Inclination and Evolution: A Stair Design
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.
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.
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.)
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.￼
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.
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.
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.
Bolting it Down
To attach the glass panels to the stair treads, a special standoff bolt was used.
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.
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.
The final result has a sculptural quality to it. The stair appears to float in space as an invitation to ascend.