Inclination and Evolution: A Stair Design
This post was originally published on The Architects’ Take blog and has been republished here with permission.
This central staircase rail went from glass, to steel, and back to glass again, in a design transformation aided by hand sketches, computer renders, and actual as-built appearance at the job site.
Many design aficionados fixate on grand spaces, “great rooms”, built-ins, and other features as design elements, without fully appreciating the importance of circulation. The master suite, the bath, the kitchen, home theaters, and other amenities are touted on design sites for their luxuriousness, but it’s the transitions, such as stairs or passageways, that tie it all together. If stairs are featured in a high-end home, it’s a grand staircase that takes up the entire main portion of the house.
A grand staircase like this one is spectacular, but where does it lead?
Existing Condition
This particular project from Mark English Architects involved a remodel to an existing three-story home in San Francisco. The building had a basement, a main floor, and an upper floor. We viewed some 3D sections of this project in a previous article that introduced the power of hand drawing. Now, we focus on one particular feature of this home, namely, a central stair as a ribbon, winding all the way from the top of the building to the basement.
The existing stair was a typical wraparound, with wooden rails and a feeling of enclosure.
image: Mark English Architects
Phase 0: Glass
The earliest conceptual design for the remodel called for a large glass wall showcasing an interior walkway on the third story. A central glass stair would run down the center of the house. The glass wall concept was rejected by San Francisco Planning, for historic reasons. Without the wall, the glass interior didn’t make as much sense from a design standpoint.
(As it turned out, the original building from 1917 was designed by a female architect named Elizabeth M. Austin of the firm Austin & Sanford. Mark English Architects commissioned an official historic report from Page & Turnbull, Inc. This report established the importance of preserving the original exterior. The final design left the home’s exterior largely the same as before.)
Initial design proposal, exterior. A central glass stair led to a glass walkway on third floor,
with light from a long row of skylights. Image: Mark English Architects
Initial design proposal, close-up of central circulation under the central skylight. Note the
glass floor on the raised walkway and stair treads. Image: Mark English Architects
Initial design proposal, interior, showing a glass staircase in the center of the building,
with glass rails and interior skylight above. Image: Mark English Architects
The final design exterior was a compromise with City Planning, and preserved the original
building’s historic appearance. Image: Mark English Architects
Phase I: Steel
With the original glass curtain wall off the table, the architect determined to re-work the interior. A subsequent stair design called for vertical rails made from steel panels. The stair treads from the main floor to the upper floor were to be fabricated from cantilevered steel. The stair from the basement up to the main floor would still have wooden treads.
Although the previous images showed the stair rail panel as a solid sheet of steel, the
designers were in fact exploring the idea of perforated steel panels. The clients designed
this pattern themselves. Mark English Architects
A physical model shows the stair’s progression through the house, from basement to
upper story. Image: Mark English Architects
Then, The Client Saw The Treads
During construction, the client saw the treads exposed. They were so excited by the look, that the architect changed the rail design from steel back to glass. “The clients were bold enough to say what they really wanted. They really got it,” observed the architect.
Construction photo showing the stair landings and clockwise ascent as seen from the
main floor. The steel treads are welded to a frame that runs along the back wall. Image:
Mark English Architects
The client saw the pattern of the steel treads and wanted them to be visible in the final
design. The stair panels and rails, which had been designed in steel, were changed back
to glass as per the initial design. Image: Mark English Architects
Phase II: Glass Rail
At this point, the technical problems involved the attachment of the glass panels to the stair treads, the shaping and mating of the panels to one another in a seamless fashion, and addressing seismic concerns. The panels also had to be joined to one another along the vertical seams. The top edge of the panels was rail-less.
Ascent from main floor to upper floor, viewed from the main floor. The glass panels
needed to be shaped and fitted together. Image: Mark English Architects
Ascent from main floor to upper floor, viewed looking down from upper story landing. The
stair rail panels would have to be fabricated. In this drawing, the callout explores the
interaction between glass panels, where the stair turns a corner. Image: Mark English
Architects
Detail of stair from basement to first floor, after changing the stair rail panels from steel
back to glass. Although the cantilevered steel stair treads didn’t extend to the basement
level, the glass panel rails did. Image: Mark English Architects
Siding on stair rail, shown from second story. The stair on the right ascends to the upper
story. A raised walkway leads off to the right, with another landing and stair descending
to the basement level. Image: Mark English Architects
Ascent from the basement to the main floor. In this portion of the stair, the stair treads
are wood, with risers. Sketch diagrams of the steel rail panels for the stair helped the
architect to understand how transitions between panels, edge of stair, and edge of floor
could occur. These early hand-drawn studies developed ideas for possible detailing.
Image: Mark English Architects
Second view of ascent from basement to first floor, viewed from the other side and
down. This stair sketch diagram shows the landing and its relationship to the
steel panels. The details of the transitions between different elements took precedence
over the actual dimensional representation in the sketch. Image: Mark English Architects
Phase III: Full-Size Physical Mockups
Full-size physical mockups built onsite out of plywood were an important part of the process of determining the glass layout. By this point, the stairs themselves were already installed, so the mockups consisted of shaping plywood panels as stand-ins for the glass panels to come. Then, the glass panels could be shaped using the plywood panels as a pattern.
Physical mockups help the architect determine the exact pattern needed for fabrication
of custom stair panels. Ascent as viewed from the main floor. Image: Mark English
Architects
Physical mockup of stair rails, showing ascent to the upper story. Image: Mark English
Architects
Bolting it Down
To attach the glass panels to the stair treads, a special standoff bolt was used.
Stair detail showing attachment of glass panels to stair treads. The treads are
cantilevered steel with wood cladding. Image: Mark English Architects
Detail of stair from basement to first floor, showing placement of attachment bolts for
the glass panels along the side of the stair. Image: Mark English Architects
Detail of stair from basement to first floor. Image: Mark English Architects
Structurally Sound Rail-Free Design
The design called for the glass panels to be attached at the bottom, and caulked together along the seams, with no attachment at the top. How would this work in earthquake country? Would the glass panels experience too much flexion along that top ridge? A structural engineering analysis created recommendations for tempered and laminated glass, showing major stress points and validating the final materials and detailing.
A structural visualization of the flexes and stresses that a rail-less glass stair rail panel
could expect to experience during a seismic event. The spots along the bottom represent
the attachment bolts, while the single spot on top represents the flexion of the tempered
and laminated glass. Image: Triumph Engineering, LLC
Flex points show for each of the stair rail attachments along the bottom. The top flex point in the center is how a rail-less glass panel attached only at the sides and bottom could be expected to behave. The panel is attached at either side to other glass panels with a specially rated flexible silicon caulking.
Final Result
The final result has a sculptural quality to it. The stair appears to float in space as an invitation to ascend.
The completed stair with wood-clad steel treads and glass panel rails. LED lights
illuminate the underside of the stair with a warm glow that complements the cooler
daylighting coming from the skylight above. Image: Mark English Architects
This model close-up looks very close to “the real thing”. Image: Mark English Architects
This post was originally published on The Architects’ Take blog and has been republished here with permission.
