cool structures and hot designs
As you may already know, the building Lab team does not shy away from geeky details. In fact, we embrace every opportunity for pushing the envelope both aesthetically and structurally when it fulfills the client's wishes and needs. From serious excavations and foundation works, to seemingly impossible cantilevers, floating stairs, and wall-to-wall glass, we relish the design and construction with much enthusiasm and glee. Certainly, we do have a secret weapon: in the person of structural engineer Nate Williams. In this issue, Nate takes time off from his busy schedule to chat with us at his Berkeley office.
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what does a structural engineer do?
The short answer: "I make sure buildings stand up, or more importantly, don't fall down." Nate and his colleagues at Mosswood Engineering take the architectural plans and create a set of structural plans which delineate the bones of the building, sort of like x-ray images. They usually include the foundation, framing (columns, beams and joists which support floors and walls), and the lateral system. The lateral system is important in California as it deals with shear walls which resist earthquakes, and in other parts of the country, hurricanes.
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stairs that not quite touch the floor?
In a dramatic remodel of a San Francisco residence, glass railing grows out of two steel plate stringers, and the whole set of stairs hovers a few inches above the landing. On the outside (facing the corridor), the smooth metal face of the stringer shows no signs of any fasteners. This effect is achieved by threading the pre-welded studs of the outside plate through the inside plate and angle tread supports (with pre-drilled holes). With the glass panel sandwiched in the center, the two identical plate stringers are fastened together tightly with nuts on the inside face (facing the wall). Near the bottom end of the stairs, a steel rod (welded to the inside steel stringer) suspends the whole ensemble from the joists of the floor above. Stay tuned for the photos of the finished stairs. You will see all the fasteners and supports disappear under the hardwood treads.
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mezzanine suspended from the ceiling!
Another interesting feature in this project is found at the lower mezzanine. Instead of being supported with a column below, one corner of the floor is suspended from the ceiling. The above drawing shows how a steel rod is attached to the joists above with a special metal holdown and secured to the floor beam below with a nut, steel plate washer, and countersink. Again, a continuous steel plate with welded thread rods is attached to each side of the floor structure to secure the glass railing. The mezzanine and the garden level (bottom) in this multi-storied residence are characterized by ample ceiling heights and shallow depths. By eliminating visual distractions such as columns, the overall space feels cleaner and more expansive.
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expand design possibilities or rein in wild schemes?
"In California, residential building designers are allowed to do remodels according to what are laid out in the code. Since the code is really restrictive, design possibilities may appear rather limited. With the help of a structural engineer, it is possible to obtain permits for unconventional features: extra large open space with beams supporting the joists; or long expanse of glass from wall to wall." We have exactly such a case at the garden level of this residence where a two-story-high wall is composed with fixed glass windows and glass sliders that open out to the patio. Nate has designed a custom fabricated moment frame (for earthquake resistance) consisted of steel columns and beams welded together on site. There is a double height glass expanse on the side and a wood beam between window systems in the middle to allow for the glass sliders (see drawings above).
Nate believes that almost any designs can be built nowadays. Most of the time, it is a matter of cost. He can advise on a solution which can achieve the desired effects and still work within the budget. In the case of the aforementioned custom moment frame, it is rather expensive. But it suits the situation better and allows for more flexibility.
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Nate Williams has been working as an engineer in the Bay Area since 1998. His experience in existing wood construction makes him familiar with local building practices and adept at combining new with existing designs. He started his business in 2006 under his own name and rebranded as Mosswood Engineering in 2010. The name comes from a local park where he often practices personal balance and equilibrium as a proficient juggler and slackliner.
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Photo captions: L-R
Profile photo: steel moment frame for a two-story-high space
1,2,3,4. details and construction photos of hanging stairs
5. details of suspended mezzanine floor
6, 7. field crews attaching steel plate to floor structure
8,9. details of steel moment frame
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