column to fit properly.
achieve this kind of accuracy, our steel expert, Dean Upton, welded
each assembly on a jig. In a 3/8-in. steel plate, Upton drilled
holes at the sleeve centers and tapped them for ½-in. bolts.
Then he tack-welded posts to the plate. These posts had been turned
to fit the inside diameter of the sleeves, and were bored to accept
the ½-in. bolts. The sleeves for each tread were cut to length
and squared. Then Upton bolted them to the jig and welded the rebar
posts that support the handrail are 1-in. OD steel tubing with a
.083-in. wall. The sleeves into which they fit are 1 1/8-in. OD
tubing with a .049 in. wall, allowing a clearance of .027 in. That
seemed a bit loose to us, but proved to be a necessary margin during
the final stair assembly.
silver-soldered a ¼-in. ring (from the sleeve material) to
each baluster to make sure they ended up at the right height. Their
tops were cut at the angle of the stairway, and as they were installed,
the balusters were rotated to match the direction of the handrail.
Once the treads were in place, we plugged the underside of the baluster
holes with Bondo (a brand of auto-body putty).
out the pipes - Material selection required some careful sleuthing.
Our basic plan called for several pipe sizes, each to fit snugly
over the next. The central column is 3½-in. pipe, schedule
40. The sleeves for the steps are 4-in. pipe, also schedule 40.
Pipe goes by the inside diameter (ID) while tubing goes by the outside
diameter (OD). With pipe, oddly enough, the OD is constant while
the ID changes with the schedule or wall thickness. A 3½-in.
pipe (schedule 40) is actually 4-in. OD by 3.548-in. ID, with a
wall thickness of .226 in. So depending on the schedule, we had
a choice of clearances between sleeve and column. With 3½-in.
pipe and a 4-in. sleeve, there is a theoretical clearance of .026
in. But this clearance did not prevail on all of the pieces so some
sleeves had to be turned down on the lathe.
technology - The concrete mix was critical for two reasons:
weight and finish texture. In order to decrease the weight of each
tread from more than 400 Ib. using concrete with standard aggregate
to about 300 Ib., we used !/2-in. Rocklite (The Lightweight Processing
Co., 715 N. Central Ave., Suite 321, Glendale, Calif. 91203), a
lightweight aggregate. But we also wanted a dense, pure white finish
on each tread. This led to our using two different batches for each
one. The outer inch or so is made up of 1 part white portland cement,
2 ½ parts 60-grit silicon sand and 2 ½ parts Cal-White
marble sand (used mainly for swimming pools), made by Partin Limestone
Products Inc. (PO Box 637, Lucerne Valley, Calif. 92356). Once we
got this outer layer of white concrete in place, we filled the core
of each tread with the lightweight mix.
carefully measured all the ingredients because slump was important-too
much slump would cause the two mixes to flow together in the form.
The lightweight mix for the core was 1 part cement, 2 1/2 parts
sand and 2 parts aggregate. To speed setting time, we added a little
calcium chloride to each batch.
we'd hoped to pour two steps per day, but found that producing one
a day was quite an accomplishment. Placing the mixes in the form
required two of us-one to tamp the outer mix and the other to keep
the core mix from migrating to the edge of the form. We placed the
concrete in layers, agitating it thoroughly after each layer to
eliminate voids. Between each pour we cleaned the form, coated all
dowels and wood insets with floor wax and sprayed the plastic laminate
and delight filled us when we stripped the form from the first tread.
The result was magnificent, but not what we'd expected. The plastic
laminate made the surface smooth as glass, and a swarm of tiny,
irregular air pockets made it look something like travertine. We
were elated with this first success.
- Once the 15 treads were cast and carted to the site, our next
hurdle presented itself-slipping the 300-lb. steps onto the steel
column. Obviously we needed some type of device to lift the treads.
It would have to be sturdy enough to carry the heavy loads, yet
also adjustable so we could fine-tune the position of each tread
over the center shaft.
solution was the homemade chain hoist shown in the drawing at left.
It consisted of a 12-ft. 4x8 I-beam and a chain hoist mounted to
a freewheeling trolley. We centered the beam over the column, and
held it up by the stair landing on one side and a sturdy frame-
work of 2x6s on the open end.
fabricated a special metal carriage and harness to carry the treads
as close as possible to their center of gravity. Each tread was
then lifted above the l0-ft. high column, and its sleeve was centered
over the shaft. Then the tread was slowly lowered into position.
Once a step was in place, we would brace its outer end with a 2x4
and then one of us would tap a baluster through the aligned sleeves
to secure the new tread to the one below it.
nearly befell us midway in the assembly. As we rolled the tenth
tread along the I-beam, a lurch in our movements caused a sudden
shift in the tread's center of gravity. Instantly the harness slipped
off the carriage and the step plummeted, bouncing against several
of the steps already in position and demolishing the bottom picket
on its way to the floor. We were stunned. Fortunately no- body had
been standing in the way of the tread when it fell. We surveyed
the damage and it appeared enormous. Chunks of concrete were knocked
off the treads in half a dozen places. We were so badly shaken that
we packed up and went home for the day, believing the project ruined.
next day we reassessed the damage and concluded that it wasn't as
severe as it had seemed. We decided to patch the damaged edges and
corners with Bondo. In some places we had to build up numerous layers
of the stuff, but it worked far better than we had dared to hope.
Because the Bondo was a different color from the pristine white
concrete, we knew we'd have to paint the final product.
the landing - The top step had a different shape from the others
because it needed to flow into the cantilevered landing. To link
the stairway to the landing, we built a triangular rebar grid to
lock the top of the central steel column rigidly to the 4x12 landing
girders. We welded sleeves for the balusters and the central column
to this grid. The grid in turn was welded to a 4-in. by 30-in. by
3/8-in. steel plate, which was bolted to the landing framing. Then
we erected a form around the grid with supports down to the floor.
We were able to use several curved components from our breakdown
form, but most of the pieces were new and had to be covered with
poured this last step with the same two mixes and care that we used
with all the other treads, and when we took off the forms it flowed
perfectly into the landing.
- Probably the most challenging part of this project was bending
a 1 7/8 -in. dia. thin-wall tube (.063 in.) into a helical handrail.
We chose this size tubing because there are stock fittings for 1½-in.
pipe that fit closely enough to be used with the tubing (1.875-in.
OD vs. 1.900-in. OD). At the top where the stairs meet the landing,
we needed a tight re- turn bend to blend the rising stair rail into
the horizontal landing rail. We made this transition with two wide-radius
elbows and a little cutting and fitting. We used another stock fit-
ting-the half-sphere cap-to finish the bottom end of the handrail,
and we used floor flanges to attach the landing rails to the wall.
radius of the stair circle was 42 in., but the radius of the line
of balusters was 40 in. The inside radius of the handrail was 40
in. less 15/16-in. (half the diameter of the tubing) or 39 1/16
in. Taking his cue from a tubing bender, Upton designed a press
that used a hydraulic jack to generate the bending force needed
to arc the straight lengths of tubing. He used an oak form block
with a radius of 38Va in., a little tighter than the required radius
to allow for some springback. As it turned out, the springback was
forming tool we tried out first had two spools about 12 in. apart.
It bent the tubing, but it also left slight dimples at each point
of contact between spool and tube. A handrail with a dimple every
6 in. was totally unacceptable (it looked like a segmented worm),
so Upton made a pair of bending shoes out of channel steel and Bondo
as an alternative. They needed periodic greasing to allow the tubing
to slip through as it was bent. This jig, shown in the drawing above
left, worked fine. A 5-ton jack supplied the pressure.
first tried to form the helix as the tube was being bent by rotating
the tube a little at each bend. But it was difficult to keep track
of the rotation. We could calculate how much rotation was required,
but to control it was tough in a small shop. Even though it wasn't
the right shape for our railing, the sculpture resulting from the
first try could be mounted on a stone block and placed in front
of a library.
learned two things from this attempt. One, the press could put a
wrinkle-free radius in our tubing, and two, trying to form both
the radius and the helix into the full-length railing was too ambitious.
Instead, Upton cut the 24-ft. tube into three 8-ft. pieces. Then
he made a plywood jig that had a radius of 39 1\6 in. (drawing,
above). This jig represented about 1/2 of a turn of the staircase,
and was tall enough to allow the rise of the handrail to be marked
diagonally on it. As he shaped each 8-ft. section, Upton checked
its bend against the jig. This worked well, and the three pieces
closely approximated the required helix plus the radius.
make final adjustments in the helical twist, each 8-ft. section
was cut into three equal pieces. After tack-welding the first section
to the bottom balusters, Upton rotated the second section slightly
to create the helix. Section two was then tack-welded in place,
and the third piece rotated slightly more than the second and so
on until all nine parts were tack-welded in place. Each piece was
aligned with its neighbor by using a short offcut of 1% in. tube
as a dowel. Before he welded the balusters to the railing (photo
above), Upton torch-welded the whole unit into one continuous piece.
Then all the welds were ground down, and any little pits were filled
with Bondo and sanded smooth. The finished rail is painted brick
red, and appears to flow as one piece from top to bottom.
Upton torch-welds the handrail to a steel baluster. Although
It appears continuous, the railing is composed of short segments
of steel tubing that were bent on a homemade press, and then
assembled on site to create the necessary helical shape.
final job was whitewashing the treads. We wanted to preserve the
texture of the concrete and to have it not look painted, so we experimented
with several finishes. We finally settled on white latex paint mixed
with a small amount of white portland cement. This gave the surface
a little roughness to the eye, but did not destroy the glass-smooth
texture to the touch. One coat completely covered the grey-green
Bondo, and we were done.
project took six weeks of concentrated effort, and it kept our attention
with a series of snags and surprises. But everybody is happy with
the way it turned out. The stairway cost almost $10,000-a lot for
one flight of stairs, but not for a sculpture that anchors a special
Dennis Allen is a general contractor living in Santa Barbara,